applied sciences

Article Reconstruction and Measurement of Irregular Karst Caves Using BLST along the Shield Metro Line

Shangqu Sun 1, Liping Li 2, Jing Wang 2,* , Shuguang Song 3, Peng He 1,* and Zhongdong Fang 2

1 Shandong Provincial Key Laboratory of Civil Engineering Disaster Prevention and Mitigation, Shandong University of Science and Technology, Qingdao 266590, China; [email protected] 2 School of Qilu Transportation, Shandong University, Jinan 250061, China; [email protected] (L.L.); [email protected] (Z.F.) 3 School of Transportation Engineering, Shandong Jianzhu University, Jinan 250101, China; [email protected] * Correspondence: [email protected] (J.W.); [email protected] (P.H.)



Received: 15 December 2019; Accepted: 2 January 2020; Published: 4 January 2020


Featured Application: Accurate exploration of karst caves and the protection of springs.

Abstract: This study investigated the application of the borehole laser scanning technology (BLST) method in the detection of both dry and water-filled karst caves. In order to solve the problem of excessive laser attenuation during the detection, we designed a test for the characteristics of multiwavelength laser attenuation in water-filled karst caves and studied the influence exerted by various factors, including different wavelengths, different laser power levels, different suspended media, and effect of turbidity on the attenuation coefficient. During the test, we discovered the existence of a “blue-green window” with low turbidity and a “near infrared window” with high turbidity in karst cave water environments. Based on the general survey results of drilling and comprehensive geophysical prospecting, a quantitative method using targeted drilling was proposed to detect the spatial morphology of karst caves in complex environments. We also investigated the effects of complex environmental factors such as suspended media and high turbidity on the laser detection distance and accuracy in karst caves, and established a quantitative matching model of laser wavelengths, laser power, and complex environmental parameters. Based on this, we obtained the best acquisition mode for detecting lasers in different karst development environments. A high-precision, three-dimensional visualized model of a real karst cave was established to quantitatively obtain the characteristic parameters, such as accurate position, three-dimensional shape, space volume, and cave filling type, which was applied to the detection of karst caves along the Jinan subway line.

Keywords: irregular karst cave; measurement; 3D parameters; BLST method; shield metro

1. Introduction As cities develop and urban populations increase rapidly, traffic congestion has become one of the greatest problems facing many cities in China. Metro systems, or subways, which makes full use of underground space and reduce congestion on the ground, have become an important part of urban infrastructure and a popular traffic choice for Chinese people in the 21st century [1–4]. Due to complex urban geologic conditions, a large number of metro tunnels have to pass through underground karst areas, such as the Jinan metro tunnel passing through water-rich hard rock karst cave areas, the Wuhan metro tunnel passing through honeycomb-shaped caves, and the Changsha metro tunnel passing through complicated underwater karst caves. The covertness of typical karst geological bodies such as karst caves increases the permeability of the rock structure and lowers relevant rock mechanical

Appl. Sci. 2020, 10, 392; doi:10.3390/app10010392 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 392 2 of 22 parameters, which may result in engineering disasters and hazards, including tunnel water bursting and inrush, water leakage, karst surface collapse, and shield cutterhead drooping, particularly when the caves bear both confined water pressure and shield tunneling disturbance [5,6]. It is no exaggeration that water inrush has become a serious threat to shield tunneling projects [7–9]. With the rapid development of computer technology, photogrammetry technology, and intelligent control technology, the detection of unfavorable geological bodies in tunnels and underground engineering projects utilizes more digital and quantitative methods and equipment with higher accuracy [10]. Currently, the common methods to detect cavity disaster sources, such as caves developed in the shallow karst areas, are drilling and geophysical exploration. The drilling method, as the most conventional and direct geological survey method, is especially suitable for detection in high-risk areas, such as underground karst caves. With a single borehole, one can obtain the approximate position, development height, and roof thickness of a karst cave at the borehole measuring point, while multiple boreholes can determine the horizontal boundary range of the karst cave. However, the drilling method lacks clear direction and is essentially a type of single-point detection method, during which the connection between boreholes can only be determined by empiricism and relevant calculation. This leads to the inevitability of blind areas, high costs, and consumption of time and labor [11]. The commonly used geophysical exploration methods, classified according to the exploration principles, include electrical methods (such as the high density electrical method), electromagnetic wave methods (such as the transient electromagnetic method and ground penetrating radar method), and seismic wave methods (such as the land sonar method) [12]. In the process of geophysical exploration, the detection capacity of geophysical exploration methods is limited by many factors, including the physical characteristics and spatial morphology of the detection medium or object, the geological structure and structural characteristics of the detection site, the hydrogeological conditions of the detection site, the topographic relief of the working face, the distribution of electromagnetic interference sources and interference bodies, the performance parameters of the devices, and the experience levels of the personnel operating the devices outdoors and processing and analyzing data indoors. Therefore, it is almost impossible to avoid fuzziness, multiplicity, and uncertainty in comprehensive geophysical exploration [13]. Consequently, drilling and comprehensive geophysical exploration methods cannot accurately detect karst cave boundaries in complex geological environments, rather can only deliver two-dimensional or even one-dimensional qualitative data with poor visualization effects and cannot provide quantitative parameters of karst caves, such as the accurate position and size information. The rapid development and wide application of contactless drilling laser measurement technology provides a solution for the exploration of karst caves. The biggest advantage of this method is that the microlaser probe can adapt to various kinds of narrow channels and boreholes and can be inserted deeply into a cave to obtain the point cloud coordinate data. This method has been widely employed in many professional instruments for the exploration of poor geological structures, such as the Cavity Monitoring System (CMS) [14] and Cavity Autoscanning Laser System (C-ALS) [15]. Li [16] took the lead in applying borehole scanning technology in goaf detection, stability calculation, monitoring, and early warning. Luo [17] investigated the formation rules of point cloud grids and maximum angle triangles, the dominant vertex segmentation strategy, and the irregular triangle optimization method, and realized the three-dimensional modeling and visualization of scattered point clouds in complex goaf sections. Many scholars have also carried out a lot of research on point cloud data acquisition, reconstruction, and visualization [18,19]. At present, the research and relevant applications are mainly focused on the detection of goaf sections in relatively favorable environments, while the occurrence environment of karst caves along subway lines is extremely complex. Many scholars have carried out studies on laser transmission characteristics in water [20–22], most of which focused on the attenuation characteristics and detection effects of lasers in ocean water and showed that a “blue-green window” exists in ocean water; that is, blue and green lasers experience the least attenuation in ocean water [23]. However, the water environment inside a water-filled cave is much more complex than that in the ocean. Most of the media in a water-filled cave, such as CaCO3, clay, and silt, are suspended. As a Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 22

Appl.the water Sci. 2020 environment, 10, 392 inside a water-filled cave is much more complex than that in the ocean.3 Most of 22 of the media in a water-filled cave, such as CaCO3, clay, and silt, are suspended. As a result, the optical reaction between the above-mentioned suspended media and the laser during its transmission result,is more the complicated, optical reaction and the between attenuation the above-mentioned characteristic of suspendedthe laser is also media more and obvious, the laser which during result its transmissionin the detection is more distance complicated, being ex andtremely the attenuation limited, and characteristic thus not meeting of the laserthe requirements is also more obvious,for karst whichcave detection. result in the Moreover, detection the distance influence being of extremely complex limited,environmental and thus factors, not meeting such as the cave requirements humidity, fordust karst concentration, cave detection. water-filled Moreover, media, the influence and turb ofidity, complex on the environmental laser point factors,cloud detection such as cave and humidity,reconstruction dust concentration, is not clear. As water-filled a result, problems media, andrelated turbidity, to multinoise, on the laser multidistortion, point clouddetection and multinail and reconstructionfactors in the karst is not cave clear. model As a result,have not problems been effectively related to solved multinoise, [24]. multidistortion, and multinail factorsIn in summary, the karst cavethe cavity model laser have scanning not been method effectively has solved the following [24]. advantages: (1) the ability to realizeIn summary,the quantitative the cavity and precise laser scanning detection method of cave has position, the following size, filling advantages: state, and (1) connectivity; the ability to(2) realizethe ability thequantitative to establish andan actual precise and detection accurate of cave cave model position, with size, a better filling visualization state, and connectivity; effect; (3) (2)the theability ability to provide to establish an accurate an actual data and basis accurate to rati caveonalize model a treatment with a better plan and visualization guide the efieldffect; workers (3) the abilityto form to reasonable provide an treatment accurate data plans. basis For to example, rationalize grout a treatment always planruns andout guidein the theprocess field workersof grouting to formthe karst reasonable caves, which treatment is mainly plans. caused For example, by groundwater grout always flow runs or cracks out in in the the process connected of grouting pipes [25]. the If karstthe boundary caves, which shape, is mainly depth, causedsize, volume, by groundwater and internal flow connectivity or cracks inof thethe connectedkarst cave pipescan be [ 25accurately]. If the boundaryobtained, shape,the stability depth, evaluation size, volume, of andthe internalsurroundi connectivityng rock in ofthe the shield karst cavetunnel can will be accuratelybe largely obtained,improved, the which stability is of evaluation great significance of the surrounding to the safe treatment rock in the of shield the karst tunnel areas. will Therefore, be largely improved,we carried whichout an isexperimental of great significance study on the to theattenuation safe treatment of a multiwavelength of the karst areas. laser in Therefore, complex karst we carried cave water out anenvironment, experimental the study results on theof which attenuation will be of used a multiwavelength to guide the selection laser in complexand optimization karst cave of water laser environment,detection in water-filled the results of caves, which in will order be usedto realiz to guidee the themaximum selection laser and detection optimization distance of laser and detection the best indetection water-filled effect caves, of water-filled in order to realizecaves. The the maximumresearch achievements laser detection will distance be also and applied the best to detectionthe Jinan esubwayffect of water-filled project. caves. The research achievements will be also applied to the Jinan subway project.

2. Multiwavelength Laser Attenuation Characteristics Test in Water-Filled Karst Caves 2. Multiwavelength Laser Attenuation Characteristics Test in Water-Filled Karst Caves The main purpose of the multiwavelength laser attenuation test system is to investigate the The main purpose of the multiwavelength laser attenuation test system is to investigate the attenuation characteristics of lasers with different wavelengths in water-filled caves in order to improve attenuation characteristics of lasers with different wavelengths in water-filled caves in order to the detection distance of lasers in water. The test system includes a laser emission module, karst improve the detection distance of lasers in water. The test system includes a laser emission module, cave water environment simulation system, and a water turbidity and laser attenuation measurement karst cave water environment simulation system, and a water turbidity and laser attenuation system,measurement as shown system, in Figure as 1shown. Four kindsin Figure of suspended 1. Four kinds media of were suspended designed media to simulate were karstdesigned water, to including silt, fine sand, clay, and CaCO3. By controlling the quality of the suspended medium to simulate karst water, including silt, fine sand, clay, and CaCO3. By controlling the quality of the simulatesuspended the medium water turbidity, to simulate five the kinds water of lasers turbidity, with five different kinds wavelengths of lasers with were different designed, wavelengths and the attenuationwere designed, characteristics and the attenuation of lasers with characteristics different wavelengths of lasers andwith power different levels wavelengths in complex water-filled and power caveslevels were in complex studied water-filled through orthogonal caves were tests. studied through orthogonal tests.

(a)

Figure 1. Cont. Appl. Sci. 2020, 10, 392 4 of 22 Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 22

(b)

FigureFigure 1. 1. TestingTesting system: system: ( (aa)) design; design; ( (bb)) actual actual device device graph. graph.

2.1.2.1. Laser Laser Attenuation Attenuation Characteristics Characteristics Test Test System System 2.1.1. Multiwavelength Laser Emission Module 2.1.1. Multiwavelength Laser Emission Module In order to study the influence of laser wavelength on attenuation characteristics more In order to study the influence of laser wavelength on attenuation characteristics more comprehensively, the laser emission module selected five kinds of lasers with typical wavelengths, comprehensively, the laser emission module selected five kinds of lasers with typical wavelengths, as shown in Table1. They were a blue-purple laser emitter (405 nm), blue laser emitter (450 nm), green as shown in Table 1. They were a blue-purple laser emitter (405 nm), blue laser emitter (450 nm), laser emitter (532 nm), near infrared laser emitter (650 nm), and far infrared laser emitter (808 nm), green laser emitter (532 nm), near infrared laser emitter (650 nm), and far infrared laser emitter (808 respectively. The emitters had a glass window laser diode, emitted continuous dot lasers, employed an nm), respectively. The emitters had a glass window laser diode, emitted continuous dot lasers, optical coated glass lens, and maintained a constant output power. Since the far infrared laser was employed an optical coated glass lens, and maintained a constant output power. Since the far infrared beyond the normal observation wavelength range, this test adopted an infrared camera (modified laser was beyond the normal observation wavelength range, this test adopted an infrared camera from Canon EOS M3 camera) with a Canon Image Stabilizer lens and an infrared filter with a 780 nm (modified from Canon EOS M3 camera) with a Canon Image Stabilizer lens and an infrared filter with wavelength. To observe the 808 nm laser beam more clearly, we used an f/4.0 aperture and set the a 780 nm wavelength. To observe the 808 nm laser beam more clearly, we used an f/4.0 aperture and exposure time to 1/160 and the ISO (International Standardization Organization) to 12800. During the set the exposure time to 1/160 and the ISO (International Standardization Organization) to 12800. test, we also utilized a laser-fixed triangle support, which ensured the stability of the fixed support During the test, we also utilized a laser-fixed triangle support, which ensured the stability of the fixed with a maximum vertical adjustment range of 2 m; and a three-dimensional servo rotating head, which support with a maximum vertical adjustment range of 2 m; and a three-dimensional servo rotating controlled the emission direction, angle, and height of the laser emitter. The head was equipped with a head, which controlled the emission direction, angle, and height of the laser emitter. The head was horizontal level to ensure horizontal emission of the emitter and avoid refraction with the glass as equipped with a horizontal level to ensure horizontal emission of the emitter and avoid refraction much as possible. with the glass as much as possible.

Table 1. Laser wavelength. Table 1. Laser wavelength. Type Wavelength (nm) Wavelength Selected (nm) Type Wavelength (nm) Wavelength Selected (nm) Blue-violetBlue-violet laser laser 405 405 405 Blue laser 450, 457, 473 450 Blue laser 450, 457, 473 450 Green laser 532 532 NearGreen infrared laser laser 635, 660, 532 670, 671 532650 NearFar infrared infrared laser laser 808, 635, 946, 660, 980, 670, 1047, 671 1064 650808 Far infrared laser 808, 946, 980, 1047, 1064 808 2.1.2. Simulation Module of Karst Cave Water Environment 2.1.2. Simulation Module of Karst Cave Water Environment This module can simulate the water turbidity under different suspended media conditions. AccordingThis module to the explorationcan simulate data the of water spring turbidity strata, it wasunder found different that most suspended of the suspended media conditions. media in Accordingwater-filled to caverns the exploration are silt, clay, data fine of sand,spring and strata calcium, it was carbonate. found that In most this test, of the four suspended kinds of suspended media in water-filled caverns are silt, clay, fine sand, and calcium carbonate. In this test, four kinds of media were used: silt, clay, fine sand, and CaCO3. Each medium was evaluated as one of five grades suspendedaccording tomedia solubility, were andused: the silt, turbidity clay, fine index sand, was and measured CaCO3. Each by a turbiditymedium was tester. evaluated as one of five grades according to solubility, and the turbidity index was measured by a turbidity tester. Appl. Sci. 2020, 10, 392 5 of 22

2.1.3. Measurement Module of Water Turbidity and Laser Attenuation Turbidity, as a parameter to evaluate water quality, is used to quantitatively measure the degree of water opacity caused by particles. There are three main kinds of turbidity measurement methods: transmission, scattering, and scattering-transmission. Scattering-transmission measurement features the highest accuracy [23], and is also the most commonly method used at present. The water turbidity measurement module used a SGZ-200BS portable turbidimeter (made by Yuefeng Device Co., Ltd. Shanghai, China). Formazine was selected to prepare turbidity liquid samples with upper and lower limits. According to the international standard ISO7027, the unit of measurement of turbidity is NTU (Nephelometric Turbidity Unit). The VLP-2000 laser power meter was selected in this test to measure the initial laser emission power and the power attenuated through the water simulator. The meter uses a pyroelectric probe and converts the laser light energy into thermal energy and then an electrical signal through the thermopile structure so that the laser power can be accurately measured. This device does not require wavelength calibration and features high resolution, fast measuring speed, convenience, and reliability. Its measuring range is 0–200 mw, and the accuracy can reach 0.1 mw. The laser power measurement module calculates the attenuation coefficient of the laser after passing through a certain distance of water by measuring the laser power before and after the attenuation of different wavelengths.

2.2. Definition of Laser Attenuation Coefficient Here, we presume a collimated laser with a wavelength of λ and radiation flux of I(λ) enters into water perpendicular to the glass. During its transmission in the water simulator, if the transmission distance is dl and the radiation flux loss caused by water scattering and absorption is dI(λ), then the water attenuation coefficient is defined as:

dI(λ) C(λ) = (1) −I(λ)dl

Thus, the radiation flux of transmission distance l of the laser in the water can be denoted as:    Zl    I( l) = I( )  C( ) l λ, λ, 0 exp λ d  (2) −  0

Presuming the attenuation coefficient remains constant during the transmission, Equation (2) can be modified as: I(λ, l) = I(λ, 0) exp[ C(λ)dl] = I(λ, 0) exp[ l/l (λ)] (3) − − 0 in which l0(λ) is also named the attenuation length. In this test system, the laser first entered perpendicularly into and through the dry analog transparent box, and the laser intensity I1 and I2 detected initially and after the attenuation by the laser power meter were recorded. The splitting ratio of the beam splitting prism was set to K1, and the attenuation rate of the glass before and after passing the transparent box was K2. Under the condition of ensuring the vertical incidence of the laser, assuming that the transmittance between the glass and the air interface remains unchanged, the laser intensity after passing through the transparent box is:

2 4 I2 = I1K1K2 T1 (4)

When the laser path remained constant, we filled the transparent box with water, and used the laser power meter to measure the light intensity at the initial stage and after attenuation, which were recorded as I10 and I20 , respectively. It was assumed that the transmittance of the laser from the air to the transparent box glass was T1, the transmittance of laser entering the water perpendicularly from the glass was T2, the average attenuation coefficient of the laser in the simulated water in the Appl. Sci. 2020, 10, 392 6 of 22 transparent box was γ, and the length of the analog box was l. The light intensity after passing through the box was: 2 2 2 γl I20 = I10 K1K2 T1 T2 e− (5) According to the Fresnel formula, under the condition of perpendicular entry of the 4n1n2 laser, T = 2 , commonly the glass refraction index n1 = 1.4985, and the water refraction (n1+n2) index n2 = 1.3228. It was assumed that the laser was attenuated to 1/1000 after passing through the dry, transparent analog box. By combining Equations (4) and (5), the average attenuation coefficient was: ! 1 I0 /I0 γ = ln 2 1 0.927388 (6) − l I2/I1 ×

3. Test Results

3.1. Attenuation Law of Lasers with the Same Wavelength in Different Suspended Media Based on the above-mentioned test system, we recorded the laser emission power and attenuation power under the influence of different laser wavelengths, different suspended media, and different water turbidity grades, and calculated the corresponding laser attenuation coefficients using the above method. (1) From the fitting curve in Figure2a, it can be seen that when the laser wavelength is 405 nm, the suspended media are CaCO3, silt, clay, and fine sand. For the laser suspended in the above media, the attenuation coefficient increases linearly with the increase of turbidity. The R2 fitting degree values are 0.9956, 0.9802, 0.9945, and 0.9965, respectively. From the slope of the fitting curve, with the increase of turbidity, the increasing rate of the laser attenuation coefficient increases from CaCO3 to clay, silt, and fine sand in turn. The particle size of CaCO3 is about 5–10 µm, and the particle sizes of clay, silt, and fine sand are greater. When the laser wavelength is 405 nm, the rate of the laser attenuation coefficient increases with the increase of the medium particle size; that is, the slope of the fitting curve is γ (CaCO3) < γ (clay) < γ (silt) < γ (sand). (2) When the laser wavelength is 450 nm, the fitting curve in Figure2b shows that the attenuation coefficient increases linearly with the increase of turbidity in the solution of CaCO3, silt, clay, and fine sand. The R2 fitting degree values are 0.954, 0.9559, 0.9562, and 0.9964, respectively. With the increase of water turbidity, the increasing rate of corresponding attenuation coefficient is different in the CaCO3, silt, fine sand, and clay media: the slope value of the fitting curve is γ (CaCO3) < γ (clay) < γ (sand) < γ (silt). (3) When the laser wavelength is 532 nm, the attenuation coefficient also increases linearly with the increase of turbidity, and the R2 fitting degree values are 0.9973, 0.9958, 0.9668, and 0.9938, respectively, as shown in Figure2c. From the slope of the fitting curve, the attenuation coe fficient rate increases from silt to CaCO3, clay, and fine sand; that is, γ (silt) < γ (CaCO3) < γ (clay) < γ (sand). (4) The attenuation coefficient of the 650 nm wavelength laser increases linearly with the increase of turbidity in the CaCO3, silt, clay, and fine sand media, as shown in Figure2d. The attenuation coefficient rate increases from silt to clay, CaCO3, and fine sand; that is, γ(silt) < γ(clay) < γ(CaCO3) < γ(sand). (5) According to Figure2e, when the laser wavelength is 808 nm, the attenuation coe fficient increases linearly with the increase of turbidity, and the R2 fitting degrees rates are all higher than 0.98. Judging from the slope of the fitting curve, with the increase of water turbidity, the order of the increasing rate of attenuation coefficient from low to high is CaCO3, fine sand, clay, and then silt. That is, the laser attenuation coefficient rate increases with the increase of the medium particle size. Appl. Sci. 2020, 10, 392 7 of 22 Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 22

(a) (b)

(c) (d)

(e)

Figure 2. AttenuationAttenuation coefficient—turbidity coefficient—turbidity curves curves using using different different wavelength lasers: lasers: ( (aa)) 405 405 nm; nm; ( (bb)) 405405 nm; ( c)) 532 532 nm; nm; ( d) 650 nm; ( e) 808 nm.

3.2. Attenuation Attenuation Law Law of of Lasers Lasers with with Differen Differentt Wavelengths in the Same Suspended Media

3.2.1. CaCO3 Suspended Medium 3.2.1. CaCO3 Suspended Medium

In CaCO3 medium, comparing the lasers with wavelengths of 405 nm, 450 nm, 532 nm, 650 nm, In CaCO3 medium, comparing the lasers with wavelengths of 405 nm, 450 nm, 532 nm, 650 nm, and 808 nm, it can be seen from Figure3a that the increasing rate of the laser attenuation coe fficient and 808 nm, it can be seen from Figure 3a that the increasing rate of the laser attenuation coefficient is the lowest when the laser wavelength is 450 nm, and all of the attenuation coefficients are lower is the lowest when the laser wavelength is 450 nm, and all of the attenuation coefficients are lower than those of other wavelengths. The cloud diagram of the laser attenuation coefficient with different than those of other wavelengths. The cloud diagram of the laser attenuation coefficient with different wavelengths and different turbidity grades is shown in Figure3b. When the laser wavelengths are wavelengths and different turbidity grades is shown in Figure 3b. When the laser wavelengths are 450 nm and 650 nm, there are two “ravines” with low attenuation coefficients. In order to compare the 450 nm and 650 nm, there are two “ravines” with low attenuation coefficients. In order to compare attenuation characteristics of 450 nm and 650 nm in depth, we adopted the above-mentioned fitting the attenuation characteristics of 450 nm and 650 nm in depth, we adopted the above-mentioned formula and solved the following two simultaneous equations: fitting formula and solved the following two simultaneous equations: YX=+Y0.1784= 0.1784 0.2174X + 0.2174 (7) (7)

YX=−0.2508 0.0796 (8) Appl. Sci. 2020, 10, 392 8 of 22

Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 22 Y = 0.2508X 0.0796 (8) Appl. Sci. 2020, 10, x FOR PEER REVIEW − 8 of 22 WhenWhen thethe turbidityturbidity isis 44 NTU,NTU, thethe attenuation coecoeffifficientscients ofof thethe twotwo laserslasers areare equal.equal. Thus, when When the turbidity is 4 NTU, the attenuation coefficients of the two lasers are equal. Thus, when thethe turbidityturbidity is is lower lower than than 4 4 NTU, NTU, the the 650 650 nm nm laser lase isr suggested, is suggested, and and when when the turbiditythe turbidity is higher is higher than the turbidity is lower than 4 NTU, the 650 nm laser is suggested, and when the turbidity is higher 4than NTU, 4 NTU, the 450 the nm 450 laser nm laser is suggested. is suggested. However, However, considering considering that that the the turbidity turbidity of purifiedof purified water water is than 4 NTU, the 450 nm laser is suggested. However, considering that the turbidity of purified water aboutis about 5 NTU5 NTU and and that that the the water water environment environment in in engineering engineering projects projects is is generally generally moremore turbidturbid thanthan is about 5 NTU and that the water environment in engineering projects is generally more turbid than purifiedpurified water,water, thethe laserlaser wavelengthwavelength ofof water-filledwater-filled cavecave detectiondetection shouldshould bebe 450450 nm,nm, requiringrequiring aa blue purified water, the laser wavelength of water-filled cave detection should be 450 nm, requiring a blue wavelengthwavelength laser.laser. wavelength laser.

(a) (b) (a) (b) Figure 3. Attenuation coefficient—turbidity curve in dissolution medium of CaCO3: (a) Two Figure 3. Attenuation coefficient—turbidity curve in dissolution medium of CaCO3: (a) Two Figuredimension; 3. Attenuation (b) Three coedimensionfficient—turbidity curve in dissolution medium of CaCO3:(a) Two dimension; (dimension;b) Three dimension (b) Three dimension 3.2.2. Silt Suspended Medium 3.2.2.3.2.2. SiltSilt SuspendedSuspended MediumMedium For silt, we compared lasers with wavelengths of 405 nm, 450 nm, 532 nm, 650 nm, and 808 nm. ForFor silt,silt, wewe comparedcompared laserslasers withwith wavelengthswavelengths ofof 405405 nm,nm, 450450 nm,nm, 532 nm, 650 nm, and 808 nm. When the laser wavelength is 650 nm, the increasing rate of laser attenuation coefficient is the lowest WhenWhen thethe laserlaser wavelengthwavelength isis 650650 nm,nm, thethe increasingincreasing raterate ofof laserlaser attenuationattenuation coecoefficientfficient isis thethe lowestlowest with the increase of turbidity, as shown in Figure 4a. However, when the turbidity is relatively small, withwith thethe increaseincrease ofof turbidity,turbidity, asas shownshown inin FigureFigure4 4a.a. However, However, when when the the turbidity turbidity is is relativelyrelatively small,small, all the attenuation coefficients of the 450 nm laser are less than those of the 650 nm laser. With the allall thethe attenuationattenuation coecoefficientsfficients ofof thethe 450450 nmnm laserlaser areare lessless thanthan thosethose ofof thethe 650650 nmnm laser.laser. WithWith thethe increase of turbidity, the 650 nm laser is more advantageous. The cloud diagram of the laser increaseincrease of of turbidity, turbidity, the the 650 nm650 lasernm islaser more is advantageous.more advantageous. The cloud The diagram cloud ofdiagram the laser of attenuation the laser attenuation coefficients with different wavelengths and turbidity grades in silt medium is shown in coeattenuationfficients with coefficients different with wavelengths different wavelengths and turbidity and grades turbidity in silt grades medium in is silt shown medium in Figure is shown4b. in Figure 4b. Figure 4b.

(a) (b) (a) (b) FigureFigure 4. AttenuationAttenuation coefficient—turbidity coefficient—turbidity curve curve in in silt silt dissolution dissolution medium: medium: (a) (a)Two Two dimension; dimension; (b) Figure 4. Attenuation coefficient—turbidity curve in silt dissolution medium: (a) Two dimension; (b) (b)Three Three dimension dimension Three dimension WhenWhen thethe laser laser wavelengths wavelengths are are 450 nm450 andnm 650 and nm, 650 there nm, are there two “ravines”are two with“ravines” low attenuation with low When the laser wavelengths are 450 nm and 650 nm, there are two “ravines” with low coeattenuationfficients. coefficients. In order to compare In order theto compare 450 nm and the 650450 nmnm attenuationand 650 nm characteristics,attenuation characteristics, we adopted thewe attenuation coefficients. In order to compare the 450 nm and 650 nm attenuation characteristics, we above-mentionedadopted the above-mentioned fitting formula fittin andg formula solved the and following solved the two followin simultaneousg two simultaneous equations: equations: adopted the above-mentioned fitting formula and solved the following two simultaneous equations: YX=−0.2531 1.2779 (9) =−Y = 0.2531X 1.2779 (9) YX0.2531 1.2779− (9) YY=+ 0.1698 0.6587 (10) YY=+ 0.1698 0.6587 (10) When the turbidity is 225 NTU, the attenuation coefficients of the two are equal. Thus, when the When the turbidity is 225 NTU, the attenuation coefficients of the two are equal. Thus, when the turbidity is lower than 22.5 NTU, the 450 nm blue laser is suggested, and when the turbidity is higher turbidity is lower than 22.5 NTU, the 450 nm blue laser is suggested, and when the turbidity is higher Appl. Sci. 2020, 10, 392 9 of 22

Y = 0.1698Y + 0.6587 (10)

When the turbidity is 225 NTU, the attenuation coefficients of the two are equal. Thus, when Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 22 the turbidity is lower than 22.5 NTU, the 450 nm blue laser is suggested, and when the turbidity isthan higher 22.5 thanNTU, 22.5 the NTU, 650 nm the near 650 nminfrared near infraredlaser is suggested. laser is suggested. This is obvi Thisously is obviously different difromfferent the fromconventional the conventional marine water marine “blue- watergreen “blue-green window”. window”. In karst In cave karst water cave waterenvironments environments with withhigh highturbidity, turbidity, the near the infrared near infrared wavelength wavelength laser suff laserers sulessffers attenuation less attenuation and can andachieve can longer achieve detection longer detectiondistances. distances. 3.2.3. Clay Suspended Medium 3.2.3. Clay Suspended Medium When the suspended medium is clay, the curve of attenuation coefficient with turbidity is shown When the suspended medium is clay, the curve of attenuation coefficient with turbidity is shown in Figure5a. We compared the lasers with wavelengths of 405 nm, 450 nm, 532 nm, 650 nm, and 808 nm. in Figure 5a. We compared the lasers with wavelengths of 405 nm, 450 nm, 532 nm, 650 nm, and 808 When the laser wavelength is 650 nm, the increasing rate of laser attenuation coefficient is the lowest nm. When the laser wavelength is 650 nm, the increasing rate of laser attenuation coefficient is the with the increase of turbidity. When the turbidity is lower than approximately 25 NTU, the 450 nm lowest with the increase of turbidity. When the turbidity is lower than approximately 25 NTU, the laser attenuation coefficients are all less than 650 nm. With the increase of turbidity, the 650 nm laser is 450 nm laser attenuation coefficients are all less than 650 nm. With the increase of turbidity, the 650 more advantageous. The cloud diagram of the laser attenuation coefficients with different wavelengths nm laser is more advantageous. The cloud diagram of the laser attenuation coefficients with different and different turbidity grades is shown in Figure5b. wavelengths and different turbidity grades is shown in Figure 5b.

(a) (b)

FigureFigure 5.5. AttenuationAttenuation coe coefficient—turbidityfficient—turbidity curve curve in in clay clay dissolution dissolution medium: medium: (a) ( Twoa) Two dimension; dimension; (b) Three(b) Three dimension dimension

WhenWhen thethe laserlaser wavelengthswavelengths areare 450450 nmnm andand 650650 nm,nm, therethere areare twotwo “ravines”“ravines” withwith lowlow attenuationattenuation coecoefficients.fficients. We adoptedadopted thethe above-mentionedabove-mentioned fittingfitting formulaformula andand solvedsolved thethe followingfollowing twotwo simultaneoussimultaneous equations:equations: Y = 0.3412X 1.6622 (11) YX=−0.3412 1.6622− (11) Y = 0.2203X + 1.602 (12) YX=+0.2203 1.602 (12) When the turbidity is 27 NTU, the attenuation coefficients of the two are equal. Thus, when the turbidityWhen is lowerthe turbidity than 27 is NTU, 27 NTU, the 450 the nm attenuation laser is suggested, coefficients and of when the two the are turbidity equal. is Thus, higher when than the 27 NTU,turbidity the 650is lower nm laser than is27 suggested. NTU, the 450 This nm follows laser theis suggested, same law and as for when silt. the turbidity is higher than 27 NTU, the 650 nm laser is suggested. This follows the same law as for silt. 3.2.4. Fine Sand Suspended Medium 3.2.4. Fine Sand Suspended Medium For a suspended medium of fine sand, we compared the lasers with wavelengths of 405 nm, 450 nm,For 532a suspended nm, 650 nm, medium and 808 of fine nm. sand, When we the compared laser wavelength the lasers iswith 450 wavelengths nm, the increasing of 405 nm, rate 450 of thenm, laser 532 nm, attenuation 650 nm, coeandffi 808cient nm. is theWhen lowest the laser with wavelength the increase is of 450 turbidity. nm, the When increasing the turbidity rate of the is lowerlaser attenuation than 40 NTU, coefficient the 450 nm is the laser lowest attenuation with the coe increasefficients of are turbidity. all less than When those the of turbidity lasers with is lower other wavelengths,than 40 NTU, as the shown 450 innm Figure laser6 attenuationa. The cloud coefficie diagramnts of are laser all attenuation less than those coe ffi cientsof lasers with with diff erentother wavelengthswavelengths, and as shown different in Figure turbidity 6a. The grades cloud in diagram fine sand of medium laser attenuation is shown coefficients in Figure6b. with In orderdifferent to wavelengths and different turbidity grades in fine sand medium is shown in Figure 6b. In order to compare the attenuation characteristics of 450 nm and 650 nm lasers in fine sand medium, we solved the following two simultaneous equations: YX=−0.30205 0.89683 (13)

YX=+0.5807 0.40815 (14) Appl. Sci. 2020, 10, 392 10 of 22 compare the attenuation characteristics of 450 nm and 650 nm lasers in fine sand medium, we solved the following two simultaneous equations:

Y = 0.30205X 0.89683 (13) − Y = 0.5807X + 0.40815 (14) Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 22

(a) (b)

FigureFigure 6. 6.Attenuation Attenuation coe coefficient—turbidityfficient—turbidity curve curve in in dissolution dissolution medium medium of of SiO SiO2:(2: a(a)) Two Two dimension; dimension; (b()b Three) Three dimension. dimension.

WhenWhen the the turbidity turbidity is is −4.74.7 NTU, NTU, the the attenuation attenuation coe coefficientsfficients of of the the two two are are equal. equal. Since Since the the water water − turbidityturbidity grades grades are are all all higher higher thanthan 0,0, whenwhen thethe suspended medium is is fine fine sa sand,nd, the the 450 450 nm nm laser laser is issuggested. suggested. FromFrom the the above above results, results, it canit can be be seen seen that that the th “bluee “blue and and green green window”, window”, which which is widely is widely used used in marinein marine water, water, is not is not suitable suitable for thefor the complex complex water-filled water-filled environment environment typical typical of karst of karst caves. caves. When When a karsta karst cave cave is filled is filled with waterwith inwater order in to order dissolve to dissolve the particles the ofparticles CaCO3 ,of silt, CaCO and clay,3, silt, the and attenuation clay, the coeattenuationfficient of coefficient the blue light of the wavelength blue light wavelength (450 nm) is (450 the lowestnm) is the when lowest the turbiditywhen the isturbidity low, and is thelow, attenuationand the attenuation coefficient coefficient in the near in infrared the near wavelength infrared wavelength (650 nm) is the(650 smallest nm) is the when smallest the turbidity when the is high.turbidity Chen is [26 high.] also Chen mentioned [26] also the mentioned existence ofthe a “nearexistence infrared of a “near window” infrared in water window” with high in water turbidity. with Whenhigh theturbidity. water containsWhen the medium water contains fine sand medium with large fine particles, sand with the large precipitation particles, rate the isprecipitation fast. However, rate someis fast. fine However, sand particles some will fine still sand be suspendedparticles will over still the be period suspended of time over when the the period attenuation of time coe whenfficient the ofattenuation the blue laser coefficient with a wavelength of the blue laser of 450 with nm a is wavelength the smallest. of 450 nm is the smallest. AtAt present, present, the the lasers lasers are extensivelyare extensively applied applied in the in detection the detection of ocean of water, ocean which water, mainly which includes mainly + 2+ 2+ aincludes variety of a ions,variety such of ions, as Na such, Mg as Na, Ca+, Mg, and2+, Ca Cl2+−, .and However, Cl−. However, abundant abundant suspended suspended particles particles in the cavein the water cave and water the particleand the size particle and masssize and can havemass greatcan have effects great on the effects laser on attenuation the laser coeattenuationfficient. Thecoefficient. characteristics The characteristics and weights ofand di ffweightserent suspended of different particles suspended will leadparticles to great will di leadfferences to great in scatteringdifferences intensity. in scattering Large wavelengthintensity. Large lasers wavelength have greater lasers penetration, have greater resulting penetration, in different resulting slopes of in thedifferent fitted curve. slopes of the fitted curve.

3.3.3.3. Attenuation Attenuation Law Law of of Lasers Lasers with with Di Differentfferent Power Power Levels Levels InIn order order to to study study the the relationship relationship between between the the laser laser attenuation attenuation coe coefficientfficient and and laser laser emission emission powerpower in in the the complex complex waterwater environmentenvironment ofof aa karstkarst cave,cave, wewe selected several laser emitters emitters with with a awavelength wavelength of of 650 650 nm nm and and emission emission power power levels levels of of100 100 mw, mw, 200 200 mw, mw, and and 300 300mw, mw, respectively, respectively, and andused used the thesame same suspended suspended medium medium in in the the water. Then, Then, the correspondingcorresponding laserlaser attenuationattenuation coecoefficientsfficients were were measured, measured, and and the the laser laser attenuation attenuatio coen coefficientfficient turbidity turbidity curve curve was was drawn, drawn, which which is is shownshown in in Figure Figure7 .7. According According to to the the curve, curve, when when the the suspended suspended media media are are CaCO CaCO3,3, silt, silt, clay, clay, and and fine fine sand,sand, the the attenuation attenuation coe coefficientfficient of of a a laser laser of of the the same same power power level level increases increases linearly linearly with with the the increase increase ofof turbidity. turbidity. When When the the turbidity turbidity remains remains constant, constant, the the larger larger the the laser laser emission emission power power of of the the same same wavelength,wavelength, the the smaller smaller the the corresponding corresponding attenuation attenuation coe coefficientfficient will will be, be, and and when when the the turbidity turbidity is is relatively high, the differences in the attenuation coefficients of lasers of different power levels are larger. Judging from the attenuation coefficients of lasers of different power levels, the lasers with higher power have lower attenuation coefficients in the water. However, high-power lasers cause harm to the operators. Therefore, the selection principle of laser power is based on ensuring the safety of personnel. It is better to select a laser with higher power to better meet the laser detection distance requirements of large water-filled karst caves. Appl. Sci. 2020, 10, 392 11 of 22 relatively high, the differences in the attenuation coefficients of lasers of different power levels are larger. Judging from the attenuation coefficients of lasers of different power levels, the lasers with higher power have lower attenuation coefficients in the water. However, high-power lasers cause harm to the operators. Therefore, the selection principle of laser power is based on ensuring the safety of personnel. It is better to select a laser with higher power to better meet the laser detection distance requirementsAppl. Sci. 2020, 10 of, x largeFOR PEER water-filled REVIEW karst caves. 11 of 22

(a) (b)

(c) (d)

Figure 7. Attenuation coefficient—turbidity curve with lasers of different power levels: (a) CaCO3; (b) Figure 7. Attenuation coefficient—turbidity curve with lasers of different power levels: (a) CaCO3;(b) silt; (c) clay; (d) SiO2. silt; (c) clay; (d) SiO2.

3.4. Optimization Scheme of Lo Longestngest Laser Detection Distance First of all, all, the the selection selection of of the the laser laser wavelength wavelength is mainly is mainly based based on the on thesuspended suspended medium medium and andturbidity turbidity of the of thecave cave water water environments. environments. Table Table 2 2gives gives the the maximum maximum detection detection ranges ofof aa multiwavelength laserlaser in didifferentfferent suspendedsuspended mediamedia andand withwith didifferentfferent turbidityturbidity levels.levels. ItIt can be seen from Table2 2 that that no no matter matter what what kind kind of of suspended suspended medium medium is is used, used, the the 450 450 nm nm wavelengthwavelength laserlaser isis suitable inin waterwater environmentsenvironments withwith low low turbidity, turbidity, while while for for clay clay and and silt silt media, media, when when the the turbidity turbidity is aboutis about 25 NTU,25 NTU, the detectionthe detection distance distance of the of 650 the nm 650 laser nm is longer.laser is Combinedlonger. Combined with the fieldwith geological the field datageological and actual data boreholeand actual data, borehole by distinguishing data, by di thestinguishing type of suspended the type mediumof suspended and turbidity medium grade and ofturbidity water-filled grade karst of water-filled caves, the karst laser caves, with thethe appropriatelaser with the corresponding appropriate corresponding wavelength is wavelength selected to increaseis selected the to maximum increase the detection maximum distance. detection distance.

Table 2. Maximum detection range and laser wavelength selection recommendation.

Turbidity (NTU) Medium 10 20 30 40 50 Length (nm) 405 1.63 0.85 0.58 0.44 0.35 450 1.95 1.03 0.70 0.53 0.43 CaCO3 532 0.83 0.54 0.40 0.32 0.27 650 1.61 0.79 0.52 0.39 0.31 808 0.85 0.57 0.43 0.34 0.29 Laser wavelength suggested (nm) 450 450 450 450 450 405 1.02 0.61 0.44 0.34 0.28 450 3.11 1.03 0.62 0.44 0.34 Silt 532 1.12 0.70 0.50 0.40 0.32 650 1.65 0.96 0.68 0.52 0.43 808 1.05 0.55 0.37 0.28 0.23 Laser wavelength suggested (nm) 450 450 650 650 650 Appl. Sci. 2020, 10, 392 12 of 22

Table 2. Maximum detection range and laser wavelength selection recommendation.

Turbidity (NTU) Medium 10 20 30 40 50 Length (nm) 405 1.63 0.85 0.58 0.44 0.35 450 1.95 1.03 0.70 0.53 0.43 CaCO3 532 0.83 0.54 0.40 0.32 0.27 650 1.61 0.79 0.52 0.39 0.31 808 0.85 0.57 0.43 0.34 0.29 Laser wavelength suggested (nm) 450 450 450 450 450 405 1.02 0.61 0.44 0.34 0.28 450 3.11 1.03 0.62 0.44 0.34 Silt 532 1.12 0.70 0.50 0.40 0.32 650 1.65 0.96 0.68 0.52 0.43 808 1.05 0.55 0.37 0.28 0.23 Laser wavelength suggested (nm) 450 450 650 650 650 405 1.26 0.71 0.49 0.38 0.30 450 2.23 0.76 0.45 0.33 0.25 Clay 532 0.64 0.43 0.33 0.26 0.22 650 1.02 0.65 0.47 0.37 0.31 808 0.86 0.54 0.40 0.31 0.26 Laser wavelength suggested (nm) 450 450 650 650 650 405 1.25 0.66 0.45 0.34 0.27 450 1.84 0.76 0.48 0.35 0.27 SiO2 532 0.59 0.36 0.26 0.21 0.17 650 0.63 0.32 0.22 0.17 0.13 808 0.74 0.52 0.40 0.32 0.27 Laser wavelength suggested (nm) 450 450 450 450 450

Secondly, in the selection of laser power, Table3 gives the maximum detection distances of lasers of various power levels in different suspended media with different turbidity grades. When only considering the maximum detection distance, a laser with higher power should be selected to meet the long detection distance requirements for water-filled caves. However, to ensure the safety of personnel, the selected laser power is generally lower than 5 milliwatt, as it will otherwise cause harm to the operator’s eyes. Many devices put an emphasis on eye protection, such as the C-ALS produced by MDL (Measurement Devices Ltd) Company (UK) adopting FDA (Food and Drug Administration) IEC (International Electrotechical Commission) first-class laser eye protection and the CMS system produced by Optech Company of Canada adopting FDA 21 CFR1040 first-class laser eye protection. On the premise of ensuring the safety of personnel, a laser with higher power should be selected.

Table 3. Maximum detection range and laser power selection recommendation table.

Turbidity (NTU) Medium 5 10 20 30 40 Power (mw) 100 2.32 1.32 0.71 0.48 0.37 CaCO3 200 4.63 2.17 1.05 0.69 0.52 300 9.55 3.22 1.39 0.88 0.65 Maximum differential times 4.12 2.45 1.96 1.83 1.77 100 3.25 1.49 0.72 0.47 0.35 Silt 200 5.78 1.93 0.83 0.53 0.39 300 6.07 3.05 1.10 0.67 0.48 Maximum differential times 1.87 2.05 1.53 1.42 1.37 100 1.70 0.96 0.52 0.35 0.27 Clay 200 3.77 1.75 0.85 0.56 0.42 300 6.16 3.39 1.23 0.75 0.54 Maximum differential times 3.63 3.52 2.37 2.12 2.01 100 1.14 0.68 0.37 0.26 0.20 SiO2 200 2.06 0.99 0.48 0.32 0.24 300 4.31 1.36 0.57 0.36 0.27 Maximum differential times 3.77 2.01 1.55 1.42 1.36 Appl. Sci. 2020, 10, 392 13 of 22

Thirdly, the evaluation of the detection environment is mainly based on the type of suspended medium in the water-filled cave and the corresponding turbidity. The specific types of suspended media can be preliminarily judged in combination with the sampled water quality. If the field conditions permit, the water quality turbidity can be sampled and detected for further quantitative grade classification [26]. Appl. Sci. 2020, 10, x FOR PEER REVIEW 13 of 22 4. Fine Measurement and Reconstruction of Complex Karst Caves 4. Fine Measurement and Reconstruction of Complex Karst Caves 4.1. Borehole Laser Scanning Technology (BLST) Automatic Laser Scanning System in Karst Caves 4.1. Borehole Laser Scanning Technology (BLST) Automatic Laser Scanning System in Karst Caves At present, laser scanning technology has been widely used in tunnels and underground engineeringAt present, projects, laser suchscanning as intechnology the acquisition has been of widely rock mass used structuralin tunnels plane and underground information, theengineering deformation projects, of roadway such surroundingas in the acquisition rock, the establishmentof rock mass ofstructural rock mass plane models, information, and even forthe noncontactdeformation monitoring. of roadway Most surrounding of the above rock, applications the establishment use vertical of laserrock scanningmass models, technology and even in large for operatingnoncontact spaces, monitoring. however Most karst of the caves above are applications usually concealed. use vertical Even laser if the scanni locationng technology and depth ofin karstlarge cavesoperating are obtained spaces, however by drilling karst and caves comprehensive are usually geophysical concealed. Even prospecting, if the location the vertical and depth laser scannerof karst cannotcaves are enter obtained the interior by drilling of the caveand comprehensive to assess the parameters geophysical due prospect to instrumenting, the size vertical limitation. laser Thus,scanner it iscannot necessary enter to the use interior a laser of detection the cave systemto assess that the can parameters enter inside due theto instrument cave through size a limitation. narrow drilling Thus, channel.it is necessary TheC-ALS to use a is laser the firstdetection laser automaticsystem that scanning can enter drilling inside the device cave developed through a narrow by MDL drilling (UK), whichchannel. consists The C-ALS of a laser is the ranging first laser module, automatic three-dimensional scanning drilling servo device mechanical developed control by system, MDL (UK), data transmissionwhich consists system, of a laser and rangin data processingg module, three-dimensional system. Its greatest servo advantage mechanical is that control the laser system, probe data is miniaturizedtransmission withsystem, a diameter and data of onlyprocessing 5 cm, whichsystem. can Its be greatest used to advant enter theage karst is that cave the through laser probe narrow is boreholesminiaturized and with carry a outdiameter three-dimensional of only 5 cm, scanningwhich can of be rock used mass to enter surfaces. the karst The maincave through structure narrow of the C-ALSboreholes is shown and carry in Figure out three-dimens8. ional scanning of rock mass surfaces. The main structure of the C-ALS is shown in Figure 8.

(a) (b)

(c) (d)

FigureFigure 8.8. BoreholeBorehole laserlaser scanningscanning technologytechnology (BLST)(BLST) method:method: ((aa)) laserlaser rangingranging module;module; ((bb)) integratedintegrated detector;detector; ((cc)) processingprocessing software; software; ( d(d)) the the whole whole system. system.

The drilling laser scanning system is based on the laser ranging principle to obtain the point cloud coordinate data. The probe has a 3D navigation system that can read the depth of the probe in real time, so as to obtain the exact location of the karst cave. Through the three-dimensional mechanical servo rotation system, the three-dimensional scanning state of the laser ranging probe are controlled, and the three-dimensional point cloud coordinates of the inner wall of the cave are measured. Finally, the point cloud model is encapsulated to form a real cave model by using point cloud processing software to realize the precise exploration of the location, shape, and volume of the karst caves. The operation process is as follows: (a) Targeted drilling. Based on the results of drilling Appl. Sci. 2020, 10, 392 14 of 22

The drilling laser scanning system is based on the laser ranging principle to obtain the point cloud coordinate data. The probe has a 3D navigation system that can read the depth of the probe in real time, so as to obtain the exact location of the karst cave. Through the three-dimensional mechanical servo rotation system, the three-dimensional scanning state of the laser ranging probe are controlled, and the three-dimensional point cloud coordinates of the inner wall of the cave are measured. Finally, the point cloud model is encapsulated to form a real cave model by using point cloud processing software to realize the precise exploration of the location, shape, and volume of the Appl.karst Sci. caves. 2020, 10 The, x FOR operation PEER REVIEW process is as follows: (a) Targeted drilling. Based on the results of drilling14 of 22 and comprehensive geophysical exploration, the location of the targeted drilling is determined and andthe drillingcomprehensive is carried geophysical out to provide exploration, the downward the loca explorationtion of the targeted channel fordrilling the laser is determined probe. (b) and The thelaser drilling probe isis loweredcarried out inside to provide the cave, the and downward the laser emissionexploration module channel is adjusted for the laser to prepare probe. the (b) caveThe laserpoint probe cloud is for lowered detection. inside (c) Pointthe cave, cloud and scanning. the laser Theemission laser rangingmodule is module adjusted emits to prepare and receives the cave the pointlaser, andcloud the for corresponding detection. (c) pointPoint cloud cloud coordinates scanning. The are calculatedlaser ranging according module to emits the three-dimensional and receives the laser,coordinates and the and corresponding angles. (d) Point point cloud cloud model coordinates processing. are Through calculated theC-ALS according software, to the the three- point dimensionalcloud model coordinates is obtained andand thenangles encapsulated. (d) Point cloud to form model a real processing. cave model, Through in order the to C-ALS obtain software, the cave thesize, point shape, cloud volume, model and is obtained other parameters, and then encapsulated as shown in Figure to form9. a real cave model, in order to obtain the cave size, shape, volume, and other parameters, as shown in Figure 9.

(a) (b)

(c) (d)

Figure 9. Cavity cloud model acquisition process: ( (aa)) drilling drilling borehole; borehole; ( (bb)) position position adjustment; adjustment; ( (cc)) scanning; ( d) cloud model reconstruction.

4.2. Solution Solution of of Complex Complex Cave Point Cloud Coordinates One side of the laser ranging module is is equipped equipped with with a a laser laser transmitter transmitter and and a a laser laser receiver. receiver. After thethe laserlaser emitteremitter emits emits the the laser, laser, the the laser laser transmitter transmitter transmits transmits the the laser laser for afor certain a certain distance distance until untilreaching reaching the wall the of wall the of karst the cave,karst wherecave, where it is then it is transmitted then transmitted back and back received and received by the laser by the receiver. laser receiver.The distance The betweendistance thebetween laser emitter the laser and emitter the measuring and the pointmeasuring of the point cave wallof the can cave be expressed wall can as:be expressed as: 1 S = c∆t (15) 1 2 Sct=Δ (15) in which S is the distance between the emitter2 and the measuring point on the cave wall, c is the intransmission which 𝑆 is speed the distance of the laser between through the the emitter media, an andd the∆ tmeasuringis the time point taken on for the the cave laser towall, be emittedc is the transmissionand received. speed of the laser through the media, and 𝛥𝑡 is the time taken for the laser to be emitted and received. After the distance 𝑆 is calculated using 𝑐 and 𝛥𝑡, the relative coordinate values of the karst cave wall point can be obtained based on the horizontal and vertical rotation angles of the laser ranging module. As shown in Figure 10, O is set as the laser emission point with the coordinates

(𝑥,𝑦,𝑧), P is set as the point to be measured on the cave wall with the coordinates (𝑥, 𝑦,) 𝑧 , 𝛼 is set as the horizontal rotation angle, and 𝛽 is set as the vertical rotation angle; then, the relative coordinate value of p is:

x = S cosβ cosα (16) Appl. Sci. 2020, 10, 392 15 of 22

After the distance S is calculated using c and ∆t, the relative coordinate values of the karst cave wall point can be obtained based on the horizontal and vertical rotation angles of the laser ranging module. As shown in Figure 10, O is set as the laser emission point with the coordinates (x0, y0, z0), P is set as the point to be measured on the cave wall with the coordinates (x, y, z), α is set as the horizontal rotation angle, and β is set as the vertical rotation angle; then, the relative coordinate value of p is:

Appl. Sci. 2020, 10, x FOR PEER REVIEW x = S cos β cos α 15 (16)of 22

y = S cos β sin α (17) y = S cosβ sinα (17) z = S sin β (18) zS= sin β (18)

(a) (b)

FigureFigure 10.10. PrinciplesPrinciples of of laser laser measurement: measurement: (a) (scanninga) scanning line; line; (b) coordinate (b) coordinate calculation calculation of the of point the pointcloud. cloud.

TheThe absoluteabsolute coordinatecoordinate value value of ofp pis: is:  x  cos β cosα 00  x    x   cos β cos α 0 0 0  x0   =+1 1Δ  βα      yy  = ctc t  00cos sin y0+  y  (19)   ∆  0 cos β sin α 0   0  (19)   2 2 · β    zzz 000 0sin sin β0 z0

TheThe factorsfactors aaffectingffecting thethe accuracyaccuracy ofof cavecave shapeshape acquisitionacquisition areare asas follows:follows: thethe highhigh irregularityirregularity ofof cavecave shapes,shapes, thethe laserlaser detectiondetection beambeam beingbeing blockedblocked byby internalinternal surroundingsurrounding rock,rock, irregularirregular rockrock columns,columns, andand irregularirregular fillings,fillings, whichwhich resultresult inin aa singlesingle detectiondetection notnot coveringcovering thethe completecomplete internalinternal boundariesboundaries ofof aa cave,cave, andand thusthus allowingallowing thethe presencepresence ofof locallocaldetection detection “blind“blind spots”.spots”. TheThe sizesize ofof aa drydry karstkarst cavecave isis generallygenerally withinwithin thethe laserlaser detectiondetection range.range. WhenWhen therethere isis aa largelarge water-filledwater-filled cave,cave, duedue toto thethe rapidrapid attenuationattenuation ofof laserslasers inin water,water, the the detection detection distancedistance isis limited, limited, andand single-stationsingle-station detectiondetection cannotcannot covercover thethe wholewhole cave.cave. MultistationMultistation detectiondetection is,is, thus,thus, requiredrequired forfor pointpoint cloudcloud datadata splicing.splicing. TheThe accuracyaccuracy ofof laserlaser rangingranging decreasesdecreases withwith thethe increaseincrease ofof detectiondetection distance,distance, resultingresulting inin lowlow detectiondetection accuracy,accuracy, as as point point cloudsclouds areare densedense nearnear karstkarst cavescaves andand areare sparsesparse farfar fromfrom thethe caves.caves. UnderUnder thesethese conditions,conditions, itit isis alsoalso necessarynecessary toto carrycarry outout multistationmultistation detectiondetection toto eliminateeliminate thethe locallocal detectiondetection blindblind spotsspots ofof thethe cavecave andand ensureensure thethe refinementrefinement andand accuracyaccuracy ofof thethe cavecave boundaryboundary shape.shape. TheThe keykey toto thethe splicingsplicing ofof multiplemultiple detectiondetection pointpoint cloudsclouds isis thethe transformationtransformation ofof thethe detectiondetection coordinatecoordinate system. system. Taking Taking the the detection detection of two of stationstwo stations as an example,as an example, it is assumed it is thatassumed the coordinate that the coordinate system of one station is 𝑜−𝑥𝑦𝑧 and the other is 𝑜′ − 𝑥′𝑦′𝑧′. In the transformation system of one station is o xyz and the other is o0 x0 y0z0. In the transformation process, the coordinate process, the coordinate system− 𝑜′ − 𝑥′𝑦′𝑧′ can be− obtained by rotating the coordinate system 𝑜−𝑥𝑦𝑧 around the three coordinate axes and with the matrix translation, which can be expressed by Equation (20). xx'  ' =+ναβγ y RyT(,,) (20) '  zz

in which the translated matrix T can be described by x0, y0, and z0, the three translation amounts in the three-dimensional axes: Appl. Sci. 2020, 10, 392 16 of 22 system o x y z can be obtained by rotating the coordinate system o xyz around the three coordinate 0 − 0 0 0 − axes and with the matrix translation, which can be expressed by Equation (20).      x0   x       y  = R( ) y  + T  0  ν α, β, γ   (20)     z0 z in which the translated matrix T can be described by x0, y0, and z0, the three translation amounts in the three-dimensional axes:    x0    T =  y   0  (21)   z0 Equations (20) and (21) include 7 parameters: 3 rotation parameters (α, β, γ), 3 translation parameters (x0, y0, z0), and a scale factor. The acquisition of the karst cave cloud coordinates can be regarded as a rigid process without any scale changes; thus, the scale factor v is 1 in the solution process. The rotation angles of the coordinate system o xyz around the x, y, and z axes are defined as − α, β, γ, respectively. The coordinate system o xyz is translated along x, y, and z axes into o x y z . − 0 − 0 0 0 The rotation matrices around the x, y, and z axes are:    1 0 0    Rx(α) =  0 cos α sin α  (22)    0 sin α cos α  −    cos β 0 sin β    Ry(β) =  0 1 0  (23)    sin β 0 cos β  −    cos γ sin γ 0    Rz(γ) =  sin γ cos γ 0  (24)  −   0 0 1   Since R(α, β, γ) = Rx(α) Ry(β) Rz(γ , the following can be obtained: · ·      1 0 0  cos β 0 sin β  cos γ sin γ 0      R(α, β, γ) =  0 cos α sin α  0 1 0  sin γ cos γ 0  (25)    −   0 sin α cos α  sin β 0 cos β  0 0 1  − −    cos β cos γ cos β sin γ sin β   −  R(α, β, γ) =  cos α sin γ + sin α sin β cos γ cos α cos γ + sin α sin β sin γ sin α cos β  (26)  −   sin α sin γ + cos α sin β cos γ sin α cos γ + cos α sin β sin γ cos α cos β  − During the scanning of the cave point clouds at two stations, the three-dimensional coordinates of the laser emission points are obtained; that is, the corresponding translation matrix T is known, and the rotation adjustment state R(α, β, γ) of the laser emission probe and the corresponding coordinate system scanning results are also known. The point cloud coordinates in the coordinate system o x y z 0 − 0 0 0 can be calculated and obtained by Equation (20). Based on this, the point cloud data of several stations can be converted and spliced, and the multistation exploration and point cloud splicing of complex karst caves and large-scale karst caves can be realized. Appl. Sci. 2020, 10, 392 17 of 22

5. Case Study in Jinan Metro

5.1. Overview of the Project Jinan is a historical and cultural city famous for various springs, which can represent the city culture. Therefore, in line with the idea of being responsible for the history and future of Jinan, during the metro construction, the spring must be properly protected, which brings more challenging requirements for the safe construction of the metro tunnels. At present, the planned Jinan rail transit line includes two levels: the urban core area fast line (Line R) is under construction, and the central urban area general line (Line M) is still in planning. The Jinan rail transit construction line is shown in Figure 11a. There are a large number of caves in the karst areas along Line R, while the planned line M will pass through the exposedAppl. and Sci. sensitive 2020, 10, x FOR areas PEER of REVIEW the spring core. A large number of springs are exposed17 in of the 22 sensitive spring area, which has a shallow limestone roof and rich karst water. This area is classified as an are exposed in the sensitive spring area, which has a shallow limestone roof and rich karst water. extreme water-richThis area is classified area with as aan large extreme number water-rich of karst area with caves, a large weathering number of fissures, karst caves, and weathering structural fissures in magmaticfissures, rocks. and Consideringstructural fissures the in complexity magmatic rocks. of groundwater Considering the in complexity spring areas of groundwater and the significance in of protectingspring the areas springs, and the there significance will be greatof protecting challenges the springs, in the there safe will construction be great challenges of the Jinanin the metro.safe construction of the Jinan metro.

(a)

(b)

(c)

Figure 11.FigureOverview 11. Overview of the of studythe study area: area: ( a(a)) JinanJinan metro metro planning planning map; map; (b) geological (b) geological cross-section cross-section of of the tunnel between Wangfuzhuang and Dayangzhuang [27,28]; (c) water inrush at the tunnel face. the tunnel between Wangfuzhuang and Dayangzhuang [27,28]; (c) water inrush at the tunnel face. The section of rail transit line R1 from Wangfuzhuang to Dayangzhuang in Jinan was constructed using the shield method. This section is located in the most water-rich degree area in the central and western part of Jinan. In this area, the water supply source is sufficient, the water output of a single fractured karst water well reaches 10,000 m3/d, and the cave is pressure-bearing. The maximum water level elevation is about 8 m above the arch of the shield tunnel. In the middle of the Appl. Sci. 2020, 10, 392 18 of 22

The section of rail transit line R1 from Wangfuzhuang to Dayangzhuang in Jinan was constructed using the shield method. This section is located in the most water-rich degree area in the central and western part of Jinan. In this area, the water supply source is sufficient, the water output of a single fractured karst water well reaches 10,000 m3/d, and the cave is pressure-bearing. The maximum water level elevation is about 8 m above the arch of the shield tunnel. In the middle of the weathered limestone under the K30 + 460.3–K31 + 362.2 mileage section, the karst in this section has been largely developed and is rich in water. The pressure-bearing karst caves were easily exposed in the shield tunnel construction process, which in the future may induce water inrush disasters, threaten the safety of workers, and impede construction progress. The interval geological profile is shown in Figure 11b. When the shield machine tunneled to K31 + 342.355, the phenomenon of high water pressure and large-flow water inrush appeared at the tunnel face, and it was found that the water inrush increased. This could result in a series of problems, such as poor slag soil properties and transportation difficulty, seriously reducing the tunneling efficiency [29]. The inrush of water from the excavation face is shown in Figure 11c, showing an obvious large flow rate that is pressure bearing.

5.2. Detection of Complex Karst Caves and Three-Dimensional Data Acquisition The main idea of this exploration is to accurately locate the karst cave based on the drilling and comprehensive geophysical prospecting methods, and then use the BLST method to obtain the morphology and characteristic parameters of the karst cave. On the basis of the results of comprehensive geophysical prospecting and drilling, the key points and abnormally complex areas are highlighted. The cross-hole resistivity CT method, owing to its high resolution, has great advantages in the detection of complex water diversion channels. Therefore, this method was adopted for fine detection to obtain the accurate positions of karst caves. In view of the fact that there are a large number of karst caves, pipelines, and cracks along the metro tunnel, only the disaster sources were selected for accurate laser scanning, such as karst caves and cracks, which are large in scale and present serious challenges in the grouting process. Targeted drilling was carried out and a karst cave point cloud model was reconstructed. The three-dimensional laser scanning detection system consists of a microlaser probe, a directional rod, cable wires, and a point cloud data acquisition and processing system. The microlaser probe entered the interior of the cave through targeted drilling and scanned the cave quickly to construct the three-dimensional shape of the cave and then extract the corresponding characteristic parameters, including the volume and depth of the karst cave. The laser probe had a built-in 3D navigation module to measure the position and direction of the probe in the cave in real time, and accurately record the x, y, and z coordinates and azimuth of the probe. The probe, which integrates a camera and an infrared lighting system, is able to monitor the movement of itself in the borehole and eschew obstacles or depressions in time to avoid any damage. The fine detection of a karst cave mainly includes two parts: field detection and indoor data processing. First, the measurement and layout of boreholes should be carried out and arranged before laser scanning and detection, so as to fully know about the on-site detection environment. Then, the detection devices are assembled and debugged, confirming that the detection system works properly. The laser probe with cable wires is lowered along the pipe vertically to monitor the internal borehole environment in real time. When the probe reaches the predetermined cave position, the depth meter is read, the data is recorded, and the mechanical scanning and accuracy parameters are set; then, the microprobe begins to measure the point clouds, and the cavity scan system simultaneously collects the point cloud data. After the scanning task is completed, the probe moves up and the field collection work finishes. The detailed steps are shown in Figure 12. Appl. Sci. 2020, 10, x FOR PEER REVIEW 18 of 22 weathered limestone under the K30 + 460.3–K31 + 362.2 mileage section, the karst in this section has been largely developed and is rich in water. The pressure-bearing karst caves were easily exposed in the shield tunnel construction process, which in the future may induce water inrush disasters, threaten the safety of workers, and impede construction progress. The interval geological profile is shown in Figure 11b. When the shield machine tunneled to K31 + 342.355, the phenomenon of high water pressure and large-flow water inrush appeared at the tunnel face, and it was found that the water inrush increased. This could result in a series of problems, such as poor slag soil properties and transportation difficulty, seriously reducing the tunneling efficiency [29]. The inrush of water from the excavation face is shown in Figure 11c, showing an obvious large flow rate that is pressure bearing.

5.2. Detection of Complex Karst Caves and Three-Dimensional Data Acquisition The main idea of this exploration is to accurately locate the karst cave based on the drilling and comprehensive geophysical prospecting methods, and then use the BLST method to obtain the morphology and characteristic parameters of the karst cave. On the basis of the results of comprehensive geophysical prospecting and drilling, the key points and abnormally complex areas are highlighted. The cross-hole resistivity CT method, owing to its high resolution, has great advantages in the detection of complex water diversion channels. Therefore, this method was adopted for fine detection to obtain the accurate positions of karst caves. In view of the fact that there are a large number of karst caves, pipelines, and cracks along the metro tunnel, only the disaster sources were selected for accurate laser scanning, such as karst caves and cracks, which are large in scale and present serious challenges in the grouting process. Targeted drilling was carried out and a karst cave point cloud model was reconstructed. The three-dimensional laser scanning detection system consists of a microlaser probe, a directional rod, cable wires, and a point cloud data acquisition and processing system. The microlaser probe entered the interior of the cave through targeted drilling and scanned the cave quickly to construct the three-dimensional shape of the cave and then extract the corresponding characteristic parameters, including the volume and depth of the karst cave. The laser probe had a built-in 3D navigation module to measure the position and direction of the probe in the cave in real time, and accurately record the x, y, and z coordinates and azimuth of the probe. The probe, which integrates a camera and an infrared lighting system, is able to monitor the movement of itself in the borehole and eschew obstacles or depressions in time to avoid any damage. The fine detection of a karst cave mainly includes two parts: field detection and indoor data processing. First, the measurement and layout of boreholes should be carried out and arranged before laser scanning and detection, so as to fully know about the on-site detection environment. Then, the detection devices are assembled and debugged, confirming that the detection system works properly. The laser probe with cable wires is lowered along the pipe vertically to monitor the internal borehole environment in real time. When the probe reaches the predetermined cave position, the depth meter is read, the data is recorded, and the mechanical scanning and accuracy parameters are set; then, the microprobe begins to measure the point clouds, and the cavity scan system simultaneously collects the point cloud data. After the Appl.scanning Sci. 2020 task, 10 ,is 392 completed, the probe moves up and the field collection work finishes. The detailed19 of 22 steps are shown in Figure 12.

Appl. Sci. 2020, 10, x FOR PEER REVIEW 19 of 22 (a) (b) (c)

(d) (e) (f)

Figure 12. IrregularIrregular karst cave model: ( (aa)) borehole borehole 17#; 17#; ( (b)) borehole borehole 20#; ( c) borehole borehole 21#; ( d) borehole 23#; (e) borehole 24#; (f) borehole 25#.25#.

Based on the results ofof fieldfield drillingdrilling andand comprehensivecomprehensive geophysicalgeophysical exploration,exploration, the borehole scanning method was used to locate and quantitatively detect karst caves at 6 6 borehole borehole locations locations (17#, (17#, 20#, 21#, 23#, 23#, 24#, 24#, 25#). 25#). The The detec detectiontion results results are are as asfollows: follows: a karst a karst cave cave with with a volume a volume of 3.02 of 3.02m3 was m3 wasmeasured measured at the at theborehole borehole 17# 17#location, location, at ata depth a depth ofof about about 15.4 15.4 m; m; th thee detailed detailed morphology and characteristic parameters of the karst cave are shownshown in Figure 1212a,a, recorded as KC1. A cave with a volume of 3.89 m3 was measured at location 20#, at a depthdepth of about 11.8 m;m; thethe three-dimensionalthree-dimensional model of the cave is shownshown inin FigureFigure 1212b,b, recordedrecorded asas KC2.KC2. A karst cavecave withwith aa volumevolume ofof 3.663.66 mm33 was measured at borehole location 21#, at a depth of about 1 m; the three-dimensional model of the karst cave is shown in Figure 1212c,c, recordedrecorded asas KC3.KC3. A water-bearing cave with a volume of about 3.84 m3 was measured at location 23#, at a depth of about 15.7 m; the three-dimensional model of the cave is shown in Figure 1212d,d, recordedrecorded asas KC4.KC4. A half water-filledwater-filled cavecave withwith aa volumevolume ofof 3.153.15 mm33 was measured atat boreholeborehole location location 24#, 24#, at at a deptha depth of of about about 28.6 28.6 m; them; the three-dimensional three-dimensional model model of the of cave the iscave shown is shown in Figure in Figure 12e, recorded 12e, recorded as KC5. as A KC5. water-filled A water-filled cave with cave a volume with a of volume 3.71 m 3ofwas 3.71 measured m3 was atmeasured borehole at location borehole 25#, location at a depth 25#, of at about a depth 22.7 of m; about the three-dimensional 22.7 m; the three-dimensional model of thecave model is shownof the incave Figure is shown 12f, recorded in Figure as 12f, KC6. recorded as KC6. According to the above-mentioned detection result results,s, the exact spatial position of the cave was obtained, including the corresponding horizontal coordinatescoordinates andand depth.depth. Combining the tunnel route design diagram, the relative spatial position relationshiprelationship between the karst cave and the tunnel was calculated. Figure 13a13a shows the exac exactt orientation and distance of the karst cave in the the circumferential circumferential direction of the tunnel; while Figure 1313bb shows the exactexact position of the karst cave in the excavation range in front of thethe tunnel,tunnel, andand thethe diagramdiagram of thethe verticalvertical sectionsection andand planeplane positionposition between the cave andand thethe tunnel. tunnel. According According to to the the spatial spatial position position relationships relationships between between karst caveskarst andcaves the and tunnel, the thetunnel, distances the distances and orientations and orientations between between the karst th cavese karst and caves the tunneland the were tunnel calculated, were calculated, as shown as in Tableshown4. in Table 4.

Table 4. Parameters of irregular caverns.

Karst Cave Orientation Vertical Distance Horizontal Actual Volume Number (°) (m) Distance (m) Distance (m) (m3) 1 317.39 2.9 2.4 5.14 3.02 2 42.89 6.5 5.8 10.09 3.89 3 31.32 3.3 0.7 4.43 3.66 4 7.67 2.6 0.8 2.65 3.84 5 170.19 3.6 1.2 3.70 3.15 6 90 0.9 0 - 3.71 Appl. Sci. 2020, 10, 392 20 of 22

Table 4. Parameters of irregular caverns.

Karst Cave Orientation Vertical Horizontal Actual Volume (m3) Number (◦) Distance (m) Distance (m) Distance (m) 1 317.39 2.9 2.4 5.14 3.02 2 42.89 6.5 5.8 10.09 3.89 3 31.32 3.3 0.7 4.43 3.66 4 7.67 2.6 0.8 2.65 3.84 5 170.19 3.6 1.2 3.70 3.15 Appl. Sci. 2020, 10, x FOR PEER REVIEW 20 of 22 6 90 0.9 0 - 3.71

(a)

(b)

FigureFigure 13. Relative 13. Relative positions positions between between karst karst caves caves and theand tunnel: the tunnel: (a) profile (a) profile diagram; diagram; (b) plane (b) plane graph. graph. 6. Conclusions 6. Conclusions(1) This paper discussed the application effect of laser point cloud scanning in dry caves and water-filled(1) This caves, paper designed discussed a multiwavelengththe application effect laser of attenuation laser point characteristic cloud scanning test in for dry water-filled caves and caveswater-filled in order caves, to solve designed the problem a multiwavelength of excessive laser laser attenuation attenuation characteristic in the detection test offor water-filled water-filled caves,caves andin order investigated to solve the problem effects of of di excessivefferent laser laser wavelengths, attenuation di inff erentthe detection laser power of water-filled levels, diff erentcaves, suspendedand investigated media, andthe effects different of turbiditydifferent grades laser wa onvelengths, the laser attenuation different laser coeffi powercient. levels, different suspended(2) The emedia,ffects of and laser differen wavelength,t turbidity power, grades and on karst the cavelaser water attenuation environments coefficient. on the attenuation characteristics(2) The effects were of revealed, laser wavelength, and the existence power, and of akarst “blue-green cave water window” environments with low on the turbidity attenuation and “nearcharacteristics infrared window” were revealed, with high and turbiditythe existence were of discovered. a “blue-green The window” laser attenuation with low coe turbidityfficients and of di“nearfferent infrared wavelengths window” increased with high linearly turbidity with were the increase discovered. of turbidity, The laser while attenuation the laser coefficients attenuation of different wavelengths increased linearly with the increase of turbidity, while the laser attenuation coefficients decreased with the increase of laser power. When the suspended media were CaCO3 and fine sand, the attenuation of the 450 nm blue laser was the lowest; when the media were silt and clay, the attenuation of the 450 nm wavelength laser was the lowest with low cave water turbidity, and the attenuation of 650 nm wavelength laser was the lowest with high turbidity, for which 23.23 NTU and 27 NTU are the critical points of high or low turbidity, respectively. (3) In this paper, an optimization scheme for the maximum laser detection distance in complex karst cave water environments was proposed, and the three-dimensional point cloud acquisition method for karst cave shapes was created. These methods can realize coordinate splicing of multistation detection point clouds in complex karst caves, and can be applied for the detection of karst caves along the Jinan metro line. Appl. Sci. 2020, 10, 392 21 of 22

coefficients decreased with the increase of laser power. When the suspended media were CaCO3 and fine sand, the attenuation of the 450 nm blue laser was the lowest; when the media were silt and clay, the attenuation of the 450 nm wavelength laser was the lowest with low cave water turbidity, and the attenuation of 650 nm wavelength laser was the lowest with high turbidity, for which 23.23 NTU and 27 NTU are the critical points of high or low turbidity, respectively. (3) In this paper, an optimization scheme for the maximum laser detection distance in complex karst cave water environments was proposed, and the three-dimensional point cloud acquisition method for karst cave shapes was created. These methods can realize coordinate splicing of multistation detection point clouds in complex karst caves, and can be applied for the detection of karst caves along the Jinan metro line.

Author Contributions: Methodology, S.S. (Shangqu Sun); writing—original draft preparation, L.L.; investigation, J.W.; validation, S.S. (Shuguang Song); data curation, Z.F.; writing—review and editing, P.H. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Science Fund for Excellent Young Scholars (NO.51722904), the National Natural Science Foundation of China (51679131), and the Transportation Technology Program of Shandong Province, China (NO. 2019B47_1). Conflicts of Interest: The authors declare no conflict of interest.

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