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This article relates to exploration of the Gaping Gill - Britain’s largest cavern. Ground penetrating radar (GPR) and ground light detection and ranging (LIDAR) surveys were tested from 18 - 24 August 2003 in Gaping Gill, Yorkshire. It was suggested that the GPR survey be used, with the aim of exploring the sediments forming the Gaping Gill Main Chambre (GGMC) cave floor. How much have these methods developed in the last two years? Share your comments with us by emailing [email protected] .

Three good reasons for carrying out this work emerged, namely: Ground light detection and ranging (LIDAR) Survey

Firstly, the unstable nature of the cave floor sediments and Previous work sections of the GGMC rock wall have been recorded for about a century. St. Amand (1999) describes an early application of laser scanner CPC ‘diggers’, such as Mole, have a burning desire to discover technology to create a 3D cave map for the Joaquin Miller Chapel new passageways. The strength of their commitment became room in the Oregon . In three days, they collected 35-million clear when they covered vast tracts of moorland with the points, capturing the detailed 3D geometry of the cave surface. GPR sledge in very little time. Could new technology find Their 3D CAD images show formations in good detail, passageways? Does the spade still reign supreme? The depth including vertical flutes on fused - columns. scale of Fig. 7 has been calibrated in 2 m spades, as a goodwill gesture. Yordas Cave has been scanned with ground LIDAR. Prior to the Several worthy geological questions could perhaps be Gaping Gill LIDAR survey described here, Graham Hunter acquired addressed by this and any subsequent surveys: a 4,7 MB LIDAR survey of Peak Cavern, to enhance the integrated modelling of Pringle et al. (2002). The putative GGMC winter wash-tub process. The possible Pleistocene stratification of the GGMC Equipment, operation and processing sediments. There are well-preserved, layered sediments elsewhere in the system. In the last few years, ground LIDAR or 3D laser mapping has been The possibility that the Gaping Gill – Ingleborough Cave replacing the previous laser theodolite or Total Station survey system could show several cycles of karstification, some technology. LIDAR technology is developing rapidly and the of which may date back to limestone deposition in the image quality of future surveys with more recent equipment Carboniferous. will doubtless be even better than that achieved at Gaping Gill in 2003. However, the market is receptive and equipment is in short Given typical maximum depths of investigation for GPR in sediments supply. Airborne LIDAR is equally sensational. of up to 25 m and a GGMC shaft depth of about 100 m, there are two levels from which new passageways might be discovered. Graham Hunter borrowed a model LMS-Z210 from Salford From a moorland survey, high-level passageways might be University. The Z210 was Riegl’s first 3D imaging sensor for imaged. industrial applications. Key parameters have a typical measurement accuracy of 25 mm with a line scan range of 80° and a frame scan From the floor of GGMC, passageways through the boulders range of 333°. Graham selected a scan rate of 18 000 points per might be imaged. The visible GGMC is approximately a triangular second. prism, shaped like a ‘Toblerone’ bar so the cavern may continue to widen with depth, hidden by the boulder fill. Compare Figs. 1, 2, 4 and 6. In which case the claim that St. Paul’s Cathedral could almost fit inside might become true, subject to excavation (for which project CPC would have the staff!).

The lower paleozoic basement unconformity beneath the great scar limestone would be an even more ambitious and significant GPR target. The unconformity is highly irregular at outcrop, so flow boundaries could control conduit development at lower levels in the cave system. Exactly how the large antennae of a GPR system capable of the necessary penetration could be moved around the passageways is a challenge for the future. Once the parameters have been Fig. 1: Gaping Gill Main Chamber LIDAR Fig. 2: Gaping Gill Main Chamber LIDAR defined the challenge shall be issued. survey 2003. Vertex cloud looking west. survey 2003. Vertex cloud looking east.

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Graham first made a LIDAR scan from the eastern edge of the Gaping Gill . This took in the surrounding moorland, the Fell Beck valley and the top of the Gaping Gill shaft. Then three pairs of orthogonal scans were made from the floor of GGMC. Reflector patches were placed in the overlap zones between surveys swaths. Altogether about 15-million xyz points were surveyed during the afternoon. The software is able to recognise the artificial reflectors and register the separate survey scans. An additional ambitious aim was to link the surface and the underground surveys. With hindsight, the surface survey needs to be taken from the northwestern edge of the sinkhole so that the central survey point from the floor of GGMC can ‘see’ reflectors or at least common features on the upper southern edge of the shaft. Then the surface and underground surveys could be joined with an overall accuracy of about 25 mm. The three pairs

of underground LIDAR scans were joined to form the data model Fig. 4: Rendered LIDAR survey results for the whole of Gaping Gill Main Chamber provided on CD-ROM. The separate image points were re-sampled and part of the lower shaft as though looking through the rock from the west. to 750 000 original vertices (860 000 with interpolation) for The western boulder slope rises towards the viewer. Note the right-triangular cross-section controlled by fault and joint surface. It is easy to suppose that rendering. The full 30 MB file covers GGMC on a spacing of about the cave walls may continue to widen downwards where hidden below the 6 cm; see Figs. 1 and 2. sediments of the cave floor.

Preliminary results Ground penetrating radar (GPR)

The 6 cm sample interval and 2,5 cm accuracy of the 2003 LIDAR Most GPR studies in have focused on detecting caves within the data model is sufficient to define the main geological features limestone from the surface, rather than the profiling of cave earth controlling the development of the exposed upper part of GGMC. deposits within the caves. The most commonly used system The main faults and joints, the main bedding planes and some is the pulseEKKO PE100. We also planned to use a PE-100 at Gaping Gill. overhangs developed below them are clearly visible. However the Typically, 25 or 50 MHz antennae are used to image down to 25 or 15 m fine detail seen in the previous Peak Cavern LIDAR survey is missing. respectively. Graham Hunter was able to use a more recent Z360 instrument with an accuracy of 12 mm to make the Peak Cavern survey. For Equipment, operation and data processing detailed geological analysis, the higher resolution image volume is needed. For present purposes a 30 MB data model is probably A commercial PE-100 system was hired and successfully tested most suitable. It is difficult to manage a 180 MB data model, at Peak Cavern. It was not working on arrival at Gaping Gill and because the rendering time between poses is tedious. it could not be repaired until early September. As a result a new and specialised GPR system was obtained - the Utsi Electronics A remarkable advantage of a LIDAR cave survey is the ability to GV2. Whereas the Canadian PE-100 system has been a GPR industry view cave morphology from the outside – as though one could see standard for well over 10 years, the Utsi Electronics GV2 represents through the solid rock; see Figs. 3 and 4. The geological features more recent technology developed in the UK. The Utsi GV2 has a controlling cavern development become much more obvious than broader frequency bandwidth from 30 - 100 MHz. Broad bandwidth when the cave is viewed from inside. The inside view is however signals are most desirable for good quality geophysical imaging. better for identifying fine detail and for comparing that detail with The Utsi antennae are about 1,1 m square and about half a metre direct observations; see Figs. 1 and 2. high. They are delicate and difficult to manoeuvre without damage. The GV2 system was transported to and from Gaping Several views of the data have been screen-grabbed. In Figs. 5 and 6 the earlier survey by the Yorkshire Ramblers Club has been overlaid for comparison with Calvert (1899, 1900).

Fig. 5: West-east LIDAR section through Gaping Gill Main Chamber (red, or grey Fig. 3: Renedered LIDAR survey results for the whole of Gaping Gill Chamber point cloud) compared with Yorkshire Ramblers Club survey of 1896. There and part of the lower shaft as though looking through the rock from the are discrepancies in the length measurements, compare the upper, LIDAR north. Dubbed HMS GG, the bow and stern are formed from the eastern (left) and lower, YRC scales. The LIDAR survey has been enlarged to fit the YRC and western (right) boulder slopes. Note sub horizontal etch marks along longitudinal section, so that the form of the floor can be compared. The YRC bedding. roof is still too high, compare Fig. 6.

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Gill using a quad bike and trailer. The antenna system remained underground for the duration of the 3-day GGMC survey.

There was overnight rain on the first evening and the waterfalls into GGMC made the entire cavern humid. As a result, the data logging laptop failed almost immediately but recovered when returned to surface to dry out. The laptop was henceforth sheathed in a plastic bag and functioned thereafter. Further problems arose from an unreliable power supply connection to the laptop.

The Utsi GV2 system was designed to use a measuring wheel to acquire data over peat terrain where a penetration depth of 11 m has been demonstrated and published. Unfortunately whilst being used on the mud floor at the western end of the main Fig. 7: Preliminary uncorrected GPR section running from north (left) to south chamber, the wheel developed a fault in the rotary encoder after past the western edge of Bar Pot entrance. The false "structure high" is the just one line of the GGMC survey. Consequently the entire survey inverted topgraphy of the valley followed by the path to Gaping Gill. strategy had to be redesigned overnight. The resulting survey was far slower to acquire and it will be much slower to process, but benefits from a rigorous quality control of every trace. were that there was very little time to look for passages beneath Preliminary tests were made in GGMC on the afternoon of the moor and there are weeks of data processing work to do 22 August, when the problems described above were tackled. The – students very welcome! last desperate act of the day was to drag the antennae across the cave floor in continuous mode, when plausible reflection events On 23 and 24 August, the two survey areas of GGMC and the moor were observed. The revised GGMC survey plan comprised three between Gaping Gill and Bar Pot were finally acquired as follows: common mid point (CMP) surveys linked by a 2D line of section along the long axis of the cave. The CMP velocity surveys are An 88 m surveyor’s tapeline was stretched along the east-west axis needed to characterise the velocity structure of the cave floor of GGMC. sediments. Every station on each CMP and on the 2D line was Three CMP velocity profiles were positioned along the tape. recorded in continuous mode with stationary antennae. These surveys took the whole working day of 23 August from 8 am to 6 pm. The three CMP surveys were centered at: So although the survey took a long time, most traces in each final CMPA: 14 m from the eastern end at the base of the section are derived from up to 300 recorded traces, each stacked eastern boulder slope. This CMP centered on the eastern 1 000 times in the recording system. A positive consequence of mud floor. the setbacks is that we have a data set with very high signal-to- CMPB: 40 m from the east end of the tape on the central noise ratio and excellent quality control. Negative consequences boulder platform. An unusual triple event appeared at around 17 m depth (using 0,1 m/ns velocity) on both CMPA and CMPB. It remains to be seen whether the triple event is a primary reflector and whether it correlates along the 2D line of section. CMPC: 81 m from the east end of the tape on the western mud bank. Acquisition was particularly slow due to glutinous mud. A 2D line of section was then run along the surveyor’s tapeline from west to east at 1 m spacing. Finally the line was completed and the GPR equipment removed from the cave by midday.

On the moor between Gaping Gill and Bar Pot four CMP sections were acquired. Several rapid 2D lines of section were also acquired in continuous mode without the benefit of the measuring wheel:

For the CMP surveys, as in GGMC, the antennae were stepped along the tape and 100 or so traces were recorded in continuous mode at each antenna-offset location. For the lines of section, the diggers led by Mole pulled the GV2 sledge across large distances logged by Aidan Karley using a handheld GPS system.

For the profile above Bar Pot, good signal was locally seen down to Fig. 6: North-south LIDAR section through Gaping Gill Main Chamber and shaft a depth of around 20 m (i.e. 10 giant spades), assuming a 0,1 m/ns (red, or grey point cloud) compared with Yorkshire Ramblers Club survey of velocity, see Fig. 7. 1896. Note the good agreement on width, but the apparent influence of railway tunnels on the perceived roofline of the earlier survey.

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The section seems geologically reasonable but the velocity profiles Acknowledgements need to be analysed in order to demonstrate primary reflection [1] Craven Pothole Club members events. There appears to be a weak multiple event at about 14 m [2] Ric Halliwell, Honorary Editor of the CPC Journal. below surface. Further processing of the 2D section should result [3] Equipment hire using funds provided by Exxon-Mobil and in a much better signal-to-noise ratio. Shell

The slow process of transcribing and quality controlling 616 files Contact Willem Marais, in Utsi format to 308 files in seg-y format was completed. Most of IS Technology Systems, the recorded gathers will be stacked to form single traces. The Tel (011) 869-4220, [email protected] single traces will then be concatenated to form the CMP gathers for velocity analysis and the 2D line of longitudinal section through GGMC. As discussed above, this course of action was forced by three considerations:

The breakage of the measuring wheel

The need to make a thorough analysis of signal-to-noise ratios to help design future surveys

The desire to image as deeply as possible beneath the surface with the equipment available.

Preliminary results

In GGMC, the impression is that GPR will prove a suitable way in which to image below the gravel. Much data processing and analysis is required before the best parameters for the next survey can be suggested. On the moor above, peat seemed to pose a problem, although the Utsi kit is designed for work on peat and 11 m penetration recorded. The CMP along the path near Bar Pot showed a strong reflection event at about 7 and 20 m depth, see Fig. 7. These events are to be compared with the lithological logs of Boothroyd and Johnstone.

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