Papers by Laurence Bentley
Water Science and Technology Library, 1994
A new joint inversion algorithm that can be applied to independent physical parameters has been d... more A new joint inversion algorithm that can be applied to independent physical parameters has been developed. The method allows the interchange of structural information between otherwise independent inversions. It is based on the principles of model-sided robust modeling and the structures in the opposing inversion model are supported and not strictly enforced. The algorithm uses unstructured meshes that allow the incorporation of topography and other information on the geometry of the subsurface. A synthetic study is done using refraction tomography and dc resistivity inversion. On one hand it is seen that an existing boundary in one parameter does not enforce the same structure in the other. On the other hand inversion results are significantly improved yielding a clearer separation of units by means of cluster analysis. The algorithm is applied to the Pine Creek site in Calgary, Alberta, Canada. The site is underlain by approximately 6 m of unsaturated gravel overlying a shaly weathered sandstone bedrock. Repeated profiles using electrical resistivity, seismic refraction and ground penetrating radar surveys were conducted in support of a groundwater study. The joint inversion reduces the ambiguity in the inversions and consequently improves the final stratigraphic model. Improved stratigraphic models will result in an improved ability to model groundwater flow and transport.
SEG Technical Program Expanded Abstracts 2002, 2002
Shallow VSP and geophysical well logging surveys were undertaken in a 127 m-deep well located at ... more Shallow VSP and geophysical well logging surveys were undertaken in a 127 m-deep well located at the Rothney Astrophysical Observatory site near Priddis, Alberta. The well was drilled through interbedded sands and shales of the Paskapoo Formation. A suite of geophysical well logs (including natural gamma-ray, normal resistivity, focusedbeam resistivity, density, neutron-neutron, caliper, temperature, and SP) was acquired in the open hole immediately after drilling. After PVC casing was inserted into the well and grouted to the formation rocks, we obtained natural gammaray and full-waveform sonic logs. The logs were useful for delineating the sandstone and shale beds, and for providing hydrogeological information at the well site. To analyze full-waveform sonic data, we formulated procedures for trace alignment, noise reduction, and automatic time picking of first arrivals. The resulting time shifts and first-break times were used with the known geometry of the full-waveform sonic tool to determine P-wave velocities of the rock formations around the well.
Environmental impacts associated with high salt concentration related to industrial activities ca... more Environmental impacts associated with high salt concentration related to industrial activities can be assessed and monitored using geophysical methods. Repeated three- dimensional electrical resistivity imaging (ERI) was used over a three-year period to map the distribution and level of the salt concentration as well as evaluate the efficiency of a system of tile drains installed as a part of the
Organic contaminants in groundwater such as those in landfill leachate or hydrocarbon spills are ... more Organic contaminants in groundwater such as those in landfill leachate or hydrocarbon spills are of concern when they impact drinking water. The delineation of these contaminants is therefore of interest and spontaneous potential (SP) has been proposed as a rapid, simple and inexpensive method for mapping a plume from the ground surface. It has been suggested that SP could be used to identify zones of high redox potential gradients that form a halo around a degrading contaminant plume. We conducted SP surveys on two hydrocarbon contaminated sites in Alberta. Neither site showed the expected negative anomaly. The computer code MIN3P was used to simulate the transport and geochemical evolution of a hydrocarbon spill. The results from one time step were used to represent the distribution of the chemical species that is the source of SP anomalies. Using a finite element code, we modelled the SP signature due to two types of hypothetical source terms. The first type is sources due to dif...
International Journal for Numerical Methods in Fluids, 1992
ABSTRACT
Hydrological Processes, 2012
Unconsolidated sediments in alpine watersheds can store glacier melt and snowmelt as groundwater,... more Unconsolidated sediments in alpine watersheds can store glacier melt and snowmelt as groundwater, which helps sustain flow in mountain rivers during dry periods. However, the amount and distribution of groundwater storage in rugged alpine terrain is not well understood, hindering our ability to predict the rate and timing of groundwater discharge into alpine streams. We show how non-invasive time-lapse microgravity surveys can be used to gauge the spatial distribution of groundwater storage changes within a large (ca 1500 Â 1000 m) moraine-talus field of the Lake O'Hara alpine watershed of the Canadian Rockies. Additional ground-penetrating radar (GPR) and seismic refraction surveys provide complementary information on subsurface bedrock topography and reveal the location of a major northwest-southeast trending depression that likely controls groundwater flow to an alpine lake contiguous with the moraine-talus field. Repeat relative gravity measurements made on a network of 80 gravity stations over and around the moraine-talus field during the summers of 2009 and 2010 reveal gravity changes of up to 25 mgal. Although the small gravity changes associated with groundwater flowing out of storage areas are noisy, significant changes are evident on the eastern side of the moraine-talus field. © Her Majesty the Queen in Right of Canada 2012.
GEOPHYSICS, 2004
Geometrically complex heterogeneities at a decommissioned sour gas plant could not be adequately ... more Geometrically complex heterogeneities at a decommissioned sour gas plant could not be adequately characterized with drilling and 2D electrical resistivity surveys alone. In addition, 2D electrical resistivity imaging profiles produced misleading images as a result of out‐of‐plane resistivity anomalies and violation of the 2D assumption. Accurate amplitude and positioning of electrical conductivity anomalies associated with the subsurface geochemical distribution were required to effectively analyze remediation alternatives. Forward and inverse modeling and field examples demonstrated that 3D resistivity images were needed to properly reconstruct the amplitude and geometry of the complex resistivity anomalies. Problematic 3D artifacts in 2D images led to poor inversion fits and spurious conductivity values in the images at depths close to the horizontal offset of the off‐line anomaly. Three‐dimensional surveys were conducted with orthogonal sets of Wenner and dipole–dipole 2D resisti...
Expanded Abstracts of 73rd Annual Internat …, 2003
Pore geometry is a difficult parameter to quantify, but it affects the elastic moduli in a fundam... more Pore geometry is a difficult parameter to quantify, but it affects the elastic moduli in a fundamental way. The uncertainty in elastic moduli is to a large degree due to the uncertainty in pore geometry. The authors inverted the velocity measurements of ninetyseven sandstone samples for the pore aspect ratio spectra. It was found that the porosity of round pores is in direct proportion to total porosity, and that the porosity of cracks is small and varies randomly for a large set of samples from different geological backgrounds but may be linearly related to total porosity for a small subset from some specific area. This pore geometrical model is central to predicting the elastic properties in sandstones. The dry elastic moduli at a given porosity vary chiefly due to cracks and the variability increases with decreasing porosity. This causes the cross-plot of dry elastic moduli versus total porosity to scatter in a triangular form, and is also responsible for higher stress sensitivity of low-porosity sandstones. The linearity between total porosity and elastic moduli results from strong correlation between crack porosity and total porosity, and it becomes more pronounced at high effective pressure or at saturation with a less compressible fluid such as water. The Vp/Vs ratio in wet sandstones is affected by the Vp/Vs ratio of rock solid and pore geometry, but the former has far more influence, which may provide the rock physics basis for lithology identification using the Vp/Vs ratio. Vp and Vs are correlated very well for both dry and wet sandstones. The two straight lines are separate at low velocities but merging toward high velocities. In addition, clay content has been found with little relationship to pore geometry and with some connection to the elastic moduli of rock solid.
SEG Technical Program Expanded Abstracts
Time-lapse seismic monitoring of reservoirs relies on changes in fluid saturation and pressure du... more Time-lapse seismic monitoring of reservoirs relies on changes in fluid saturation and pressure due to petroleum reservoir exploitation causing observable changes in seismic response. The Gassmann equation can be used to estimate changes in the bulk modulus of the reservoir for given changes in bulk moduli of dry rock and fluids. Gas, oil and water bulk moduli are approximated using published results and commonly available petroleum reservoir data. The lower and upper limits of the fluid mixture bulk modulus are calculated from the saturation-weighted harmonic and arithmetic averages, respectively. The change in the dry bulk and shear moduli due to changes in effective pressure is approximated for sandstones using published data. Estimated changes in the bulk modulus, shear modulus and fluid-saturated density are used to calculate new compressional and shear wave velocities. Percentage changes in velocities and acoustic impedance can be inspected for significance. Finally, synthetic shot gathers are generated which can be compared for changes in seismic attributes such as reflection coefficient, AVO effects or frequency changes.
The bulk and shear moduli of dry sandstone increase with effective pressure and decrease with tem... more The bulk and shear moduli of dry sandstone increase with effective pressure and decrease with temperature and the rate of change varies with effective pressure and temperature. In order to calculate the effect of effective pressure, a rock physics model based on pore aspect ratio spectra (KT model) is adopted in computation of elastic moduli and velocities. The pore aspect ratio spectra for a set of water-saturated sandstone samples are first assumed to be proportional to that of the standard sample, and are then adjusted to fit velocity measurements. Dry bulk and shear moduli at different pressures are calculated with the optimized pore aspect ratio spectra by setting the bulk moduli of contained fluid equal zero. It is found that the exponential relationship exists between the rate of change of elastic moduli and effective pressure as follows:
In order to manage a reservoir we need to be able to predict how fluids will flow in response to ... more In order to manage a reservoir we need to be able to predict how fluids will flow in response to the production and injection of fluids. Reservoir simulation allows us to predict fluid flow. Before production, a reservoir is characterized using data from geology, seismic exploration, logging, well tests, core and laboratory work. During production, pressure, saturation and temperature change, and the change is predicted by reservoir simulation. Dynamic data collected during production and 4-D seismic surveys can be used to improve reservoir simulation. As a first step, reservoir simulation parameters are adjusted so that the model predictions approximate the bottom hole pressure (BHP), water cut, gas oil ratio (GOR) and other field observations. Reservoir parameters can be further adjusted so that the predicted changes in seismic response approximate the results from 4-D seismic surveys. We are currently searching for candidate 4-D research sites.
Geophysical data and interpretations can make important contributions to hydrological studies. Th... more Geophysical data and interpretations can make important contributions to hydrological studies. The keys to success are integrating geophysical and hydrogeological survey planning, clearly defining hydrological study objectives, using hypothesis driven planning and interpretation and using clearly defined petrophysical models to link geophysical responses with hydrological processes and geologic settings. Auxiliary measurements such as temperature as well as using multiple geophysical methods also reduce the ambiguities that often plague geophysical interpretations.
Cluster analysis is used to construct fluid flow zones from seismic attributes. The steps are: 1.... more Cluster analysis is used to construct fluid flow zones from seismic attributes. The steps are: 1. Remove grid points that contain outliers in any seismic attribute; 2. Scale each attribute to zero mean and unit variance; 3. Use principal component analysis to transform the scaled attributes to uncorrelated principal component attributes; 4. Principal component attributes are grouped into categories of similar seismic response using cluster analysis; 5. Upscale the seismic grid to the computational grid scale using a weighted voting procedure (morphing); 6. Spatially filter cluster assignments to remove small, isolated spots using a weighted voting scheme; 7. Assign a seismic zone to spatially connected elements with the same cluster category. Using a Gulf of Mexico test case, the zoning procedure produced useful computational zones and reduced the time required for history matching.
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Papers by Laurence Bentley