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Seismic Cone Penetration Test and Vertical Seismic Profiling in Permafrost

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Seismic Cone Penetration Test and Vertical Seismic Profiling in Permafrost Permafrost, Phillips, Springman & Arenson (eds) © 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7 Seismic cone penetration test and vertical seismic profiling in permafrost A.-M. LeBlanc & R. Fortier Département de géologie et de génie géologique and Centre d’études nordiques, Université Laval, Sainte-Foy (Québec), Canada M. Allard Département de géographie and Centre d’études nordiques, Université Laval, Sainte-Foy (Québec), Canada ABSTRACT: Seismic Cone Penetration Tests (SCPTs) were carried out to a depth of 24 m in a permafrost mound near the Inuit community of Umiujaq in Nunavik, Canada, to study its cryostratigraphy in terms of pene- tration resistance and seismic velocities. The tip load, friction, temperature, inclination and electrical resistivity were measured with sensors embedded in the penetrometer. The cone penetration was stopped periodically to add a new rod and to perform multi-offset Vertical Seismic Profiling (VSP). A Swept Impact Seismic Technique (SIST) was used as a seismic source directly in contact with the thawing front and the seismic signal was recorded with a module of triaxial accelerometers, also embedded in the penetrometer, and with a seismograph. The seis- mic velocities were evaluated from the propagation time of the seismic signal in permafrost. The permafrost con- ditions are characterised by variation in seismic velocities with depth, depending on the ground temperature and sequence of frozen soil layers and ice lenses. 1 INTRODUCTION 2 STUDY SITE The study of the dynamic behaviour of frozen soils is The fieldwork was conducted on the east coast of the fundamental in cold regions engineering for the Hudson Bay, near the Inuit community of Umiujaq design of vibrating machinery on frozen ground, the (56°N, 76°W) in Nunavik, Canada, in the discontinu- response of frozen soil to dynamic loading induced by ous and scattered permafrost zone (Fig. 1). The study earthquakes and the excavation of frozen ground. The site is a permafrost mound located in a deep valley major part of the investigation into the dynamic char- and formed in marine sediments of the Tyrell Sea acteristics of frozen soils has been conducted in a lab- (Allard & Seguin 1987). This permafrost mound has oratory on artificial frozen samples or on undisturbed a diameter and height of about 70 and 4 m respectively permafrost samples (Nakano & Froula 1973, Stevens (Fig. 2). Ostioles and gelifluxion process affect the 1975, King et al. 1982, Zimmerman & King 1986). In summit and sides of the permafrost mound. Previous the field, various investigations have used seismic- studies (Fortier & Allard 1998) have shown that the refraction and seismic-reflection surveys (Barnes active layer thickness is close to 1.5 m and the per- 1963, Hunter 1973, MacAulay & Hunter 1982, Miller mafrost base is about 21.5 m deep. The permafrost in et al. 2000) and vertical seismic profiling (Skvortsov the valley is qualified as warm since the temperature et al. 1992) not only for the detection and delineation at a depth of 10 m is close to Ϫ1°C. The area is char- of permafrost, but also for the measurement of acterised by a subarctic climate and the mean annual dynamic properties of frozen ground such as seismic air temperature is about Ϫ3.5°C. velocities of both compressional and shear waves. However, in situ studies that consider the cryostrati- graphy and heterogeneity of dynamic properties of 3 FIELD METHODOLOGY AND EQUIPMENT permafrost are scarce. Multi-offset vertical seismic profiling (VSP) was Deep seismic cone penetration tests (SCPTs) including carried out in a permafrost mound near the Inuit com- multi-offset vertical seismic profiling (VSP) were car- munity of Umiujaq in Nunavik, Canada, to study the ried out down to a depth of 24 m below the permafrost dynamic properties and determine the cryostratigra- base in the permafrost mound. The penetrometer used phy of permafrost. In this paper, the field methodology is a Vertek cone with a 10 cm2 cross sectional area of based on a seismic cone penetration test (SCPT) and the tip base and a 60° cone angle, a 100 cm2 friction the results from both SCPT and VSP in permafrost are sleeve, an electrical resistivity module, an inclinometer, reported. a temperature sensor at the tip and a module of triaxial 633 80 75 70 65 ˚ ˚ ˚ ˚ 60 60 ˚ ˚ Umiujaq ? 55 ˚ Legend (after Allard & Seguin, 1987) Discontinuous but Sporadic permafrost widespread permafrost Discontinuous and Continuous permafrost Scattered permafrost Figure 3. Penetration rate-controlled SCPT in perma- frost. The linear pushing system is made of two ball screws Study site Tree line controlled by two electrical servo-motors. The overall Figure 1. Location of study site. Umiujaq, Nunavik, height of the system is about 2.5 m. Canada. 2700) was used for the data acquisition and a graphical program developed in LabVIEW (National Instru- ments) was used for the monitoring. The LabVIEW graphical interface gives data access in real-time. The cone penetration was stopped at depth intervals of 1 m to add a new penetration rod and to perform a multi-offset vertical seismic profiling (VSP). A cross configuration of seismic source locations at the sur- face, formed by two axial lines of twenty seismic shot points each, was used to carry out the VSP. The shot points of each axial line were 1 m apart and the SCPT was at the intersection of axial lines. The seismic source, VibSIST-20 from Vibrometric, stood directly on the thawing front at a depth of about 0.75 m for bet- Figure 2. The study site is a permafrost mound formed in ter mechanical contact and to avoid the signal attenu- the marine sediments of the Tyrell Sea. The photograph ation in unfrozen ground (Figs 4a, b). Forty steel scale is given by a Logan’s tent in the center. striking plates were buried in the ground, following the cross configuration of seismic source locations, accelerometers. A linear pushing system, recently for allowing the VSP to be done faster and increasing developed at Laval University (Buteau & Fortier 2000) the work efficiency. To avoid any thermal disturbance for penetration rate-controlled SCPT in permafrost of the active layer and the permafrost, the excavated (Fig. 3), was used to control the penetration rate accu- material and vegetation were put back in place. Plastic rately. A constant penetration rate of 0.1 cm/s was used tubes were used to give permanent access to the strik- to carry out the deep SCPT below the permafrost base. ing plates and protect them against the collapse of During an SCPT, tip load, friction, temperature, incli- surrounding material (Fig. 4b). Therefore, the seismic nation and electrical resistivity were measured with source (Fig. 4a) could be moved quickly from one shot sensors embedded into the penetrometer and automati- point to another to complete 40 seismic shots in less cally recorded at an interval of 5 seconds using a data than thirty minutes before continuing the SCPT down acquisition system. A multimeter (Keithley model to another meter. During a seismic shot, the ball 634 8 seconds were used to monitor the seismic signal. The total number of impacts during a sweep varied between 125 and 135. The energy delivered by the seismic source was about 2.5 kJ with 20 J/impact. 4 PERMAFROST SAMPLING On June 8th 2001, one borehole was drilled a few meters away from the SCPTs to a depth of 4.6 m in the permafrost mound to measure the physical properties and determine the cryostratigraphy of the permafrost. The marine sediments of the Tyrell Sea are clayey silt with a few sand beds. The depth of the thawing front was 0.75 m and a few ice lenses 0.5–1 mm thick were present in the frozen active layer below the thawing front. The permafrost table at a depth of 1.64 m is clearly marked by an increase in ice content. A com- plex reticulate network of horizontal and vertical ice lenses from 1 to 4 cm thick characterise the cryofacies of permafrost. The average density of the active layer Figure 4. (a) Seismic source VibSIST-20 made of a power- and permafrost is 2.1 and 1.75 kg/m3, respectively, controlled Bosch hammer. (b) A striking plate standing on while the total water content is lower than 20% for the the thawing front. The access to the striking plate is ensured active layer and higher than 40% for the permafrost. by an plastic tube for protecting against the collapse of the surrounding soil. 5 CRYOSTATIGRAPHY OF PERMAFROST extremity of the impact rod of the VibSIST (Fig. 4a) rests on the ball joint, a concave cavity in the striking Three deep SCPTs were carried out in the permafrost plate. mound in June 2001 to a depth of 24 m below the per- The seismic source used in the present study is based mafrost base. The cryostratigraphy of this permafrost on the Swept Impact Seismic Technique (SIST) devel- mound has been defined in terms of its mechanical and oped by Park et al. (1996), which is a combination electrical properties. The results of the SCPT carried of the Vibroseis swept-frequency and the Mini-Sosie out on June 16th 2001 are given in Figure 5. Cone multi-impact techniques (Crawford et al. 1960, Barbier resistance or tip load (qc) is a measure of the soil resist- et al. 1976). The seismic signal produced by the ance to the cone penetration while the friction ratio is VibSIST is a series of short pulses according to a deter- a measure of the mobilised friction along the friction ministic coding scheme 10 seconds long, in which the sleeve of the penetrometer normalised by the cone rate of impacts increases linearly with time.
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