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Ground penetrating radar surveys of peatlands for oilfield pipelines in Canada
1[10 ELSEVIER
Journal of Applied Geophysics 34 (1995) 109-123
Ground penetrating radar surveys of peatlands for oilfield pipelines in Canada Harry M. Jol, Derald G. Smith * Department of Geography, Universi~ of Calgary, Calgar)', Aim. T2NIN4, Canada Received 5 December 1993; accepted 11 August 1995
Abstract
Placement of buried pipelines in thick peat deposits is difficult because of the low bearing strength and high water content of the material for support of heavy construction equipment. Previously, ground penetrating radar (GPR) has been used to assess thickness and volume of peat as a fuel resource and horticultural material in Scandinavia and Canada. To our knowledge, GPR has not been applied to the site assessment and placement of pipelines crossing peatlands. Field experiments were conducted in the Mitsue oilfield operated by Chevron Canada Resources Ltd., located immediately southeast of Lesser Slave Lake in north-central Alberta Province, Canada. Surficial deposits consist of Holocene, linear, sandy beach ridges separated by peatlands underlain by sand. Several GPR surveys assessed the thickness of the peat along two oil pipeline right-of-ways. Results show the peat-sand contact as irregular and undulating, ranging from 0 to 3.7 m deep. Each survey, 460 and 550 m long, was completed in two hours. Such results from 1 m station spacings (sampling interval) can considerably reduce the uncertainties in planning and placement of oil, gas, and water pipelines crossing peatlands. Results indicate that thickness variations of peat can be detected more effectively in terms of quality of results, lower cost, and less time with GPR than with a peat probe or by coring.
1. Introduction
The problem in constructing pipelines across peatlands is the low bearing strength and high (up to 90% ) water content of the material. Saturated peat is unable to support the weight of the heavy construction equipment necessary for pipeline placement. Peatlands cover 50% of northern Alberta (S. Zoltai, pers. commun,, 1993), which coincidentally seem to occur at the surface of most oilfields in the Province (G. McCullough, pers. commun., 1993). In Canada, pipelines in peat are usually placed in midwinter when the frozen peat provides a relatively firm working surface for construction equipment for trenching, transportation, welding, plac* Corresponding author. 0926-9851/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved
SSD10926-9851(95)00018-6
ing, and burying. In thicker peat ( 2 - 4 m) construction activities sometimes result in equipment falling through the upper frozen layer (0.6-1.1 m thick). At times scuba divers are required to place chains and cables onto the submersed equipment before vertically hoisting it back onto the frozen surface (G. McCullough, pers. commun., 1993). Such delays are very costly and time consuming. Over the years field experience has shown that extra care and risk assessment is necessary in areas with thicker peat. Previously, ground penetrating radar (GPR) has been applied to detect the thickness of peat, the nature of the underlying mineral soil-peat contact, and the internal structure of the peat (Bjelm, 1980; Ulriksen, 1981, 1982; Remote Applications Inc., 1982; Tolonen et al., 1982; Tiuri et al., 1983, 1984; Chernetsov et al.,
110
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Fig. 1. Location of the Mitsue oilfield study area near the southeast end of Lesser Slave Lake in north-central Alberta, Canada.
1988; Warner et al., 1990; H/inninen, 1990, 1992a,b; Jol and Smith, 1991; Finnish Geotechnical Society, 1992; Sutinen, 1992). The relative permittivity of peat varies proportionally with the degree of decomposition or humification; therefore, GPR greatly facilitates examination of the developmental history of a peat deposit (Finnish Geotechnical Society, 1992). Peat is commercially mined for fuel and as a horticultural material. In peat mining, it is extremely important to know the locations of changes in sub-peat topography since they might otherwise cause contamination of the peat or damage to the equipment employed in the mining process. In Finland, sparks arising from a contact between peat mining machinery and underlying granite bedrock have caused fires in drained, dry peat (Hanninen, 1992a). A GPR cross-section profile provides an excellent picture of the thickness of a deposit. Combined with global positioning system (GPS) data, GPR can provide accurate cross-sectional profiles of peat depth and extent. A very close relationship occurs between the depth of peat obtained from drilling results and those
inferred from GPR (H~inninen, 1992b). Therefore, GPR provides an exceptional method to assess peat. Radar also yields information concerning the gyttja layer (organic-rich clay) and other sediments under the peat. If glacial till ties under the peat there will be a large amount of radar signal energy scattering due to the boulders; if the sublayer is clay there will be a strong signal (reflection) return from the high conductivity interface. No energy should be transmitted through thick clay so signals appearing from below the can be considered as system noise. If the clay layer is very thin, some energy may pass through it (Ulriksen, 1982). Reflections from below the peat-gyttja and mineral soil contact may also indicate sand or gravel. Ulriksen (1982) suggested that GPR measurements of peat thickness are substantially more accurate than those obtained by drilling or probing. He concludes that radar yields more trustworthy data than conventional sampling of the peat thickness and "thus becomes a method that is so accurate that there is no adequate means to measure its quality" (Ulriksen, 1982). The depth data obtained using the GPR method are also 10-20% less costly than than those yielded by traditional methods of probing and coring (H~inninen, 1992b). The pipeline study area reported in this paper is located in the Mitsue oilfield, near the town of Slave Lake southeast of Lesser Slave Lake, north-central Alberta Province, Canada (Figs. 1 and 2). The Mitsue field produces both oil and gas and has numerous feeder pipelines crossing peatlands from wells to collection batteries. As part of a larger study, the GPR results presented in this paper were surveyed across a peatland between two Holocene former sandy beach ridge complexes active about 8000 years ago (Figs. 1 and 2). Sand underlies the peat and a thin layer of gyttja (510 cm) occurs at the basal peat-sand contact. The objective of this study was to test whether GPR could identify the contact between peat and underlying mineral soil (in this case it was sand) with higher accuracy and at less cost and time than probing. Most important was the detection of areas with peat thicker than 1.5 m; such information will be useful for siting anticipated new pipelines and in deciding on the type of equipment to be used for trenching and filling during pipeline placement.
H.M. Jol, D.G. Smith/Journal of Applied Geophysics 34 (1995) 109-123
2. Methodology Ground penetrating radar allows continuous subsurface profiling at 1 m intervals. It is relatively rapid to implement which, in turn, reduces field work time and costs. The short-duration transmitted pulse produces a high-resolution (0.1-1 m) cross-sectional profile of the subsurface which can be used to determine structures from a few centimeters to tens of metres in size and depth. Digital GPR profiles are similar in appearance to seismic profiles, except GPR uses an electromagnetic energy source while seismic employs an acoustic source. The theory and methodology of GPR are adequately explained elsewhere (Morey, 1974; Annan and Davis, 1976; Ulriksen, 1982; Davis and Annan, 1989). A pulseEKKOT M IV radar system was used with 100 MHz antennas (1 m transmitter-to-receiver antenna separation) and a 1000 V transmitter in the reflection survey mode. At each 1 m station spacing, the digitized traces were vertically stacked 64 times with a sampling
Ill
time interval of 800 picoseconds. Profiles were processed and plotted using pulseEKKOT M IV (version 3.1 ) software. An average near-surface velocity of 0.04 m / ns was determined for the peat using the common midpoint survey (CMP) technique (Annan and Davis, 1976). Both low (50 and 100 MHz) and high (200 and 400 MHz) frequency antennas can be used in peatland investigations. Low-frequency antennas lend themselves best to the measurement of peat layer thickness, whereas high frequency antennas provide data on the near surface layers and moisture differences between peat layers (Finnish Geotechnical Society, 1992). A peat probe with 1.5 m extensions was used to determine peat thickness above the mineral soil substrate, which in this case was sand. The probe was inserted into the peat at 20 m intervals along 200 m of one GPR survey to confirm the GPR results and to determine the sub-peat sediment. Peat probes have limited accuracy if the peat sediment contact consists of gyttja or clay. In spite of the fact that the peat probe is
Fig. 2. An aerial photograph of the study area showing the scrub forest-coveredpeatlands and slightly arcuate patterns of sandy beach ridge complexes (see Fig. 1 for location). The broken lines are cleared right-of-waysused for pipelines which were surveyedby GPR shown in Figs. 3 and 5. Photo AS 342-5513 No. 234 taken August 10, 1952 (courtesy of the Governmentof Alberta).
I 12
H.M. Jol, D.G. Smith/Journal of Applied Geophysics 34 (1995) 109-123
an imprecise device, it is still the least difficult method by which to verify the depth of peat.
3. Results and interpretation Two GPR surveys were conducted parallel to two different existing pipelines along two cleared right-ofways in the Mitsue oilfield in northern Alberta (Figs. 1 and 2). In this sector of the Mitsue field, terrain units consist of sandy beach ridges, peatlands, sandy siltcovered river meander point bar sandy silts, and shallow lakes with a gyttja layer in the bottom (Fig. 2). Scrub stands of black spruce, birch and larch trees cover most of these peatlands (Fig. 2). The first GPR profile in Fig. 3 shows 460 traces at 1 m spacings. Profile A shows reflections to depths of 300 ns translated into a 5-6 m depth using an average near-surface velocity of 0.04 m/ns. The two uppermost continuous reflections represent the air wave (upper) and ground wave and water level (lower). At the time of the survey the groundwater level was at the surface. The deeper, nearly horizontal reflections are interpreted as peat stratigraphy. These reflections may record growth layers, previous drying cycles, or potentially, peat quality (H~inninen, 1990, 1992b; Warner et al., 1990). This profile is an excellent example of the quality of data that can be obtained using GPR in peatlands. A sandy beach ridge occurs on the right side (east) of the profile. These sand bodies have similar stratigraphy and inclined reflections (radar facies) as the beach ridges reported by Jol and Smith (1991) and Smith and Jol (1992). From positions 395 to 450 m, the uppermost continuous, inclined wavy reflection is interpreted as former offshore bars. The undulating nearly continuous reflections between traces 270 and 395 m represent the contact between peat and sand. The interface may contain some gyttja, common at the base of many peat bogs (S. Zoltai, pers. commun., 1993). The peat-buried sand ridge at 270 m may represent an ice-pushed offshore sand bar which never evolved into a beach ridge complex; the slopes are rather steep for a wave-deposited offshore bar. Most of the peat is between 1.5 and 3.3 m thick, excluding the ridge at the 270 m trace. The scrub forest cleared right-of-way crossing the peatland in Fig. 4 shows the ground conditions for GPR field operations. Cleared right-of-ways reduce GPR
survey time by 70% as compared to GPR surveys through the adjacent forested sites. Besides the trees and downfallen logs on the ground, the irregular ground surface in peatland make radar surveying difficult. The second GPR profile, shown in Fig. 5, contains 550 traces at 1 m spacings. The profile in Fig. 5A shows reflections to depths of 350 ns which translates into depths of 6-7 m using the average near-surface velocity of 0.04 m/ns. At the time of the survey the water level was about 10-30 cm below the surface, represented by the prominent continuous reflection below the broken irregular reflection or ground wave (Fig. 5A and B). In order to improve our confidence in the GPR results shown in Fig. 5A, we carried out a peat probe survey to verify the depth of contact between the peat and suspected underlying sand. Results shown in Fig. 5B and Table 1 indicate a close association between the GPR and peat probe methods to determine the thickness of peat. At all locations the probe encountered sand; however, at some sites the probe may have penetrated the sand to a depth below the peat-sand contact. The propagation speed of the GPR signal varies from one medium to another. The cross-sections in Figs. 3 and 5 used an average near-surface velocity of 0.04 m / ns determined only for the peat layer using the CMP method. Since the study was to determine the thickness of peat no correction was carried out for the underlying sand which has an average near-surface velocity of 0.07 m/ns.
4. Risk considerations in pipeline construction In Fig. 3B the thickness of peat between trace 330 and 375 m is greater than 2.5 m and, over much of the profile, the peat is thicker than 1.5 m. In this 45 m sector, extra care should be taken to reduce overloading by heavy construction equipment to prevent breaking through the frozen peat, sinking, and becoming immobile. In an average year undisturbed peat will freeze to 0.6-4).7 m; in a disturbed condition it will freeze to about 0.9-1.1 m deep (S. Zoltai, pers. commun., 1993). These depths are greater at more northerly locations. Preventative measures such as removing the snow cover in early winter to allow for a greater depth of freezing may deepen and improve the bearing strength of the frozen peat. However, such measures also cause greater difficulty in trench excavation for
H.M. Jol, D. G. Smith/Journal of Applied Geophysics 34 (1995) 109 123
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H.M. Jol, D.G. Smith/Journal of Applied Geophysics 34 (1995) 109-123
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Fig. 4. Photograph showing a right-of-way for a recently buried oil pipeline crossing 460 m of peatland shown in Fig. 3. View is looking northwest taken with a telephoto lens from a beach ridge.
pipeline placement. If time is limited, the use of lighter weight construction equipment is an alternative strategy in pipeline placement. If equipment does become stuck or sinks into the peat, the costs of downtime and later cleanup and repairs can be very expensive costing up to ten of thousands of dollars (G. McCullough, pers. commun., 1993). Fig. 5B shows a 110 m wide high-risk zone between trace 330 and 440 m where the peat exceeds 2.5 m deep. Here, the peat reaches a maximum depth of 3.7 m. As is shown in Fig. 3, caution should be taken in the high-risk zone by the pipeline contractor to reduce the chances of equipment breaking through the frozen peat and getting stuck. Safety risks to equipment operators is minimal should equipment fall through thick peat. The operating cabs of most heavy equipment are between 1.5 and 2.5 m above ground level, thus greatly reducing injury risks to operators. Discussions with pipeline field managers indicate that for short feeder lines from oil wells to collection batteries very little prior assessment of ground and subsurface conditions is carried out (G. McCullough, pers. commun., 1993). Construction risks are often determined from prior experience in the area. However, during the construction of major pipelines from collection batteries to processing facilities and from there to main trunk lines little technology exists for low-cost, quick assessment of peatlands. Present methods of peat assessment consist of peat probes and coring devices,
both of which are time consuming, expensive, and limited in accuracy. From our experience in peatlands, GPR is an extremely fast, low cost and accurate geophysical method that can be used to examine a wide variety of sites. Provided that suitable field conditions are present, GPR produces low cost, continuous subsurface profiles which cannot be matched by drilling. Moreover, a GPR grid survey can provide a general three-dimensional impression of the subsurface structures in sub-peat sediment and peat thickness. However, there are two problems, (1) equipment access in uncleared areas with dense forest, and (2) radar signal attenuation in saline and/or clay-rich zones (Finnish Geotechnical Society, 1992; Sutinen, 1992). Although there are few problems involved in placing feeder steel pipelines (8.9 cm diameter) in peat, serious problems occur when placing larger pipelines (30 cm diameter) from collection batteries to processing facilities and major trunk lines (0.9-1.2 m diameter) to southern markets. Heavy trenchers and wide pad crawler tractors are necessary for such pipe placements. In these circumstances, GPR can play a greater role, perhaps being most effective as a pre-assessment stage to a selective drilling program. Work in the Mitsue oilfield suggested that GPR has other potential applications in peatland assessments (Fig. 6). Power line poles placed in peat are susceptible to falling over unless deeply buried in the sub-peat
118
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Table 1 Peat probe spacing and depths along the eastern half of the GPR survey in Fig. 5 Position
Depth (m)
250 270 290 300 310 330 350 364 390 410 430 450 470 490 510 532
2.9 2.8 0.8 1.4 1.6 2.1 2.8 3.1 4.0 3.2 2.1 2.1 1.7 1.8 1.6 0.0
tling), while a peat probe survey at 20 m intervals over the same distance required four hours, and at times was subjective in locating the peat-sand contact. The latter survey technique lacks depth accuracy and continuity. (4) From this study of feeder oil and gas pipelines right-of-ways, we conclude that GPR is the lowest cost and most effective method to determine extent, continuity, and thickness of peatlands necessary for risk evaluation prior to pipeline placement. Several other ongoing field experiments in northern peatlands indicate that GPR can determine sub-peat topographic highs of mineral soil and peat thickness, necessary in the location and construction of roads, setting powerline poles, and locating drill rig pads, pipeline battery collection sites, processing plants, and major pipelines. GPR can also determine the potential peat compaction beneath gravel roads and drill pads (Fig. 6).
Acknowledgements sediment. GPR can provide a quick assessment of peat thickness for such installations. In the Mitsue oilfield several of the pipelines carried freshwater for downhole oil reservoir flooding to enhance oil recovery. Moreover, the amount of gravel fill (pit run) necessary to build up roads and drill pads could be estimated by knowing the thickness of peat and the amount of compaction (Fig. 6). Such information could aid considerably in the planning process to determine the volume of gravel necessary for new developments.
5. Conclusions ( 1 ) A prominent, nearly-continuous GPR reflection in the Mitsue Oilfield marks the peat-sand contact which was verified by a peat probe survey. Continuity of the reflection is broken when the contact interface is too steep (position 294) or silt and clay sediments (gyttja) are encountered between 377 and 411 m in Fig. 5. (2) GPR profiles indicate that the peat-sand contact ranges in depth from 0 to 3.7 m. Generally, the nearly horizontal reflections represent peat stratigraphy while inclined and wavy reflections indicate sand deposited as former offshore bars and beach faces. (3) The 550 m wide (Fig. 5) peatland required a two hour GPR survey (includes setup and disman-
The Natural Sciences and Engineering Research Council (NSERC) of Canada provided funds to purchase the GPR system and to support the field experiments. An NSERC Scholarship, an Alberta Heritage Scholarship Fund Award, a Steinhauer Graduate Research Fellowship, and a Petro-Canada Graduate Research Award were provided to Jol to carry out graduate research on GPR. Graham McCullough of Chevron Canada Resources, Ltd. and Steven Zoltai of Northern Forestry Centre, Government of Canada are thanked for advice on the Mitsue oilfield and peat conditions in northern Alberta.
References Annan, A.P. and Davis, J.L., 1976. Impulse radar sounding in permafrost. Radio Sci., 11: 383-394. Bjelm, L., 1980. Geological interpretation with subsurface interface radar in peatlands. Proc. 6th Int. Peat Cong., Int. Peat Soc., Duluth, MN, pp 7-8. Chernetsov, E.A., Beletsky, N.A. and Baev, M.Yu., 1988. Radar profiling of peat and gyttja deposits. Proc. 8th Int. Peat Cong., Int. Peat Soc., Leningrad, pp. 15-21. Davis, J.L. and Annan, A.P., 1989. Ground penetrating radar for high resolution mapping of soil and rock stratigraphy. Geophys. Prospect., 37: 531-551. Finnish Geotechnical Society, 1992. Ground Penetrating Radar Geophysical Research Methods. Finn. Geotechn. Soc., Tampere, 64 pp.
H.M. Jol, D.G. Smith/Journal of Applied Geophysics 34 (1995) 109-123
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Fig. 5. A 550 m ground penetrating radar survey paralleling several feeder pipelines across a peatland (Figs. 1 and 2). Sand underlies fhe at the east end (right side). Thickness of the peat varies from 0 to 3.7 m between trace 293 and 519 m. Large dots along the profile repn probe survey. The rttdar and probe transect lines were parallel and spaced 7 m apart. Steel feeder pipelines 8.9 cm diameter are buried 1.5 n to a nearby collection battery. Other oilfield pipelines transport water across the peatlands for downhole oil reservoir flooding to enh~mce r
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H.M. Jol, D.G. Smith/Journal of Applied Geophysics 34 (1995) 109-123
123
Fig. 6. Ground photo showing a forested peatland (3-4 m thick) taken near the easternmost well head in Fig. 1. A 1.5 m thick gravel-fill (pit run) roadway has subsided 1 m into the peat. A pipeline is located 3 m to the right of the roadside and buried 1.5 m. Power line poles with horizontal basal supports prevent poles from falling over. In the background is a 2 m (subsided 1.5 m) thick gravel-fill oil well drill pad. Hanninem P., 1990. Use of an impulse radar in peat research. 3rd Int. Conf. Ground Penetrating Radar, Abstr. Techn. Meet., Lakewood, CO, May 14-18, p. 37. H~inninen, P., 1992a. Application of ground penetrating radar techniques to peatland investigations. In: P. H~inninen and S. Autio (Editors L 4th Int. Conf. Ground Penetrating Radar, June 8-13, Rovaniemi. Geol. Surv. Finl. Spec. Pap., 16: 217-221. H~inninen, P., 1992b. Application of ground penetrating radar and radio wave moisture probe techniques to peatland investigations. Geol. Surv. Finl. Bull., 1, 71 pp. Jol, H.M. and Smith, D.G., 1991. Ground penetrating radar of northern lacustrine deltas. Can. J. Earth Sci., 28: 1939-1947. Morey, R.M., 1974. Detection of subsurface cavities by ground penetrating radar. US Highway Geol. Symp. Proc., 27: 28-30. Remote Applications Inc., 1982. The use of impulse radar techniques for depth profiling of peat deposits. Natl. Res. Counc. Can., Div. Energy Res. Dev., Ottawa. Smith, D.G. and Jol, H.M., 1992. Ground penetrating radar results used to infer depositional processes of coastal spits in large lakes. In: P. H~inninen and S. Autio (Editors), 4th Int. Conf. Ground Penetrating Radar, June 8-13, Rovaniemi. Geol. Surv. Finl. Spec. Pap.. 16: 169-177.
Sutinen, R., 1992. Glacial deposits, their electrical properties and surveying by image interpretation and ground penetrating radar. Geol. Surv. Finl. Bull., 359, 123 pp. Tiufi, M., Toikka, M., Marttilla, I. and Tolonen, K., 1983. The use of radio wave probe and subsurface interface radar in peat resource inventory. Proc. Symp. Remote Sensing in Peat and Terrain Resource Survey, Int. Peat Soc., Aberdeen, pp. 131-143. Tiuri, M., Toikka, M., Tolonen, K. and Rummukainen, A., 1984. Capability of new radiowave moisture probe in peat resource inventory. Proc. 7th Int. Peat Congress, Dublin, 1, pp. 517-525. Tolonen, K., Tiuri, M., Toikka, M. and Saarilahti, M., 1982. Radiowave probe in assessing the yield of peat and energy in peat deposits in Finland. Suo, 33:105-112. Ulriksen, C.P.F., 1981. Investigation of peat thickness with radar. Proc. 6th Int. Peat Congress, Int. Peat Sot., Duluth, MN. Ulriksen, C.P.F., 1982. Application of impulse radar to civil engineering. Ph.D. Dissertation. Lund Univ. Technol., Lund (republished by Geophysical Survey Systems Inc.. Hudson, NH). Warner, B.G., Nobes, D.C. and Theimer, B.D, 1990. An application of ground penetrating radar to peat stratigraphy of Ellice Swamp, southwestern Ontario. Can. J. Earth Sci., 27: 932-938.
Journal of Applied Geophysics 34 (1995) 109-123
Ground penetrating radar surveys of peatlands for oilfield pipelines in Canada Harry M. Jol, Derald G. Smith * Department of Geography, Universi~ of Calgary, Calgar)', Aim. T2NIN4, Canada Received 5 December 1993; accepted 11 August 1995
Abstract
Placement of buried pipelines in thick peat deposits is difficult because of the low bearing strength and high water content of the material for support of heavy construction equipment. Previously, ground penetrating radar (GPR) has been used to assess thickness and volume of peat as a fuel resource and horticultural material in Scandinavia and Canada. To our knowledge, GPR has not been applied to the site assessment and placement of pipelines crossing peatlands. Field experiments were conducted in the Mitsue oilfield operated by Chevron Canada Resources Ltd., located immediately southeast of Lesser Slave Lake in north-central Alberta Province, Canada. Surficial deposits consist of Holocene, linear, sandy beach ridges separated by peatlands underlain by sand. Several GPR surveys assessed the thickness of the peat along two oil pipeline right-of-ways. Results show the peat-sand contact as irregular and undulating, ranging from 0 to 3.7 m deep. Each survey, 460 and 550 m long, was completed in two hours. Such results from 1 m station spacings (sampling interval) can considerably reduce the uncertainties in planning and placement of oil, gas, and water pipelines crossing peatlands. Results indicate that thickness variations of peat can be detected more effectively in terms of quality of results, lower cost, and less time with GPR than with a peat probe or by coring.
1. Introduction
The problem in constructing pipelines across peatlands is the low bearing strength and high (up to 90% ) water content of the material. Saturated peat is unable to support the weight of the heavy construction equipment necessary for pipeline placement. Peatlands cover 50% of northern Alberta (S. Zoltai, pers. commun,, 1993), which coincidentally seem to occur at the surface of most oilfields in the Province (G. McCullough, pers. commun., 1993). In Canada, pipelines in peat are usually placed in midwinter when the frozen peat provides a relatively firm working surface for construction equipment for trenching, transportation, welding, plac* Corresponding author. 0926-9851/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved
SSD10926-9851(95)00018-6
ing, and burying. In thicker peat ( 2 - 4 m) construction activities sometimes result in equipment falling through the upper frozen layer (0.6-1.1 m thick). At times scuba divers are required to place chains and cables onto the submersed equipment before vertically hoisting it back onto the frozen surface (G. McCullough, pers. commun., 1993). Such delays are very costly and time consuming. Over the years field experience has shown that extra care and risk assessment is necessary in areas with thicker peat. Previously, ground penetrating radar (GPR) has been applied to detect the thickness of peat, the nature of the underlying mineral soil-peat contact, and the internal structure of the peat (Bjelm, 1980; Ulriksen, 1981, 1982; Remote Applications Inc., 1982; Tolonen et al., 1982; Tiuri et al., 1983, 1984; Chernetsov et al.,
110
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Fig. 1. Location of the Mitsue oilfield study area near the southeast end of Lesser Slave Lake in north-central Alberta, Canada.
1988; Warner et al., 1990; H/inninen, 1990, 1992a,b; Jol and Smith, 1991; Finnish Geotechnical Society, 1992; Sutinen, 1992). The relative permittivity of peat varies proportionally with the degree of decomposition or humification; therefore, GPR greatly facilitates examination of the developmental history of a peat deposit (Finnish Geotechnical Society, 1992). Peat is commercially mined for fuel and as a horticultural material. In peat mining, it is extremely important to know the locations of changes in sub-peat topography since they might otherwise cause contamination of the peat or damage to the equipment employed in the mining process. In Finland, sparks arising from a contact between peat mining machinery and underlying granite bedrock have caused fires in drained, dry peat (Hanninen, 1992a). A GPR cross-section profile provides an excellent picture of the thickness of a deposit. Combined with global positioning system (GPS) data, GPR can provide accurate cross-sectional profiles of peat depth and extent. A very close relationship occurs between the depth of peat obtained from drilling results and those
inferred from GPR (H~inninen, 1992b). Therefore, GPR provides an exceptional method to assess peat. Radar also yields information concerning the gyttja layer (organic-rich clay) and other sediments under the peat. If glacial till ties under the peat there will be a large amount of radar signal energy scattering due to the boulders; if the sublayer is clay there will be a strong signal (reflection) return from the high conductivity interface. No energy should be transmitted through thick clay so signals appearing from below the can be considered as system noise. If the clay layer is very thin, some energy may pass through it (Ulriksen, 1982). Reflections from below the peat-gyttja and mineral soil contact may also indicate sand or gravel. Ulriksen (1982) suggested that GPR measurements of peat thickness are substantially more accurate than those obtained by drilling or probing. He concludes that radar yields more trustworthy data than conventional sampling of the peat thickness and "thus becomes a method that is so accurate that there is no adequate means to measure its quality" (Ulriksen, 1982). The depth data obtained using the GPR method are also 10-20% less costly than than those yielded by traditional methods of probing and coring (H~inninen, 1992b). The pipeline study area reported in this paper is located in the Mitsue oilfield, near the town of Slave Lake southeast of Lesser Slave Lake, north-central Alberta Province, Canada (Figs. 1 and 2). The Mitsue field produces both oil and gas and has numerous feeder pipelines crossing peatlands from wells to collection batteries. As part of a larger study, the GPR results presented in this paper were surveyed across a peatland between two Holocene former sandy beach ridge complexes active about 8000 years ago (Figs. 1 and 2). Sand underlies the peat and a thin layer of gyttja (510 cm) occurs at the basal peat-sand contact. The objective of this study was to test whether GPR could identify the contact between peat and underlying mineral soil (in this case it was sand) with higher accuracy and at less cost and time than probing. Most important was the detection of areas with peat thicker than 1.5 m; such information will be useful for siting anticipated new pipelines and in deciding on the type of equipment to be used for trenching and filling during pipeline placement.
H.M. Jol, D.G. Smith/Journal of Applied Geophysics 34 (1995) 109-123
2. Methodology Ground penetrating radar allows continuous subsurface profiling at 1 m intervals. It is relatively rapid to implement which, in turn, reduces field work time and costs. The short-duration transmitted pulse produces a high-resolution (0.1-1 m) cross-sectional profile of the subsurface which can be used to determine structures from a few centimeters to tens of metres in size and depth. Digital GPR profiles are similar in appearance to seismic profiles, except GPR uses an electromagnetic energy source while seismic employs an acoustic source. The theory and methodology of GPR are adequately explained elsewhere (Morey, 1974; Annan and Davis, 1976; Ulriksen, 1982; Davis and Annan, 1989). A pulseEKKOT M IV radar system was used with 100 MHz antennas (1 m transmitter-to-receiver antenna separation) and a 1000 V transmitter in the reflection survey mode. At each 1 m station spacing, the digitized traces were vertically stacked 64 times with a sampling
Ill
time interval of 800 picoseconds. Profiles were processed and plotted using pulseEKKOT M IV (version 3.1 ) software. An average near-surface velocity of 0.04 m / ns was determined for the peat using the common midpoint survey (CMP) technique (Annan and Davis, 1976). Both low (50 and 100 MHz) and high (200 and 400 MHz) frequency antennas can be used in peatland investigations. Low-frequency antennas lend themselves best to the measurement of peat layer thickness, whereas high frequency antennas provide data on the near surface layers and moisture differences between peat layers (Finnish Geotechnical Society, 1992). A peat probe with 1.5 m extensions was used to determine peat thickness above the mineral soil substrate, which in this case was sand. The probe was inserted into the peat at 20 m intervals along 200 m of one GPR survey to confirm the GPR results and to determine the sub-peat sediment. Peat probes have limited accuracy if the peat sediment contact consists of gyttja or clay. In spite of the fact that the peat probe is
Fig. 2. An aerial photograph of the study area showing the scrub forest-coveredpeatlands and slightly arcuate patterns of sandy beach ridge complexes (see Fig. 1 for location). The broken lines are cleared right-of-waysused for pipelines which were surveyedby GPR shown in Figs. 3 and 5. Photo AS 342-5513 No. 234 taken August 10, 1952 (courtesy of the Governmentof Alberta).
I 12
H.M. Jol, D.G. Smith/Journal of Applied Geophysics 34 (1995) 109-123
an imprecise device, it is still the least difficult method by which to verify the depth of peat.
3. Results and interpretation Two GPR surveys were conducted parallel to two different existing pipelines along two cleared right-ofways in the Mitsue oilfield in northern Alberta (Figs. 1 and 2). In this sector of the Mitsue field, terrain units consist of sandy beach ridges, peatlands, sandy siltcovered river meander point bar sandy silts, and shallow lakes with a gyttja layer in the bottom (Fig. 2). Scrub stands of black spruce, birch and larch trees cover most of these peatlands (Fig. 2). The first GPR profile in Fig. 3 shows 460 traces at 1 m spacings. Profile A shows reflections to depths of 300 ns translated into a 5-6 m depth using an average near-surface velocity of 0.04 m/ns. The two uppermost continuous reflections represent the air wave (upper) and ground wave and water level (lower). At the time of the survey the groundwater level was at the surface. The deeper, nearly horizontal reflections are interpreted as peat stratigraphy. These reflections may record growth layers, previous drying cycles, or potentially, peat quality (H~inninen, 1990, 1992b; Warner et al., 1990). This profile is an excellent example of the quality of data that can be obtained using GPR in peatlands. A sandy beach ridge occurs on the right side (east) of the profile. These sand bodies have similar stratigraphy and inclined reflections (radar facies) as the beach ridges reported by Jol and Smith (1991) and Smith and Jol (1992). From positions 395 to 450 m, the uppermost continuous, inclined wavy reflection is interpreted as former offshore bars. The undulating nearly continuous reflections between traces 270 and 395 m represent the contact between peat and sand. The interface may contain some gyttja, common at the base of many peat bogs (S. Zoltai, pers. commun., 1993). The peat-buried sand ridge at 270 m may represent an ice-pushed offshore sand bar which never evolved into a beach ridge complex; the slopes are rather steep for a wave-deposited offshore bar. Most of the peat is between 1.5 and 3.3 m thick, excluding the ridge at the 270 m trace. The scrub forest cleared right-of-way crossing the peatland in Fig. 4 shows the ground conditions for GPR field operations. Cleared right-of-ways reduce GPR
survey time by 70% as compared to GPR surveys through the adjacent forested sites. Besides the trees and downfallen logs on the ground, the irregular ground surface in peatland make radar surveying difficult. The second GPR profile, shown in Fig. 5, contains 550 traces at 1 m spacings. The profile in Fig. 5A shows reflections to depths of 350 ns which translates into depths of 6-7 m using the average near-surface velocity of 0.04 m/ns. At the time of the survey the water level was about 10-30 cm below the surface, represented by the prominent continuous reflection below the broken irregular reflection or ground wave (Fig. 5A and B). In order to improve our confidence in the GPR results shown in Fig. 5A, we carried out a peat probe survey to verify the depth of contact between the peat and suspected underlying sand. Results shown in Fig. 5B and Table 1 indicate a close association between the GPR and peat probe methods to determine the thickness of peat. At all locations the probe encountered sand; however, at some sites the probe may have penetrated the sand to a depth below the peat-sand contact. The propagation speed of the GPR signal varies from one medium to another. The cross-sections in Figs. 3 and 5 used an average near-surface velocity of 0.04 m / ns determined only for the peat layer using the CMP method. Since the study was to determine the thickness of peat no correction was carried out for the underlying sand which has an average near-surface velocity of 0.07 m/ns.
4. Risk considerations in pipeline construction In Fig. 3B the thickness of peat between trace 330 and 375 m is greater than 2.5 m and, over much of the profile, the peat is thicker than 1.5 m. In this 45 m sector, extra care should be taken to reduce overloading by heavy construction equipment to prevent breaking through the frozen peat, sinking, and becoming immobile. In an average year undisturbed peat will freeze to 0.6-4).7 m; in a disturbed condition it will freeze to about 0.9-1.1 m deep (S. Zoltai, pers. commun., 1993). These depths are greater at more northerly locations. Preventative measures such as removing the snow cover in early winter to allow for a greater depth of freezing may deepen and improve the bearing strength of the frozen peat. However, such measures also cause greater difficulty in trench excavation for
H.M. Jol, D. G. Smith/Journal of Applied Geophysics 34 (1995) 109 123
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pipeline placement. If time is limited, the use of lighter weight construction equipment is an alternative strategy in pipeline placement. If equipment does become stuck or sinks into the peat, the costs of downtime and later cleanup and repairs can be very expensive costing up to ten of thousands of dollars (G. McCullough, pers. commun., 1993). Fig. 5B shows a 110 m wide high-risk zone between trace 330 and 440 m where the peat exceeds 2.5 m deep. Here, the peat reaches a maximum depth of 3.7 m. As is shown in Fig. 3, caution should be taken in the high-risk zone by the pipeline contractor to reduce the chances of equipment breaking through the frozen peat and getting stuck. Safety risks to equipment operators is minimal should equipment fall through thick peat. The operating cabs of most heavy equipment are between 1.5 and 2.5 m above ground level, thus greatly reducing injury risks to operators. Discussions with pipeline field managers indicate that for short feeder lines from oil wells to collection batteries very little prior assessment of ground and subsurface conditions is carried out (G. McCullough, pers. commun., 1993). Construction risks are often determined from prior experience in the area. However, during the construction of major pipelines from collection batteries to processing facilities and from there to main trunk lines little technology exists for low-cost, quick assessment of peatlands. Present methods of peat assessment consist of peat probes and coring devices,
both of which are time consuming, expensive, and limited in accuracy. From our experience in peatlands, GPR is an extremely fast, low cost and accurate geophysical method that can be used to examine a wide variety of sites. Provided that suitable field conditions are present, GPR produces low cost, continuous subsurface profiles which cannot be matched by drilling. Moreover, a GPR grid survey can provide a general three-dimensional impression of the subsurface structures in sub-peat sediment and peat thickness. However, there are two problems, (1) equipment access in uncleared areas with dense forest, and (2) radar signal attenuation in saline and/or clay-rich zones (Finnish Geotechnical Society, 1992; Sutinen, 1992). Although there are few problems involved in placing feeder steel pipelines (8.9 cm diameter) in peat, serious problems occur when placing larger pipelines (30 cm diameter) from collection batteries to processing facilities and major trunk lines (0.9-1.2 m diameter) to southern markets. Heavy trenchers and wide pad crawler tractors are necessary for such pipe placements. In these circumstances, GPR can play a greater role, perhaps being most effective as a pre-assessment stage to a selective drilling program. Work in the Mitsue oilfield suggested that GPR has other potential applications in peatland assessments (Fig. 6). Power line poles placed in peat are susceptible to falling over unless deeply buried in the sub-peat
118
H.M. Jol, D.G. Smith/Journal of Applied Geophysics 34 (1995) 109-123
Table 1 Peat probe spacing and depths along the eastern half of the GPR survey in Fig. 5 Position
Depth (m)
250 270 290 300 310 330 350 364 390 410 430 450 470 490 510 532
2.9 2.8 0.8 1.4 1.6 2.1 2.8 3.1 4.0 3.2 2.1 2.1 1.7 1.8 1.6 0.0
tling), while a peat probe survey at 20 m intervals over the same distance required four hours, and at times was subjective in locating the peat-sand contact. The latter survey technique lacks depth accuracy and continuity. (4) From this study of feeder oil and gas pipelines right-of-ways, we conclude that GPR is the lowest cost and most effective method to determine extent, continuity, and thickness of peatlands necessary for risk evaluation prior to pipeline placement. Several other ongoing field experiments in northern peatlands indicate that GPR can determine sub-peat topographic highs of mineral soil and peat thickness, necessary in the location and construction of roads, setting powerline poles, and locating drill rig pads, pipeline battery collection sites, processing plants, and major pipelines. GPR can also determine the potential peat compaction beneath gravel roads and drill pads (Fig. 6).
Acknowledgements sediment. GPR can provide a quick assessment of peat thickness for such installations. In the Mitsue oilfield several of the pipelines carried freshwater for downhole oil reservoir flooding to enhance oil recovery. Moreover, the amount of gravel fill (pit run) necessary to build up roads and drill pads could be estimated by knowing the thickness of peat and the amount of compaction (Fig. 6). Such information could aid considerably in the planning process to determine the volume of gravel necessary for new developments.
5. Conclusions ( 1 ) A prominent, nearly-continuous GPR reflection in the Mitsue Oilfield marks the peat-sand contact which was verified by a peat probe survey. Continuity of the reflection is broken when the contact interface is too steep (position 294) or silt and clay sediments (gyttja) are encountered between 377 and 411 m in Fig. 5. (2) GPR profiles indicate that the peat-sand contact ranges in depth from 0 to 3.7 m. Generally, the nearly horizontal reflections represent peat stratigraphy while inclined and wavy reflections indicate sand deposited as former offshore bars and beach faces. (3) The 550 m wide (Fig. 5) peatland required a two hour GPR survey (includes setup and disman-
The Natural Sciences and Engineering Research Council (NSERC) of Canada provided funds to purchase the GPR system and to support the field experiments. An NSERC Scholarship, an Alberta Heritage Scholarship Fund Award, a Steinhauer Graduate Research Fellowship, and a Petro-Canada Graduate Research Award were provided to Jol to carry out graduate research on GPR. Graham McCullough of Chevron Canada Resources, Ltd. and Steven Zoltai of Northern Forestry Centre, Government of Canada are thanked for advice on the Mitsue oilfield and peat conditions in northern Alberta.
References Annan, A.P. and Davis, J.L., 1976. Impulse radar sounding in permafrost. Radio Sci., 11: 383-394. Bjelm, L., 1980. Geological interpretation with subsurface interface radar in peatlands. Proc. 6th Int. Peat Cong., Int. Peat Soc., Duluth, MN, pp 7-8. Chernetsov, E.A., Beletsky, N.A. and Baev, M.Yu., 1988. Radar profiling of peat and gyttja deposits. Proc. 8th Int. Peat Cong., Int. Peat Soc., Leningrad, pp. 15-21. Davis, J.L. and Annan, A.P., 1989. Ground penetrating radar for high resolution mapping of soil and rock stratigraphy. Geophys. Prospect., 37: 531-551. Finnish Geotechnical Society, 1992. Ground Penetrating Radar Geophysical Research Methods. Finn. Geotechn. Soc., Tampere, 64 pp.
H.M. Jol, D.G. Smith/Journal of Applied Geophysics 34 (1995) 109-123
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Fig. 5. A 550 m ground penetrating radar survey paralleling several feeder pipelines across a peatland (Figs. 1 and 2). Sand underlies fhe at the east end (right side). Thickness of the peat varies from 0 to 3.7 m between trace 293 and 519 m. Large dots along the profile repn probe survey. The rttdar and probe transect lines were parallel and spaced 7 m apart. Steel feeder pipelines 8.9 cm diameter are buried 1.5 n to a nearby collection battery. Other oilfield pipelines transport water across the peatlands for downhole oil reservoir flooding to enh~mce r
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123
Fig. 6. Ground photo showing a forested peatland (3-4 m thick) taken near the easternmost well head in Fig. 1. A 1.5 m thick gravel-fill (pit run) roadway has subsided 1 m into the peat. A pipeline is located 3 m to the right of the roadside and buried 1.5 m. Power line poles with horizontal basal supports prevent poles from falling over. In the background is a 2 m (subsided 1.5 m) thick gravel-fill oil well drill pad. Hanninem P., 1990. Use of an impulse radar in peat research. 3rd Int. Conf. Ground Penetrating Radar, Abstr. Techn. Meet., Lakewood, CO, May 14-18, p. 37. H~inninen, P., 1992a. Application of ground penetrating radar techniques to peatland investigations. In: P. H~inninen and S. Autio (Editors L 4th Int. Conf. Ground Penetrating Radar, June 8-13, Rovaniemi. Geol. Surv. Finl. Spec. Pap., 16: 217-221. H~inninen, P., 1992b. Application of ground penetrating radar and radio wave moisture probe techniques to peatland investigations. Geol. Surv. Finl. Bull., 1, 71 pp. Jol, H.M. and Smith, D.G., 1991. Ground penetrating radar of northern lacustrine deltas. Can. J. Earth Sci., 28: 1939-1947. Morey, R.M., 1974. Detection of subsurface cavities by ground penetrating radar. US Highway Geol. Symp. Proc., 27: 28-30. Remote Applications Inc., 1982. The use of impulse radar techniques for depth profiling of peat deposits. Natl. Res. Counc. Can., Div. Energy Res. Dev., Ottawa. Smith, D.G. and Jol, H.M., 1992. Ground penetrating radar results used to infer depositional processes of coastal spits in large lakes. In: P. H~inninen and S. Autio (Editors), 4th Int. Conf. Ground Penetrating Radar, June 8-13, Rovaniemi. Geol. Surv. Finl. Spec. Pap.. 16: 169-177.
Sutinen, R., 1992. Glacial deposits, their electrical properties and surveying by image interpretation and ground penetrating radar. Geol. Surv. Finl. Bull., 359, 123 pp. Tiufi, M., Toikka, M., Marttilla, I. and Tolonen, K., 1983. The use of radio wave probe and subsurface interface radar in peat resource inventory. Proc. Symp. Remote Sensing in Peat and Terrain Resource Survey, Int. Peat Soc., Aberdeen, pp. 131-143. Tiuri, M., Toikka, M., Tolonen, K. and Rummukainen, A., 1984. Capability of new radiowave moisture probe in peat resource inventory. Proc. 7th Int. Peat Congress, Dublin, 1, pp. 517-525. Tolonen, K., Tiuri, M., Toikka, M. and Saarilahti, M., 1982. Radiowave probe in assessing the yield of peat and energy in peat deposits in Finland. Suo, 33:105-112. Ulriksen, C.P.F., 1981. Investigation of peat thickness with radar. Proc. 6th Int. Peat Congress, Int. Peat Sot., Duluth, MN. Ulriksen, C.P.F., 1982. Application of impulse radar to civil engineering. Ph.D. Dissertation. Lund Univ. Technol., Lund (republished by Geophysical Survey Systems Inc.. Hudson, NH). Warner, B.G., Nobes, D.C. and Theimer, B.D, 1990. An application of ground penetrating radar to peat stratigraphy of Ellice Swamp, southwestern Ontario. Can. J. Earth Sci., 27: 932-938.