High-resolution seismic imaging over thick permafrost at the 2002 Mallik drill site

Transcription

1 High-resolution seismic imaging over thick permafrost at the 2 Mallik drill site D.R. Schmitt 1, M. Welz 1, and C.D. Rokosh 1 Schmitt, D.R., Welz, M., and Rokosh, C.D., 5: High-resolution seismic imaging over thick permafrost at the 2 Mallik drill site; in Scientific Results from the Mallik 2 Gas Hydrate Production Research Well Program, Mackenzie Delta, Northwest Territories, Canada, (ed.) S.R. Dallimore and T.S. Collett; Geological Survey of Canada, Bulletin 585, 13 p. Abstract: A short, high-resolution, two-dimensional profile and a single-fold, pseudo three-dimensional, reflection-seismic volume were acquired at the 2 Mallik drill site on Richards Island in the Mackenzie Delta. The two-dimensional profile displays horizontal high-amplitude variations of limited lateral extent (i.e. <100 m) that are present both within and above the gas hydrate stability zone; these observations are in agreement with earlier seismic surveys at the site. The current pseudo three-dimensional survey showed the bright spot and dimmed zoned variations, and confirmed lateral heterogeneity of reflectivity within the permafrost zone and the gas hydrate stability zone. The reflectivity variations may be due to small-scale changes in pore-space saturation, both within and above the gas hydrate stability zone, resulting in substantially changed compressional-wave speeds. Future work will include detailed modelling of expected seismic responses based on variations between pore fluid and solid. Résumé : Nous avons enregistré un profil sismique bidimensionnel à haute résolution de courte étendue, ainsi qu un volume pseudo-tridimensionnel obtenu par réflexion sismique à couverture simple, au site de forage Mallik 2, sur l île Richards, dans le delta du Mackenzie. Le profil bidimensionnel montre des variations horizontales de grande amplitude, mais d étendue limitée (moins de 100 m), à l intérieur et au-dessus de la zone de stabilité des hydrates de gaz. Ces observations concordent avec les résultats des levés sismiques effectués antérieurement sur le site. Le levé pseudo-tridimensionnel montre des points brillants et des zones d atténuation, et confirme l inégalité latérale de la réflectivité dans la zone de pergélisol et la zone de stabilité des hydrates de gaz. Les irrégularités de la réflectivité pourraient être dues à des variations à petite échelle de la saturation de l espace interstitiel dans la zone de stabilité des hydrates de gaz et au-dessus de cette zone, produisant d importants changements des vitesses des ondes de compression. Les travaux prévus pour l avenir comprennent une modélisation détaillée des réponses sismiques attendues selon les variations des proportions de fluides et de solides dans les pores. 1 Institute for Geophysical Research, Department of Physics, University of Alberta, Edmonton, Alberta, Canada T6G 2J1 1

2 GSC Bulletin 585 INTRODUCTION The JAPEX/JNOC/GSC et al. Mallik 3L-38, 4L-38, and 5L-38 scientific wells were drilled in the Canadian Arctic (Fig. 1a) during 2 in order to study the feasibility of methane gas production from permafrost-related gas hydrate. Many research projects were initiated during this phase of the project, including coring of the gas hydrate zones, extensive geophysical and mud-gas logging, reservoir-evaluation testing, cross-well tomographic seismic studies, and vertical seismic profiling (Dallimore and Collett, 5). This paper describes part of the project field program: the acquisition of a high spatial resolution, two-dimensional seismic profile and a single-fold, closely spaced, pseudo three-dimensional coverage over the wells. Seismic studies have been used to map the permafrost zones. Hunter and Hobson (1974) described the use of refraction techniques in detecting subsea permafrost. Walker and Stuart (1976) measured the seismic-interval velocities in a number of wells drilled in the Mackenzie Delta by Imperial Oil, using a crystal cable of spaced hydrophones in conjunction with check-shot measurements. They observed highly variable velocities within the permafrost zones, ranging from 2290 to 4120 m/s, that were substantially higher than velocities of 1520 to 1980 m/s measured in deeper nonfrozen sections. Hatlelid and MacDonald (1982) used the existence of seismic refractions and reflections to infer the velocity and thickness of the permafrost. A number of earlier seismic studies were carried out at this site. Using reprocessed large-scale industrial surveys acquired in 1984 by Imperial Oil Ltd., Collett et al. (1999) studied a deeper, large-scale anticlinal structure that both traps gas and serves as a source reservoir for the methane that constitutes the gas hydrate. This survey was acquired with dynamite at an average common midpoint (CMP) spacing of 20 m at 15 fold (the number of traces with a common midpoint that have been added together to increase the signal/noise ratio). These data were typical of exploration surveys in the mid-1980s and, while they provided good regional coverage, their target zone was substantially below the permafrost and gas hydrate zones. As such, they cannot be used reliably to study the nearer surface stratigraphy to the bottom of the gas hydrate stability zone (at a depth of about 1100 m). Sakai (1999) and Walia et al. (1999) described more detailed vertical-seismic-profile measurements in the 1998 JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well. Their work provided in situ seismic velocities for both compressional (P) and shear (S) waves, and evidence for seismic reflectivity along the profile. In 1999, both P- and horizontal S-wave, high-resolution, two-dimensional seismic profiles (Fig. 1b) were acquired close to the 2 JAPEX/JNOC/GSC et al. Mallik 5L-38 gas hydrate production research well site (Hunter et al., 1999, 0; Miller et al., 0, 3, 5). Their unique survey first obtained high-frequency and spatial-resolution profiles, each with CMP spacings of 2.5 m at folds as great as 30 and coverage lengths of about 1.2 km. In contrast to the regionalscale deep surveys contributed by Collett et al. (1999), they concentrated on the gas hydrate stability zone and showed evidence, in both raw shot gathers and the final processed seismic section, that the gas hydrate zones are laterally discontinuous. The work described in this paper had several purposes. First, earlier drilling, vertical seismic profiling (VSP; Sakai, 1999), and conventional (Collett et al., 1999) and high-resolution (Miller et al., 3, 5) seismic measurements have indicated that the permafrost and gas hydrate stability zones display some degree of lateral heterogeneity. Gas hydrate is trapped in a broad anticlinal structure composed of thick, young, unconsolidated, Tertiary deltaic sand and silt. Log evaluation of Imperial Oil Mallik L-38 and JAPEX/JNOC/GSC Mallik 2L-38 shows the strata to be relatively uniform in the immediate area of the Mallik wells. Recent seismic profiling acquired in 1 (Miller et al., 3), however, shows horizontal variations in seismic amplitudes that are likely indicative of changing gas-water-ice pore saturations. Vertical-seismicprofile images obtained in the Mallik 2L-38 gas hydrate evaluation well, drilled in 1998, could also show this heterogeneity in seismic amplitudes (Sakai, 1999; Walia et al., 1999), although these workers did not specifically make this observation. The present survey was carried out in order to examine the detailed structure, to the degree it may be deduced from high-resolution seismic imaging, with two-dimensional and pseudo three-dimensional coverage centred immediately on the three 2 wells (JAPEX/JNOC/GSC et al. Mallik 3L-38, 4L-38, and 5L-38) and the corresponding vertical seismic profile (Milkereit et al., 5). Second, although the gas hydrate dissociation process itself remains an active area of research, the simple fact that free methane will be liberated into the pore space during the breakdown of the gas hydrate suggests there will be a large change in the overall seismic properties of the producing gas hydrate zone. Substitution of highly compressible gas for the liquids or solids in the pore space produces a large decrease in the seismic P-wave velocity, the bulk of the decrease actually occurring at gas saturations of only a few per cent. Much of the essential physics of this effect is described by Gassmann s relation (Gassmann, 1951), where the overall fluid compressibility takes on that of the gas, while the density remains that of total pore saturation. Gassmann s theory does not consider other changes in the material, particularly those that alter the frame stiffness of the rock due to damage of the grain-to-grain contacts, which could lower the velocity even further. This diminished velocity leads also to a smaller elastic impedance (i.e. the product of the seismic velocity and the mass density), which is the physical property that most influences the seismic reflectivity of a given horizon. There are numerous examples of the effect of diminished velocity, including seismic bright-spot analyses that, in many cases, provide a direct indicator of free gas within a formation and the effect of steam injected into heavy oil reservoirs (e.g. Schmitt, 1999). Given that current knowledge of the dissociation process is imperfect, attempts to model expected seismic responses would be highly speculative at best, but the likelihood that the dissociation-related changes in seismic properties will be large suggests that time-lapse seismic measurements may be one way to remotely study the hydrate dissociation process. The initial 2

3 Northing m: UTM Zone 8 Miller et al Line 2 D.R. Schmitt et al. high-resolution seismic measurements were also carried out to better understand some practicalities of conducting time-lapse surveys of hydrate dissociation. Time-lapse data were acquired, but these suffer from rig-noise problems at the site and are still being analyzed. This paper reports on the acquisition and processing of the field seismic data. The resulting conventional, two-dimensional seismic profile and the novel, single-fold, pseudo threedimensional seismic volume both display the lateral changes in amplitudes seen by earlier workers. Although more work is required to understand these variations, they are likely due to changes in the saturation state across the short profile. SEISMIC PROFILE SITE Three wells at the Mallik site (i.e. Mallik 3L-38, 4L-38, and 5L-38) were drilled during 2 on Richards Island in the Mackenzie Delta region of the Northwest Territories, at latitude N and longitude W. The locations of these wells and the earlier Mallik L-38 and 2L-38 wells are given in Table 1 and Figure 1. Since the site is located near the shore of the Beaufort Sea, the topography over the seismic experiment does not vary significantly and is near 1 m above sea level. The seismic experiments were conducted between late February and early March 2. The weather conditions at the site during the recording periods were typical for the season, with wind-chill temperatures ranging from -55 C to -20 C. Wind conditions varied substantially from calm to greater than 50 km/h, which would Table 1. Surveyed well locations at the Mallik drill site. Well designation Latitude Longitude Easting (m) Northing (m) L '44"N '25"W 2L '40.71"N '30.37"W L '38.32"N '41.61"W L '40.29"N '36.18"W L '39.30"N '38.90"W Note: All eastings and northings are in North UTM zone 8 with central meridian at 135 W Mallik 3L-, 4L- and 5L-38 2L-38 L-38 Mackenzie Bay Yukon Territory Northwest Territories Taglu staging site Beaufort Sea Inuvik (field lab) Tuktoyaktuk Aklavik km a) Recording tent Receivers XL244 CMP 143 Pseudo 3-D midpoint coverage XL53 4L 5L 2L IL168 3L 1L CMP 007 2D midpoint coverage Shot points Easting m: UTM Zone 8 IL110 Miller et al Line 1 SP 137 N b) Figure 1. a) Mackenzie Delta area, showing the location of the Mallik drill sites. The red and green lines are roads. b) Geometry of the seismic experiment at the JAPEX/JNOC/GSC et al. Mallik 5L-38 gas hydrate production research well. Surface positions of the Mallik 3L-58, 4L-58, and 5L-58 wells are shown as solid circles. The two-dimensional midpoints for the profile extend from common midpoint number (CMP) 7 to 143. Pseudo three-dimensional coverage is indicated by the rectangle extending between in-lines (constant shot lines) 110 and 168, spaced at 3 m intervals, and between cross-lines (constant receiver lines) 53 and 244, spaced at 2 m intervals. The southwest corner of the map is at UTM 8, E, N. 3

4 GSC Bulletin 585 produce white-out conditions. The data in this paper were acquired on February 27, 2, during the period of the lowest wind-chill recorded during the drilling program. GEOLOGY This section concentrates on a few structural, petrophysical, and stratigraphic features of the gas hydrate reservoirs, especially where related to the geophysical properties of the rocks and therefore contributing to variations in seismic velocity and reflectivity. Ten sedimentary sequences have been identified in the Upper Cretaceous to Quaternary strata of the Beaufort-Mackenzie area (James and Baxter, 1988; Dixon et al., 1992; Dallimore and Collett, 1999). Surficial Quaternary sediments are composed of modern deltaic deposits overlying older fluvial and glacial deposits to a depth of less than 100 m. Gas hydrate at Mallik is trapped in a broad anticlinal structure (Collett et al., 1999) consisting of sand-dominated Tertiary strata that represent a generally northward- or northeastward-prograding deltaic facies. Three Tertiary sequences are identified on logs and in core (i.e. Iperk, Mackenzie Bay, and Kugmallit), with gas hydrate occurring in the upper Kugmallit Sequence (Dallimore and Collett, 1999; Uchida et al., 1999) and, to a lesser degree, near the base of the lower Mackenzie Bay Sequence (depending on where the top of the Kugmallit Sequence is placed). The hydrate is encased in framework-supported, high-porosity (32 45%) sand and in matrix-supported sandy gravel with a porosity of 23 to 29%. The degree of consolidation of the sediments is variable, but they are generally weakly consolidated, hence affecting P- and S-wave velocities through variations in bulk framework moduli. Shale content, calculated from well logs, is generally less than 10% in the poorly consolidated, quartz-rich (approx. 90%) reservoir sand. Log evaluation shows that at least six coal beds, approximately 1 to 3 m thick, are present in the approximately 580 m thick Kugmallit Sequence (Miyairi et al., 1999, Fig. 5). According to well-log evaluation of Mallik 2L-38, approximately 111 m of net gas hydrate pay occur between and m below surface, comprising four gas-bearing sequences that range in thickness from about 20 to 35 m, each resting on water-saturated sediments (Dallimore and Collett, 1999). The clathrate is generally fine grained (<2 mm) and fills intergranular pores or coats mineral grains (Uchida et al., 1999), along with rare thin veins (1 2 mm) and clasts or nodules (up to 0.5 mm, more rarely 2 mm). Gas hydrate saturation has been calculated from well logs at more than 60% in the gas-hydrate-bearing zones and peaks at slightly more than 90% (Miyairi et al., 1999; Collett et al., 1999; Dallimore et al., 1999). Free hydrocarbons have also been identified on well logs immediately below the calculated gas hydrate stability zone in one thin sand between and m. The base of the permafrost zone at the Mallik site is at a depth of approximately 640 m within the middle of the Mackenzie Bay Sequence and dips to the northeast at about 4 m/km (Majorowicz and Smith, 1999; Collett et al., 1999). Miller et al. (3) observed no clear seismic reflection at the base of the permafrost, where velocity dropped from 4 to 1500 m/s, and suggested that the permafrost base is gradational over about 50 ms. Miller et al. (3) also observed a strong reflection in the permafrost zone, at about ms two-way travel time, that occurs at about the position of the disconformable contact between the Iperk and Mackenzie Bay sequences. The calculated depth to the base of the gas hydrate stability zone is about 1100 m at the Mallik site. The gas hydrate base dips steeply (at 35 to 50 m/km) from the southwest toward the Mallik 2L-38 well site and, toward the northeast, exhibits a shallower slope of about 10 m/km (Judge and Majorowicz, 1992; Dallimore and Collett, 1999, Fig. 5). Detecting this dip over the small coverage of the current profiles would be difficult. High-amplitude reflections within the gas hydrate zone (Miller et al., 3) were observed as laterally discontinuous. The breadth of the reflectors within the main zone of gas hydrate concentration is generally a few hundred metres, with the reflectors reappearing laterally after a few tens of metres. Miller et al. (3) suggested that the lack of lateral reflection continuity might relate to variations in gas hydrate concentration or changes in clay content rather than to rock types. Nevertheless, laterally discontinuous high-amplitude reflectors are also seen in the Miller et al. (3) data and in the data set from this study above the zone of gas hydrate concentration. Lateral continuity of reflectors is perhaps not expected in this area owing to laterally complex geology at the scale of a few hundred metres. For example, Collett et al. (1999) proposed that bottom-simulating reflectors (BSR), as observed in seismic sections over marine gas hydrate accumulations, should not be expected in the Mackenzie Delta. In marine settings near the continental-shelf edge, gas hydrate is found in laterally continuous and vertically homogeneous silt and clay, whereas gas hydrate accumulations in the Mackenzie Delta are located in complexly interbedded sandstone and shale sequences (Collett et al., 1999, p. 370), with the hydrate dominantly accumulating in high-porosity sand. Hence, the geometry or architecture of the geological environments near the Mallik site (i.e. fluviodeltaic) may not lend itself to continuous reflectors such as a BSR, either within a sequence or at the base of the gas hydrate. According to Hill et al. (1), submarine distributary channels in the modern Mackenzie Delta are up to approximately 10 m thick and a few hundred metres wide, while the seismic reflections in the Miller et al. (3) Tertiary sequences similarly span a maximum of a few hundred metres. FIELD EXPERIMENTS Geometry The seismic experiment itself was constrained by a number of environmental and physical factors that limited operation of the seismic source to the ice road leading directly to the Mallik drill site. Therefore, a T receiver layout was employed (Fig. 1b) in order to obtain both a two-dimensional profile and an unconventional, single-fold, three-dimensional volume. The shot-line trends east-northeast, pointing toward the main Mallik 5L-38 borehole. The extent of the shot line was constrained to the west by the camp itself and to the east by 4

5 D.R. Schmitt et al. the geometry of the ice road and the energy of the seismic source. The T array was laid out on the tundra to the west of the drill site such that the Mallik 3L-58, 4L-58, and 5L-58 wells would fall at the source-receiver midpoints. The array was placed using differential GPS (DGPS) positioning after the source line had been surveyed, in order that the two-dimensional profile would be centred near the Mallik 5L-38 well. The 61 common shot gathers along the base of the T that was used to provide the two-dimensional profile had offsets ranging from 510 m to 1070 m, the offsets being controlled by the access to the Mallik site (as noted above). The University of Alberta IVI minivibrator source, consisting of a high-frequency vibrator of 0 pounds force mounted on a weighted 3-ton truck (Fig. 2), was used to provide seismic energy on February 27, 2. The source operated with linear sweeps, of 14 seconds duration, from 12 Hz to 180 Hz at 5500 pounds force; the source vibration was repeated eight times at each location. The force level of the vibrator plate remained uniform over the entire sweep, indicating good coupling with the ice road. A 6 m source spacing was used, with each of the final 61 shotpoints being located with DGPS. The two-dimensional line was acquired using 48 channels along the stem of the T, and the resulting subsurface coverage extends for m, centred on the Mallik 5L-38 well, with 137 CMP traces at a 2 m spacing, the fold varying from 1 to 36 (Fig. 3). The single-fold, pseudo three-dimensional coverage was obtained from 192 channels laid out along the top of the T. Standard OYO Instruments 14 Hz geophone singles were spaced at 4 m intervals along both the stem and top of the T ; each of the 240 receiver locations was determined using DGPS. This geometry results in a 2 m CMP spacing for the two-dimensional line and for the in-line (or shot-line) trace spacings for the pseudo three-dimensional volume. Placement of the receivers involved cutting down through the snow to the surface, which consisted mainly of ice at the top of frozen lakes but less so over small islands, and drilling a small hole in the surface into which the receiver could be placed (Fig. 4). The small pit around the geophone was quickly refilled with blowing snow; the snow providing some degree of isolation from airborne noise. Seismic traces were acquired using the University of Alberta 240-channel semidistributed system that consists of ten 24-channel field boxes (Geode system manufactured by Geometrics Ltd.) linked via a field intranet to the control and storage computer. The computer and operators were housed in an offsite, heated, double-walled tent near the geophone array. Correlation of the radio-transmitted sweep occurred on the field boxes after each sweep and prior to vertical stacking. The filtered ground motion, as measured by accelerometers on the vibrator plate and provided directly by the IVI vibrator control system, was used as the sweep. Excellent coupling of the plate to the ice road was achieved, as indicated by monitoring of the force for each shot. Each field box requires its own battery, and battery life in the extreme cold had been an important concern prior to the field measurements. Nevertheless, the system appeared to run efficiently and no recording time was lost due to low battery voltages. Figure 2. University of Alberta IVI Minivibrator seismic source on the ice road leading to the Mallik camp. GSC Fold Common midpoint number Figure 3. Plot of fold versus common midpoint for two-dimensional profile, 2 Mallik drill site. Figure 4. Typical pit on ice for placement of geophones, 2 Mallik drill site. The pit is about 60 cm square and 50 cm deep, and the arrow points to a geophone head that is about 6 cm by 4 cm. GSC

6 GSC Bulletin 585 Data processing This study focuses on reflected seismic energy and, as such, both rig-produced noise and source-generated Rayleigh surface waves contaminated the records. An example of a raw seismic record (Fig. 5, left panel) shows that both the rig noise and the dispersive source-generated surface-wave move out at speeds of less than 2500 m/s. These nearly parallel arrivals suggest the rig noise is transferred to the receivers as a surface wave and not through the air. The rig noise is most evident prior to the arrival of the direct wave, which moves out at a velocity near 0 m/s, as can be traced from to 260 ms over the range from trace 1 to trace 48 in Figure 5. For the offsets associated with shot 137, the Rayleigh wave train, which was among the strongest arrivals, clearly interfered with the desired reflected energy at times from 450 to 550 ms on trace 1. The speeds of both the Rayleigh and the direct waves were relatively high due to their transmission through the frozen ground. A two-dimensional f-k filter was applied to each of the shot gathers in order to attenuate both the rig noise and the Rayleigh wave; the filtered version of the raw shot gathers (Fig. 5, middle and right panels) indicates that much but not all of this undesired energy was removed. Although velocity analysis was carried out on these filtered data, the most successful velocity model was developed using the interval velocities derived from the vertical seismic profiling observations obtained in the Mallik 4L-38 well by Milkereit et al. (3). The normal velocity analyses were difficult because of the relatively short range of shot-receiver offsets available and the limited lateral extent of the events. This problem was compounded by the high near-surface velocities of the permafrost, which resulted in only small moveout over a given CMP gather and therefore larger uncertainty in the stacking velocity. The topmost measurement in their two-way travel time to depth curve (Fig. 6) was from 250 m depth, and the times extrapolated back reasonably to the surface. The trend of the two-way times derived from the earlier measurements below a depth of 500 m by Walia et al. (1999) in the 1998 Mallik 2L-38 drillhole agree well but are systematically shifted to shorter times by 75 ms. Both of the vertical seismic travel-time curves, however, show a noticeable change in slope consistent with higher interval velocities at depths shallower than m, and are in good agreement with inferred permafrost thicknesses. The trend with depth of the near-vertical VSP interval velocities (Fig. 7) agrees well with the P-wave sonic-log information obtained during the logging program at the Mallik Offset 737 A) Raw B) Filtered C) Filtered/AGC CMP Time (Ms) 1: SHOT_POINT_NO [48] 1: SHOT_POINT_NO [48] 1: SHOT_POINT_NO [48] Figure 5. A 1 ms long, two-dimensional shot-gather record (with root-mean-square normalization only) produced by shotpoint 137 near the JAPEX/JNOC/GSC et al. Mallik 5L-38 gas hydrate production research well: A) as observed ( Raw ); B) after debiasing and application of an f-k filter ( Filtered ); and C) after application of the same f-k filter and debiasing but with a 300 ms automatic gain control applied ( Filtered/AGC ). 6

7 Contents Index 0 MacKenzie Bay Kugmalik Checkshot two-way travel time (s) Depth below kelly bushing (m) 300 Permafrost? Iperk Depth below kelly bushing (m) D.R. Schmitt et al. P-VSP P-stacking S-sonic P-sonic Figure 6. Plot of depth versus two-way travel times as obtained from first-arrival picking of the near-source, offset, vertical seismic profile obtained in the JAPEX/JNOC/GSC et al. Mallik 4L-38 well (data provided by Milkereit et al., 5). Thicknesses of the various geological formations and of the ice-bonded permafrost are shown Velocity (m/s) Figure 7. Plots of vertical seismic profiling (VSP) and logcompressional, log-shear, and calculated root-mean-square (RMS) velocities versus depth for the JAPEX/JNOC/GSC et al. Mallik 4L-38 and 5L-38 wells (data provided by Milkereit et al., 5). Mallik 5L-38 CMPXC NE SW MacKenzie Iperk Bay Permafrost CMPYC Kugmalit Time (ms) 100 m Figure 8. Two-dimensional seismic profile passing near the JAPEX/JNOC/GSC et al. Mallik 5L-38 well (see Fig. 1b for location). Common midpoint spacing was 2 m. A 300 ms window automatic gain control was applied to this section for display. The colour bar ranges from black to red, corresponding to positive and negative amplitudes, respectively. Abbreviations: CMPYC, common-midpoint northing; CMPXC, common-midpoint easting. 7

8 GSC Bulletin 585 Table 2. Processing streams for two-dimensional profile and single-fold pseudo three-dimensional volume. Two-dimensional profile Pseudo three-dimensional volume Acquisition (SEG2) Broadside acquisition (SEG2) Add geometry Add geometry to headers Convert to SEGY Convert to SEGY Field static corrections Field static corrections De-bias De-bias RMS mean RMS mean f-k filter shot gathers f-k filter on common receiver gathers NMO correction NMO correction RMS mean RMS mean CMP stacking Save SEGY RMS mean Display volume Display Abbreviations: f-k, frequency (f) wavenumber (k); NMO, normal moveout; RMS, root mean square; SEG2, SEGY, digital data acquisition formats approved by the Society of Exploration Geophysicists (SEG) Technical Standards Committee; CMP, common midpoint. 5L-38 well. These velocities were used to directly calculate the root-mean-square (RMS) velocity versus two-way vertical time function that was necessary to apply the normal moveout correction to the traces in order to eliminate the effects of offset. This RMS function atypically decreased with depth (or two-way time) due to the very high velocities encountered in the uppermost permafrost layer. It was found during processing, however, that this velocity function needed to be adjusted to slightly higher velocities, by about 10%, in order to properly flatten events. No special accommodation was made in this study for the influence of offset with anisotropy. This anisotropy might be a consequence of the velocity anisotropy also observed by Milkereit et al. (5) in their more oblique offset VSP measurements. The source of the anisotropy is currently unknown. Alternatively, the discrepancy may arise from use here of the standard hyperbolic reflection moveout assumption. Poley and Lawton (1991) modelled the ray paths and travel times expected for a number of marine surveys over permafrost and concluded that a nonhyperbolic normal moveout correction may be required beneath the ice-bearing permafrost. Whether this problem would apply to zones within the permafrost needs to be investigated. Aside from obtaining the velocity function from a modification of the seismic band VSP interval velocities, the two-dimensional profile was processed in a standard fashion. The 4 m receiver spacing enforced a 2 m CMP spacing; this was retained and accounts for minor crooked-line CMP binning using the seismic- processing software Vista. The processing stream is outlined in Table 2. The resulting two-dimensional profile is shown in Figure 8. The pseudo three-dimensional volume is unique in that it is single fold, with the source shooting broadside into the top of the T array with 196 geophones. There is minimal processing of these single-fold data (Fig. 8). Again, to ameliorate the strong Rayleigh wave and rig noise, the data were resorted into common receiver gathers (CRG). The moveout of both the noise and the Rayleigh wave are apparent in this projection of the data, and an f-k filter was applied to the CRG after the normal moveout (NMO) correction had been applied. This had the effect of further separating the noise from the desired reflections in the f-k domain, allowing for a cleaner filter. This data volume was then saved in an appropriate format for transfer to seismic-data-interpretation software (Kingdom Suite ). A series of cutaway views of this seismic volume is shown in Figure 9, and the corresponding movie is included on the accompanying CD-ROM (Schmitt, 5). RESULTS AND DISCUSSION Localization of seismic energy The unprocessed shot gathers of Figure 6 show a rapid decay in signal amplitude from the surface, indicating that a large portion of the seismic energy is localized within the permafrost zone. This is also apparent in the amplitude-envelope attribute of the ungained two-dimensional profile (Fig. 10). In general, the amplitude remains strong until approximately ms, which corresponds closely to the inferred bottom of the ice-bonded permafrost zone. This effect was predicted by modelling carried out by Milkereit et al. (5), and is problematic in the sense that the seismic reflections expected from the gas hydrate zone will be of relatively lower amplitude. Lateral amplitude variations A more interesting observation in Figures 8 and 10 is the lateral variations in seismic amplitude (i.e. a number of bright spots appearing). At face value, these variations occur along a given horizon and appear, in general, to extend no more than 100 m. These bright zones appear at all times in the section; it must be remembered that the profile shown in Figure 8 has had an AGC gain applied that tends to weaken the events near the surface due to the very strong early events immediately below 100 ms. Some examples of these zones include one at 300 m to the left of the centre of the image, and another at 550 ms immediately to the left of the Mallik 5L-38 well. Some of these bright spots are likely artifacts of surface-wave contamination; for example, the bright reverberations beginning at 500 ms at the northeast end and at 700 ms at the southwest end of the profile could be due to stacking of remaining surface-wave energy where the fold is relatively low. Nevertheless, imperfect surface-wave removal cannot explain shallower events. Similar characteristics are seen in the single-fold pseudo three-dimensional seismic volume. Figure 9 displays a series of cutaway views, with the in-line face progressively moving to the southwest with in-line number. Although best viewed in the movie on the accompanying CD-ROM, one bright anomaly is seen in Figure 9 near 300 ms in panel 120; this anomaly appears wider in panel 130 but shifts to the left (southeast) by panel 150. Other examples of short-range (<100 m extent) amplitude anomalies are also present. The blooming and decaying of these amplitude anomalies are much more apparent in animated visualizations, which unfortunately cannot be provided here. These data have been subject to minimal processing aside from the application of an NMO correction and of the f-k filter to attenuate noise along the cross-line (common receiver) direction. Although the f-k 8

9 D.R. Schmitt et al. XL 053 XL 148 XL Figure 9. Series of pseudo three-dimensional chair-cut views down into the single-fold seismic volume, looking from southeast to northwest (see Fig. 1b for additional orientation information) near the JAPEX/JNOC/GSC et al. Mallik 5L-38 gas hydrate production research well. In-lines (shot lines) 110 to 168 trend to the northwest at 2 m spacings and cross-lines (receivers) 53 to 244 trend to the southwest at 3 m spacings. The volume is 1 second thick. Thin white lines provide geographic reference to north-south, east-west, and time axes. The east-west axis has been stretched relative to the north-south axis in order to make the features more visible. In each frame, the in-lines are moved progressively to the southwest, with in-lines 110, 120, 130, 140, 150, and 160 shown. The first panel shows cross-lines 53, 148, and 244. The black arrows point to the bright anomaly mentioned in the text. 9

10 GSC Bulletin CMPXC CMPYC NE Mallik 5L SW Amplitude Time (ms) m Figure 10. Amplitude envelope of a two-dimensional seismic profile with no gains applied, JAPEX/JNOC/GSC et al. Mallik 5L-38 gas hydrate production research well. Colour mapping was applied logarithmically, with red representing amplitudes 60 decibels smaller than purple. filter can produce some smearing along the cross-line direction, as is apparent in the smoothed appearance of the crosslines in Figure 9, it cannot influence adjacent traces in the in-line (common shot) frames; each in-line trace is independent of the next, particularly since each was also acquired using single geophone groups that are essentially point detectors. Furthermore, these data rely on close proximity of detectors and midpoints to highlight continuity and to reduce the degree of smearing that occurs during CMP stacking, not on stacking of numerous corrected traces. The continuity of the bright spots from adjacent but independent traces further suggests that these amplitude anomalies are not acquisition or processing artifacts. Although it is tempting to interpret these bright spots as actual geological features, there are some physical constraints that must be accounted for. One issue that must be considered is the resolution of the seismic image, both laterally and temporally. Conventional analyses consider lateral resolution to be measured by the first Fresnel zone, and the temporal resolution to be measured by the Rayleigh criterion. Taken at face value, the former suggests that the smallest resolvable feature with lateral dimensions will have dimensions of 2r F, where r F is the radius of the first Fresnel zone, approximated by, 1 rf z 2 λ where z is the depth and λ is the predominant wavelength. An estimate of the total width of the Fresnel zone, assuming a predominant frequency of 50 Hz (Fig. 11) and using the average velocity to a given depth as determined from the vertical seismic profiling, shows that the width of the Fresnel zone over the depth ranges of interest is greater than 100 m. The Rayleigh-criterion (λ/4) vertical resolution, calculated using the same predominant frequency and the VSP interval velocities of Figure 3, remains relatively uniform, varying from 10 to 14 m in thickness. At face value, both of these criteria suggest that resolution of features smaller than this would be difficult. This may be the case, but it must be realized that the detection of an anomaly is not the same as being able to resolve it in a structural sense. The amplitude anomalies seen in Figures 8 to 10 are smaller than the purported resolution limit. The resolution limits as calculated do not, however, consider the effect of amplitude variations on thin beds (for further discussion, see Schmitt 1999), and it is these changes in amplitude that are likely being detected in the data. That said, it is likely that what is observed is a product of nonspecular scattering of the seismic wavefield as it impinges on features with dimensions at or near the seismic wavelengths, so the observed anomalies are likely due, in part, to diffractions. Without further study, it would be dangerous for the interpreter to obtain measures of the size of the anomalous features based on their face-value dimensions within either the two-dimensional profile or the pseudo three-dimensional volume. Interpretation Although a conclusive interpretation of the seismic observations and, in particular, the amplitude anomalies cannot be provided at this point, it remains interesting to speculate on what their source may be. Since the amplitude anomalies must be caused by changes in the elastic properties of the material, these data may lead to a better understanding of the structure of such thick permafrost. Permafrost is known to contain talik (thaw zones), cryopeg (zones of saline waters in which the salinity has been enhanced by freezing of the adjacent regions), and free gas. Numerous studies, both experimental and theoretical, have demonstrated large changes in 10

11 D.R. Schmitt et al. seismic velocity based on saturation. The seminal study by Timur (1968) of the freezing of saline waters in the pore spaces of rocks illustrates the importance of freezing on the overall compressional-wave velocities in porous sandstone. The velocity of dry sandstone changed little with temperature over the range from +26 o C to -36 o C, whereas velocities in the same rocks saturated with distilled water increased by 32.9% in a Berea sandstone (porosity of 18.1%) and nearly 51.2% in a more porous Boise sandstone (porosity of 26.4%). These observations have been confirmed by numerous other workers (e.g. King, 1977). Conversely, substitution of gas for water into the pore space, as discussed earlier, has an opposite but equally significant effect in weakly consolidated porous rocks. One early example of this effect was given by Domenico (1977), who showed the compressional-wave velocity of a water-saturated Ottawa sand (porosity of approx. 38.0%) under a confining pressure near 7 MPa decreased by 45.1% a) Depth below kelly bushing (m) b) Depth below kelly bushing (m) Width of first Fresnel zone (m) Rayleigh criterion vertical resolution (m) Figure 11. a) Width of lateral resolution versus depth, as determined by the conventional first Fresnel zone /4 criteria, using average velocities to depth and a predominant frequency of 50 Hz. b) Vertical resolution versus depth, as determined by the Rayleigh /4 criteria, using interval velocities determined by vertical seismic profiling and a predominant frequency of 50 Hz. upon replacement of the water with gas. The elastic properties of a given porous formation can therefore vary dramatically based on whether a few percentage points of ice, unfrozen water, or free gas are contained in the pore space; variation in the saturation state of the materials is one possible explanation for such strong lateral changes in the seismic reflectivity. Walker and Stuart (1976) also suggested that shale units within the permafrost have lower velocities than sand, based on an assumption that less permeable shale does not freeze, but this idea needs further testing. CONCLUSIONS A high spatial resolution, two-dimensional, seismic-reflection profile (250 m in length) and a single-fold, pseudo threedimensional seismic volume were acquired, centred over the Mallik 3L-58, 4L-58, and 5L-58 wells. The common midpoint spacing of the data sets is 2 m for the two-dimensional profile traces and a 2 m by 3 m binning for the single-fold traces that produced the pseudo three-dimensional volume. Good lateral continuity is seen in the data sets, although both display lateral variations in reflection strength over distances of less than 100 m. These observations agree well with the earlier detailed seismic surveys described by Hunter et al. (1999, 0) and Miller et al. (3), and may be indicative of the variable velocities observed by Walter and Stuart (1976) using well-bore seismic surveys. Issues related to the resolution of the seismic experiment, as well as a lack of knowledge of the detailed structure of the permafrost and gas hydrate stability regimes, preclude a detailed interpretation of these amplitude variations at present. Nevertheless, it is likely that reflectivity variations result from small-scale changes in saturation, given that porous sections of these zones can be saturated concomitantly with ice, unfrozen water, and free gas, as at Mallik, and that these differing saturations result in substantially changed compressional-wave speeds. Future work will include detailed modelling of expected seismic responses based on variations in the proportions of pore fluid and solid. ACKNOWLEDGMENTS This work could not have occurred without the assistance in field deployment of M. Lazorek and L. Tober, and infrastructural assistance at the field site by M. Riedel. Additional field assistance was provided by D. Banerjee, B. Milkereit, E. Adam, G. Pratt, and K. Bauer. The support of the staff, both at the Mallik drill site and in Inuvik, was also critical to the success of the project. The authors acknowledge the international partnership that undertook the Mallik 2 Gas Hydrate Production Research Well Program: Geological Survey of Canada (GSC), Japan National Oil Corporation (JNOC), GeoForschungsZentrum Potsdam (GFZ), United States Geological Survey (USGS), India Ministry of Petroleum and Natural Gas (MOPNG), BP-ChevronTexaco-Burlington joint venture parties, and United States Department of Energy (USDOE). The program received financial support from the International Continental Scientific Drilling Program (ICDP). The authors also acknowledge Seismic Image Software Ltd. for access to the Vista 11

12 GSC Bulletin 585 seismic-data-processing program. This work was primarily funded by the senior author s NSERC discovery grant and the Canada Research Chairs program. The manuscript was improved through the careful reviews of S. Pullan and an anonymous reviewer, and the professional editing of Bob Davie. REFERENCES Collett, T.S., Lee, M.W., Dallimore, S.R., and Agena, W.F. 1999: Seismic- and well-log-inferred gas hydrate accumulations on Richards Island; in Scientific Results from JAPEX/JNOC/GSC Mallik 2L-38 Gas Hydrate Research Well, Mackenzie Delta, Northwest Territories, Canada, (ed.) S.R. Dallimore, T. Uchida, and T.S. Collett; Geological Survey of Canada, Bulletin 544, p Dallimore, S.R. and Collett, T.S. 1999: Regional gas hydrate occurrences, permafrost conditions, and Cenozoic geology, Mackenzie Delta area; in Scientific Results from JAPEX/ JNOC/GSC Mallik 2L-38 Gas Hydrate Research Well, Mackenzie Delta, Northwest Territories, Canada, (ed.) S.R. Dallimore, T. Uchida, andt.s. 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