CHARACTERIZATION AND MAPPING OF THE PERMAFROST ZONE ON LAND BASED SEISMIC REFLECTION DATA, CANADIAN ARCTIC ISLANDS

T. A. Brent, J. C. Harrison

Geological Survey of , 3303 33 St. N.W. , Calgary, Alberta, T2L 2A7. e-mail: [email protected] e-mail: [email protected]

Abstract

Industry seismic profiles have been utilized to locate the base of ice-bonded permafrost (BIBP) throughout the Canadian Arctic Islands in porous Devonian through Tertiary bedrock to 1008 m below surface. Seismically- defined BIBP features, located at 160 to 480 milliseconds (ms), have been correlated with conventional BIBP as picked on petrophysical logs, and thermal profiles of studied exploration wells. Identifying seismic features include: 1) a continuous reflector from the base of permafrost ice that transects inclined stratal reflections; 2) amplitude decay of stratal reflectors at the BIBP; 3) bending of stratal reflectors that pass through the BIBP; 4) amplitude anomalies associated with a lithology and porosity dependent transitional BIBP and; 5) step-down of sub-permafrost reflectors that extend beyond the shoreline limit of permafrost. These data can provide an improved understanding of regional heat flow and porosity variation in bedrock, and insight into the growth and decay of the permafrost layer.

Introduction Recognition of the base of ice-bonded permafrost (BIBP) Determining the base of deep permafrost from petro- physical logs frequently provides ambiguous results, Down-hole petrophysical techniques indicate that the especially for fine grained rocks (Hnatiuk and Randall, BIBP occurs throughout land areas of the Canadian 1977), and may define levels in the subsurface that fail Arctic Islands at depths to 960 m (Hardy, 1984). The to coincide with the 0¡ isotherm as provided by inde- phase transition is primarily recognized by a break pendent thermal profiling. In the absence of drill holes, downsection to lower sonic velocity and lower resisiti- permafrost thickness determination has been, at best, vity (Hnatiuk and Randall, 1977), and by thermal sur- educated guesswork. This paper introduces an alterna- veys which locate the 0¡ isotherm (Taylor et al., 1989). tive method of identifying and mapping the base of ice- Our work shows the BIBP can be identified on seismic bonded permafrost by utilizing industry-acquired seis- profiles by: 1) a distinct near-horizontal semi-continu- mic reflection profiles. ous reflector, located at 210 to 480 ms in porous sand- stones, that transects inclined primary (stratal) reflec- Previous work on this subject is limited. Merritt (1979) tions; 2) an abrupt down-dip fall off of acoustic impe- identified a near-horizontal reflection within inclined dance at 160 to 300 ms on inclined stratal reflections Triassic sandstones at approximately 300 ms on located where, for example, high porosity sand layers (Figure 1), and correlated this surface are encased in low porosity shales; 3) an abrupt down- with a break on resistivity and sonic logs near 610 m in dip rise in acoustic impedance, also at 160 to 300 ms, the nearby Robert Harbour K-07 well. Harrison (1995) on inclined stratal reflections where, for example, high provided additional examples from Melville Island and porosity sands overlie cemented sandstones or lime- suggested that the base of permafrost may account for a stones; 4) seismic amplitude anomalies (Òbright spotsÓ) down-dip amplitude decay observed at 230 ms (380 m) on dipping stratal reflections where ice levels differ in on inclined primary reflections from frozen sand layers interbedded lithologies. Other permafrost-associated encased in shale. Our subsequent research, and prelim- features include: 5) an abrupt decrease in apparent atti- inary examination of the Canadian Arctic Islands data tude of primary reflections and decrease in time thick- set (35,000 line kms), confirms the validity of the earlier ness of some units that pass upwards through the BIBP; reports, and indicates that such features can be used to and 6) a 25 to 160 ms step-down, beneath shorelines, of locate and map the base of ice-bonded permafrost sub-permafrost reflections on seismic profiles that throughout the polar regions. extend into the offshore (Figure 2). These phenomena are now recognized on sixteen major islands in the

T. A. Brent, J. C. Harrison 83

80¡00'

76¡00' 78¡00' 2.30 km/s 2.2-2.9 3.9-4.3 2.8-2.9 3.2-3.4 2.9-4.9 3.1-3.2 2.5-2.9 Ellesmere Island Unfrozen 210 T 480 Foshiem Peninsula D

3.6 90¡00' 2.55 2.9-4.3 4.3-4.4 3.0-3.2 2.9-3.1 3.1-5.0 4.1-4.2 4.0-4.4 3.7-4.3 km/s D T Frozen 250 480

Devon Island

00'

¡ 86 fine 3.1-5.4 2.9-5.5 fine fine fine coarse coarse coarse 4.0-4.4 3.7-4.3 coarse coarse coarse coarse Lithology P C 185 J T D K T J age Bedrock Cornwall Island Cornwallis Island Model interval velocities from sonic logs and crystal cable surveys of selected wells on Sabine Peninsula (for Permian to Cretaceous strata) and representative interval velocities for Carboniferous and Devonian coarse clastic rocks of the Tertiary, Canadian Arctic Islands. Tertiary Triassic Jurassic Permian Cretaceous Devonian Carboniferous T

J 94¡00' 290

210 98¡00' Bathurst Island Amund Ringnes Island 260 T 375 K D 230 K D 375 290 200 412

102¡00' D D 160 280 D D D 260 465 387 Cameron Island 320 120 T T D 280 K 165 Alexander Island 300 D * 260 185 405 J J KILOMETERS 80 350 D Vanier Island 290 310 D D 325

106¡00' D 40 Richardson Point G-12 370 * 110¡00' Sabine Peninsula D 400 375 K D 360 0 C K J T Mackenzie D 230 195 265 412 J 350 325 450 D D D map inset 310 Location of Fig. 2 Sherard Bay F-14 K K 195 J T Drake D-73

114¡00' 230 265 412 Location of Fig. 4a Melville Island J 210 230 K Location of seismic projected to D-73 BIBP 10 km Eglinton Island Sabine Peninsula

77¡00' 75¡00' Banks Island

Figure 1. Distribution in the Canadian Arctic Islands of seismic phenomena attributable to the base of ice bonded permafrost (BIBP). Boxed values represent two-way travel times in milliseconds (ms) to the BIBP. Age of strata featuring these seismic phenomena are provided by circled letters (see table). Conversion to approximate depth can be obtained by reference to tabulated velocity data.

84 The 7th International Permafrost Conference Canadian arctic in strata of Middle Devonian to Tertiary age (Figure 1).

Conversion to depth from two-way travel times of seismically identified features is a matter of common practice, but may be a nontrivial exercise utilizing velocities from all sources including sonic logs and check-shot (crystal cable) surveys (Walker and Stuart, 1976; Merritt, 1979). Representative velocities are tabu- lated on (Figure 1). The thickest known development of seismically-defined deep permafrost, as provided by the data collected for the present report, occurs in Middle and Upper Devonian sandstones on southwest Ellesmere Island where a BIBP reflection is observed at 480 ms or 1008 m at 4.2 km s-1.

Figure 2. Step-down of reflectors at the permafrost limit beneath a shoreline Figure 3. Base of ice bonded permafrost on part of Panarctic seismic profile (location on map inset on Figure 1) 2204 and correlation of a BIBP reflector to deflections on resistivity and sonic logs, from the Richardson Point G-12 well, northeastern Melville Island. Correlation into wells the profile, and in the Cretaceous, is seen as a down-dip reflection termination at 230 ms. Although some stratal BIBP IN DEVONIAN SANDSTONES (MELVILLE ISLAND) reflectors have been displaced by two small faults, it is Near the Richardson Point G-12 well of northeastern clear that some reflectors (in the uppermost part of the Melville Island, a near-horizontal reflection at 325 ms is Triassic section for example) have elevated amplitudes attributable to the BIBP and is continuous throughout below the BIBP. Other reflectors in the Jurassic and eight seismic profiles in a 75 km2 area of open folded Cretaceous appear to decay in strength, or terminate Middle and Upper Devonian sandstones (Figure 3). The downsection, at the BIBP. tectonic inclination of strata is recognized by primary stratal reflections lying both above and below the near- Correlation from seismic profiles to wells has been horizontal BIBP reflector, and in bedrock outcrops at the attempted at the Panarctic Dome et al. Sherard F-14 surface (Harrison, 1995). The BIBP reflector correlates well (Figure 5). The seismic BIBP, identified by down- with a sharp break on both the resistivity and sonic dip reflection amplitude decay from a ten metre sand- velocity logs for the well, and with the BIBP as picked stone bed embedded in Cretaceous shales, is located at by Hardy (1984) at 747 m. There is no shallow gas or 288 ± 14 m below kelly bushing (KB). In contrast, the gas hydrate in this well. BIBP, picked in the shales on the resistivity and crystal cable survey, is located at only 183 m below KB.

BIBP IN PERMIAN TO CRETACEOUS ROCKS (MELVILLE ISLAND) Many seismic profiles on the Sabine Peninsula display To reconcile the apparent depth discrepancy observed features associated with the BIBP. The geology in this for the BIBP at Sherard F-14, correlation from seismic area has been described by Harrison (1995), and fea- data has also been studied at the Drake Point D-73 well tures a gently inclined, northerly-dipping panel of low- (Figure 6). The seismically-derived BIBP, picked at a grade sedimentary rocks that are Permian in age in the down-dip amplitude decay on a primary ice-bonded south and range through to Cretaceous and Paleocene sandstone reflector in Cretaceous shale, is located at in the north (Figure 4a). A prominent near-horizontal 260m ± 12 m below ground level (GL) which is close to reflection, attributed to the BIBP, transects inclined the 0¡C isotherm located by down-hole temperature stratal reflectors and is correlated over a distance of 20 measurements at 288 m below GL (Taylor et al., 1982). km. This seismically-defined phase transition is best In contrast, the BIBP, picked in shales on velocity sur- observed at 380 ms in porous Lower Triassic sand- vey data in the D-73 well, is probably located at 180 m stones. The reflector weakens to the north, but is readily below GL. The implication of these discrepancies in cal- identified climbing section through the Jurassic part of culated depth to BIBP is that porous Cretaceous sand-

T. A. Brent, J. C. Harrison 85 Figure 4. a) Seismic expression of base of permafrost-related phenomena (at 200 to 400 ms) transecting northerly-dipping stratification on Panarctic seismic pro- file 1921-2674, Sabine Peninsula, northeastern Melville Island; b) synthetic seismic profile of stratification and permafrost related phenomena as seen on profile 1921-2674; c) derivative geological model used to generate the synthetic traces. (Figure 1 for interval velocities and line of profile) stone beds are fully frozen down to near the 0¡C Slope region lay up to 155 m above the measured 0¡C isotherm, and to depths that exceed (by about 100 m) isotherm. the well log picks for the BIBP in the surrounding shales. This observation supports similar results obtained by Collett et al. (1989) who found that the log- picked BIBP in fine grained rocks in the Alaska North

86 The 7th International Permafrost Conference Figure 6. Picks of the BIBP in Cretaceous shales and sand, the 0¡ isotherm, and the TBGH in the South Drake D-73 well, Sabine Peninsula. Figure 5. Position of the seismically-defined BIBP in Cretaceous sandstones and the published log pick of the BIBP in Cretaceous shales as displayed on velocity and resistivity logs for the Sherard Bay F-14 well, Sabine Peninsula. The down-dip decay of stratal reflection amplitude is a feature best displayed on bed ÒAÓ, where a thin layer Seismic modelling of ice-bonded sand (3.0 km s-1), embedded in frozen Cretaceous shale (2.5 km s-1), passes down-section into A synthetic seismic section (Figure 4b), intended to unfrozen strata (sand 2.7 km s-1; shale 2.35 km s-1) at reproduce various features observed on Panarctic pro- 250 m. Down-dip amplitude decay is also featured in file 1921-2674 (Figure 4a), has been generated using a 26 the Jurassic where thin shales lie within thick sands. Hz Ricker wavelet, 5% noise and velocities for various Up-dip amplitude decay is best illustrated where low frozen and unfrozen rocks as obtained from available porosity sand at the base of the Triassic overlies cemen- sonic logs and crystal cable surveys (Table, Figure 1). ted sandstones in the highest beds of the Permian. The derivative lithologic model features an unfaulted panel of interbedded sands and shales carrying a uni- A feature intended to account for the shallower BIBP form dip of 3¡, vertically exaggerated in Figure 4c. in wells that penetrate shale, is carried through the entire northern (left) half of the geological model. The A general feature of the synthetic data is the variation amplitude anomaly (Òbright spotÓ) circled on bed ÒBÓ in apparent attitude of all units that pass upwards arises from the assumption in the model of a 100 m through the BIBP. This apparent decrease in attitude is a higher BIBP only in fine-grained lithologies, creating an consequence of sand-domination of the lower part of acoustic transition zone. This transition may also the stratigraphic succession and substantial velocity dif- account for other amplitude bright spots near the ferences between frozen and unfrozen strata. imaged BIBP surface on profile 1921-2674. The near-horizontal reflector identified above 400 ms in the sand-dominated Triassic part of profile 1921- Thoeretical base of gas hydrates (TBGH) 2674, is simulated in the synthetic profile by modelling a sharp transition from ice-bonded sand (4.1 km) to The seismic expression of the base of the gas hydrate zone has been widely interpreted in the marine realm water-saturated sand (3.1 km s-1) at 750 m below sur- (Kvenvolden, 1993). Thermal regimes in the Arctic face at the base of the Triassic, rising to 400 m below Islands are also favourable for the stability of methane- surface at the top of the Triassic. hydrates ranging to depths of about 2000 m (J.

T. A. Brent, J. C. Harrison 87 Majorowicz, personal communication, 1997). However, 3. These phenomena are recognized on sixteen of the the relatively recent emergence by post-glacial isostatic larger islands in the Canadian Arctic, to depths ranging recovery of some coastal areas of the arctic, coupled up to 1008 m, and in Devonian through Tertiary age with low thermal conductivities of bedrock, can pro- strata. Similar features can be anticipated in all high la- duce conditions where gas hydrates are limited to the titude land areas where bedrock, regardless of age, has shallow subsurface or are absent near some low-lying sufficient porosity. coast lines. The position of the TBGH in the Drake D-73 well is identified in Figure 6 along with various per- 4. Variable rock properties of interlayered porous and mafrost-related picks. Wells further inland and at impermeable strata can account for up to 100 m of com- higher elevations feature a TBGH that is much deeper plex local relief on the BIBP. than picks for the BIBP. Conversely, wells close to sea level may feature coincident TBGH and BIBP. 5. Permafrost-related seismic phenomena can be mod- elled and used as tools for the regional analysis of heat The principal conclusion to be drawn from this is that flow and stratal variation in rock properties between some seismic phenomena attributable to the BIBP may drill holes. locally coincide with the TBGH. However, significant shallow gas has not been detected during mud logging of local exploratory wells and, in the present study, there has been no evidence found to indicate that any of the documented seismic phenomena are hydrate related.

Conclusions

1. The base of ice-bonded permafrost has been identi- fied at 160 to 480 ms on land based industry-acquired seismic reflection profiles of the Canadian Arctic Islands, and has been correlated with conventional base-permafrost picks on resistivity and sonic logs, and on thermal profiles of selected exploration wells.

2. Six distinct seismic phenomena can be attributed to the existence of permafrost.

References

Collett, T.S., Bird, K.J., Kvenvolden, K.A and Magoon, L.B. Merritt, R.K. (1979). Geophysical measurements in the per- (1989). Map showing the depth to the base of the deepest mafrost zone - Canadian Arctic Islands. In Proceedings of a ice-bearing permafrost as determined from well logs, Symposium on Permafrost Field Methods, Ottawa, Canada. North Slope, Alaska. United States Geological Survey Map, pp 56-72. OM-222, scale 1:1,000,000. Taylor, A.E., Burgess, M., Judge, A.S. and Allen, V.S. (1982). Hardy & Associates (1984). Study of well logs in the Arctic Geothermal data collection - northern wells, 1981; Islands to outline permafrost thickness and/or gas Geothermal Series, 13, Earth Physics Branch, EMR (153 p.). hydrate occurrence. Earth Science Branch Open File, 84-8 EMR (374 p.). Taylor, A., Judge, A. and Allen, V. (1989). The automated well temperature measuring system installed at Cape Allison Harrison, J.C. (1995). Melville IslandÕs salt based fold belt, C-47, offshore well, Arctic Islands of Canada. Part 2 - data Arctic Canada. Geological Survey of Canada, Bulletin 472, retrieval and analysis of the thermal regime. Journal of (331 pp). Canadian Petroleum Technology, 28 (2), 95-101. Hnatiuk, J. and Randall, A.G. (1977). Determination of per- Walker, J.H.D. and Stuart, A.J. (1976). Permafrost investiga- mafrost thickness in wells in northern Canada. Canadian tions by crystal cable surveys, Mackenzie Delta, N.W.T. Journal of Earth Sciences, 14, 375-383. Proceedings of the Society of Professional Well Log Analysts Symposium, Section J, pp. 38-55. Kvenvolden, K.A. (1993). Gas hydrates - geological perspec- tive and global change. Reviews of Geophysics, 31 (2), 173- 187.

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