Neil Lennart Anderson

Glacial Loading and Unloading: A Possible Cause of Rock Salt Dissolution in the Western Canada Basin

Each of the five main bedded Devonian rock salts in the Western Canada Basin has been leached in places more-or-less continuously since shortly after deposition. For the most part, salt dissolution has been self-sustaining and relatively slow. At various times however, leaching has been significantly accelerated by large-scale external processes such as regional uplift and erosion (during the pre-Cretaceous hiatus), and regional faulting (during the mid-Late Cretaceous). The most recent phase of accelerated leaching occurred during the Quaternary, probably as a result of glacial loading and unloading.Two principal lines of evidence support the thesis that accelerated rates of salt dissolution occurred during the Quaternary. First, there is an apparent correlation between surface drainage patterns and the active dissolution margins of the Devonian rock salts. A significant percentage of the lakes and rivers in proximity to active salt dissolution margins are situated along the near-zero edge of the respective salt bodies. Second, salt-related subsidence features at the pre-Quaternary subcrop level have been mapped on seismic data in the immediate proximity of active salt dissolution margins. The Quaternary subsidence features associated with the Prairie Formation rock salt for example, parallel the active salt margin and can be up to several kilometers wide. The areal extent of these shallow subsidence features and the high volume of rock salt dissovled at these sites suggest that accelerated rates of leaching occurred during the Quaternary (relative to the average rate of dissolution throughout the Cretaceous). The magnitude of these shallow structural features demonstrates clearly that subsidence associated with Recent salt dissolution can influence surficial drainage patterns to the extent suggested by the distribution of the lakes and rivers in proximity to the salt dissolution margins.The accelerated rates of salt dissolution during the Quaternary are attributed to glacial loading and unloading, and associated processes. Glacial loading could result in the compaction of subsurface strata, an increase in geothermal gradient, the thermal expansion of the deep aquifers, and the expulsion of interstitial water. These effects could increase both the rate of centrifugal fluid flow within a basin and the rate of salt dissolution. Glacial unloading could also affect the rate of salt dissolution. This rapid decrease in applied load is manifested as glacial rebound, a process which could increase the porosity and fracture permeability of the subsurface, and decrease the geothermal gradient. Potentially, the rate of centripetal fluid flow (at least around the basin margins) could be increased. Increased centripetal fluid flow, accompanied by an influx of understurated glacial melt waters would enhance the rate of salt dissolution.

Differential compaction of Winnipegosis reefs: A seismic perspective

Winnipegosis Formation reefs in southern Saskatchewan are typically encased in the thick, apparently incompressible salts of the Prairie Evaporite. These reefs are characterized by raised rims and reverse drape along the top of the salt. Both features, clearly visible on seismic data, are primarily due to postdepositional compaction. The rims developed principally as a result of differential compaction within the different reef environments; the structural low at the Prairie Evaporite level is attributed to differential compaction between the reef and the encasing salt. If these salts are effectively incompressible, the rim and lagoonal facies are estimated to have been compacted by at least 30 and 44 percent, respectively. This paper illustrates the usefulness of seismic data to separate postdepositional compaction features from primary features to determine the primary morphology of a reef better and to determine the relative amounts of postdepositional compaction within the different reef environments....

Ground-penetrating radar surveying in support of archeological site investigations

Abstract In April and July of 1996, ground-penetrating radar (GPR) surveys were conducted in support of archeological investigations at Flagstaff, Arizona and Sebastian, Florida, respectively. A GSSI SIR System 8 radar unit with a 500-MHz monostatic antenna was used for both surveys. The Flagstaff, Arizona survey was conducted at Elden Pueblo Ruins. The site is located in a coniferous forest and characterized by a myriad of surficial and subsurface features. Surficial features consisted mostly of pottery shards and the remnants of rock walled structures. The subsurface features consist mostly of rock lined pits, stone walls, and grave sites covered by a soil layer of variable thickness. The soil is derived from volcanic clastics and the underlying Kaibab Limestone bedrock. GPR profiles were acquired across various locations, some of which had been previously excavated and backfilled by archeologists. The main objectives were to determine the utility of the GPR technique with respect to locating subsurface features of archeological interest, determine the optimum field parameters in the area, and direct further field work. The Sebastian, Florida survey was conducted along the Atlantic coastline. Data were acquired along five beaches and one coastal sand dune. The beaches and dunes of the area are composed of a medium to coarse grained sand, containing quartz grains and carbonates. The principle objective of the Sebastian, Florida survey was to locate wreckage from a Spanish treasure fleet. A secondary objective was to determine the utility of GPR in a near shore marine environment.

Westhazel General Petroleums Pool; case history of a salt-dissolution trap in west-central Saskatchewan, Canada

The Westhazel General Petroleums (GP) Pool of west‐central Saskatchewan, Canada, produces from the GP member of the Lower Cretaceous Mannville Group. This reservoir is structurally closed across the updip, eastern dissolutional edge of the underlying Middle Devonian rock salt of the Leofnard Member, Prairie Formation. The leaching of these salts occurred in post‐Mannville time in the Westhazel area and caused the regional southwest dip of the General Petroleums member to be locally reversed. The Westhazel GP Pool, from a geophysical perspective, is characteristic of many of the shallow Lower Cretaceous pools situated along the dissolutional edge of the Prairie salt. The thin, 10 m reservoir facies at Westhazel does not exhibit a diagnostic signature on either seismic or gravity data. Rather, it is the updip edge of the salt across which the reservoir is closed that can be mapped using geophysical techniques. On seismic data, the dissolutional edge of the Prairie salt is characterized by: (1) a subtle decr...

Mapping subvertical discontinuities in rock cuts using a 400-MHz ground penetrating radar antenna

Hidden subvertical discontinuities oriented parallel to subparallel to the exposed faces of outcropping sandstone were effectively mapped at three different study sites in central Missouri using a ground-penetrating radar system (GPR) equipped with a 400-MHz monostatic antenna and a survey wheel. At each site, a suite of 2-D ground-penetrating radar profiles were acquired along multiple closely spaced traverses on relatively smooth exposed rock surfaces. Time-zero correction was applied to the raw GPR data which were then processed using band-pass filtering, range and display gain, color transformation, and deconvolution techniques. Pseudo 3D images of each identified discontinuity at each site were constructed based on the interpretation of the nonmigrated ground-penetrating radar profiles. These pseudo 3D images were hand-migrated and transformed into true 3D images which depict variable depths at “perpendicular horizontal distance” to each discontinuity relative to the exposed rock face. The results demonstrate that GPR can be used to detect and map hidden discontinuities. This information can then be used for rock slope stability analysis and rock engineering purposes.

Active-passive Array Surface Wave Inversion and Comparison to Borehole Logs in Southeast Missouri

In May 2002, both active- and passive-source surface wave data were acquired using 4-channel arrays at six selected bridge sites in southeast Missouri. Processing of acquired data (increase of signalto-noise ratio, estimation of phase velocities) was carried out and dispersion curves of Rayleigh wave phase velocities were constructed. Each fundamental mode dispersion curve was then inverted by linearised optimization to a layered shear-wave velocity profile to depths of up to 60 m. The estimated shear-wave velocity profiles were compared to other geotechnical data that had been previously acquired at each test site for the Missouri Department of Transportation (MoDOT) including cone penetrometer test (CPT) data, borehole lithologic control, seismic cone penetrometer test (SCPT) shear-wave data and cross-borehole (CH) shear-wave data. The surface wave models, although smoother than the destructive test logs, are accurate and consistent (17% average difference with CH results on two sites), and, moreover, provide information on lithology above the water table and at depths beyond the SCPT and CH limitations, in a more logistically-easier and costeffective manner.

Impact Origin of the Newporte Structure, Williston Basin, North Dakota

The Newporte field is located just south of the United States-Canada border in Renville County, North Dakota, in the north-central portion of the Williston basin. Integration of seismic, well-log, and core data supports the interpretation of an impact origin for the Newporte structure. The structure involves both Precambrian basement and lower Paleozoic sedimentary units. Oil and gas production began in 1977 from brecciated basement rocks along the rim of the 3.2-km-diameter circular structure. Both well logs and seismic data were used to determine thickness changes of sedimentary units overlying the structure. Resulting isopach maps reveal a circular, bowl-shaped feature with a recognizable rim. Microscopic shock metamorphic features in quartz and feldspar are visible in basement clasts that form a mixed breccia with Cambrian Deadwood sandstone within the western rim of the structure. A Late Cambrian-Early Ordovician age is suggested for the structure because of the presence of flat-lying Deadwood sandstone overlying mixed basement/sandstone breccia along portions of the rim. Identification of the Newporte structure as an impact crater adds to the gro ing base of evidence revealing the relevance of impact craters to petroleum exploration.

Natural recession of the eastern margin of the Leofnard salt in Western Canada

The Lloydminster area (T35-65, R15W3M-10W4M) of east‐central Alberta and west‐central Saskatchewan, Canada, is dissected by the north‐northwest trending updip active dissolution margin, of the Devonian Leofnard Member rock salt. West of this margin, up to 150 m of rock salt is preserved; updip and to the east, the salt has mostly been leached from the rock record. The margin is up to 40 km wide and characterized by extreme local variations in net salt thickness. The dissolution of the Leofnard rock salt in the Lloydminster area has resulted in the entrapment of significant hydrocarbon accumulation. Stratigraphic traps, for example, have formed where reservoir facies were either preferentially deposited or preserved in salt‐dissolution lows. Structural traps, in contrast, have formed where reservoir facies are draped across residual salt or collapse features. It has been estimated that three trillion barrels of oil (mostly of high viscosity and unrecoverable) are entrapped along the eastern dissolution mar...

Preliminary Analysis using Surface Wave Methods to Detect Shallow Manmade Tunnels

Abstract : Multi-channel Rayleigh wave data were acquired across a 1 meter diameter spillway tunnel along three parallel traverses with surface to tunnel separations of 0.90 meters, 2.15 meters and 3.13 meters depth, respectively. These surface wave data were acquired by placing a 24- channel geophone array perpendicular to the center-line of the spillway tunnel and incrementally moving the array across the tunnel. The near source-receiver offset was 6 meters; the 4.5 Hz geophones were spaced at 0.5 meters. The tunnel locations were identified visually on velocity-filtered common-shot gathered field records. Tunnel locations were also identified by analyzing common shot-gathered records using two newly-developed automated interpretation programs: Spiking Filter Analysis and Attenuation Analysis of Rayleigh Waves (AARW). Electrical resistivity data was acquired along each traverse for comparison purposes. The engineering geophysics community has recently focused on the use of Rayleigh (surface) wave methods, such as Multichannel Analysis of Surface Waves (MASW) and Refraction Micrometer (ReMi), to detect manmade tunnels in the earths shallow subsurface. Successful tunnel-detection applications of these methods have been reported. However, further experimental and analytical investigations are required to comprehend all significant aspects of the observed surface wave data. This case study reports on three alternate surface wave methods that were used to locate a 1 meter diameter tunnel: visually-identified diffracted/reflected surface-wave energy; the Attenuation Analysis of Rayleigh Wave (AARW); and Spiking Filter Analysis.

Salt dissolution and surface subsidence in central Kansas: A seismic investigation of the anthropogenic and natural origin models

Gradual subsidence in the Punkin Center study area, northeast Reno County, Kansas, has resulted in ponding of surface waters, abandonment of at least one oil well, and damage to county roads. Because the Punkin Center area is within the Burrton oil field and is underlain by the Hutchinson Salt Member, surface subsidence historically has been attributed to salt dissolution of anthropogenic origin. Subsidence that occurred significant distances from any known well sites has been attributed to unrecorded abandoned wells or complex asymmetric patterns of salt dissolution that originated at a drillhole. To ascertain the validity of the widely accepted anthropogenic salt‐dissolution model, a 4-km seismic reflection profile was acquired along an east‐west‐oriented county road adjacent to an abandoned oil well. The expectation was that the residual Hutchinson Salt Member would be anomalously thin where maximum recorded surface subsidence occurred and thicker elsewhere. Contrary to expectations, the residual Hutch...