Laurence R. Bentley

Two‐ and three‐dimensional electrical resistivity imaging at a heterogeneous remediation site

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 resistivity survey lines. The ...

Critical steps for the continuing advancement of hydrogeophysics

Special hydrogeophysics issues published by hydrology and geophysics journals, special sessions and workshops at conferences, and an increasing number of short courses demonstrate the growing interest in the use of geophysics for hydrologic investigations. The formation of the hydrogeophysics technical subcommittee of AGUs Hydrology section adds further evidence of the recognized significance of this growing interdisciplinary field. Given the clear value of nondestructive and nonintrusive imaging for subsurface investigations, we believe the advances in the adoption of existing geophysical methods, the development of novel methods, and the merging of geophysical and other data made in hydrogeophysics could be applied to a wide range of geological, environmental, and engineering applications.

Integrated hydrogeological and geophysical study of depression-focused groundwater recharge in the Canadian prairies

[1] In the northern prairie region of North America, numerous seasonal wetlands and ephemeral ponds form as snowmelt water is trapped in small topographical depressions. A detailed hydrogeological investigation is combined with electrical resistivity imaging (ERI) to evaluate the roles of the wetlands and ponds on depression-focused groundwater recharge at the St. Denis National Wildlife Area in Saskatchewan, Canada. The analysis of groundwater samples indicated two distinct geochemical zones: a zone of salt leaching under Wetland 109 and zones of salt accumulation under the adjacent uplands. Two intermediate zones were identified as partially leached or mixed leached-unleached and premodern or mixed saline-nonsaline water. The resistivity images of the areas around 109 were classified using the correlation between ERI-derived electrical conductivity (EC) and the groundwater EC. The ERI data clearly showed that depression-focused recharge occurs under all depressions regardless of size. Leaching is observed to the depth of a regional intertill aquifer, indicating that depression-focused recharge contributes to the regional groundwater system. The ERI data also revealed the complex pattern of salt distribution that could not have been recognized by hydrogeological observations alone. The complex distribution of salts appears to be caused by interaction between wetlands and variations in topography. INDEX TERMS: 1829 Hydrology: Groundwater hydrology; 1045 Geochemistry: Low-temperature geochemistry; KEYWORDS: depression-focused recharge, electrical resistivity imaging, groundwater recharge, prairie wetland

Resolution of 3-D Electrical Resistivity Images from Inversions of 2-D Orthogonal Lines

Three-D electrical resistivity imaging (ERI) using sets of orthogonal of 2-D survey lines provides an efficient and cost effective tool for site characterization in environmental and engineering investigations. A 3-D survey design using sparse sets of lines reduces the survey time at the expense of the resolution. The effects of line spacing on the resolution of 3-D electrical resistivity images were investigated using numerical modeling with synthetic and field data for two standard configurations, dipole-dipole and Wenner arrays. Synthetic data studies indicate that dipole-dipole configuration produces a more accurate map of the subsurface than the Wenner configuration. A severely under-sampled 3-D survey could result in introducing small-scale shallow spurious artifacts at the surface of the resistivity model caused by the projection of the anomalies located in the deeper parts of the model. Results from inversion of the real and synthetic data showed that lines should be separated by no more than four...

Time-lapse electrical resistivity monitoring of salt-affected soil and groundwater

Summary Time-lapse electrical geophysics is used to monitor the remediation of oilfield brines in soil and groundwater. Challenges with comparing geophysical surveys done during different seasons at different temperatures were overcome through temperature monitoring and geophysical processing techniques. The background electrical conductivity structure captured by the geophysics compared well with push tool conductivity logs collected and had strong correlation with salt samples taken. However the changes in electrical conductivity observed in the timelapse geophysics provided information about the remediation effectiveness that was not available from the logs and point measurements. From the observed changes in the geophysics we interpret that depression focused recharge has a large impact on remediation effectiveness. This research indicates that time-lapse electrical geophysics is an effective and low cost remediation monitoring method at appropriate sites.

Capturing Geological Realism in Stochastic Simulations of Rock Systems with Markov Statistics and Simulated Annealing

ABSTRACT Simulated annealing is a numerical algorithm that can be used to impose statistical structures on numerical grids representing heterogeneous rock or sediment. In this paper, we use the flexibility of simulated annealing to generate grids with Markov statistical structures. Our purpose is to transmit the rich geological information captured in Markov statistics into stochastic grids while maintaining the flexibility of annealing to honor field data. Performance issues that compromise annealing grids imbued with Markovian properties include scales of bedding or rock bodies relative to grid size, and the amount of geological complexity in the embedded Markov structures. The remedies to these issues include proper selection of grid size, careful choice of annealing type, and consideration of an alternative annealing stopping rule based on a chi-squared test statistic. If performance issues are overcome, complex stratal patterns such as higher-order dependency, cyclicity, and directionality can be replicated in grids by this method. In addition, accounting for variations in depositional rate allows for transference of Markov structures obtained from vertical boreholes to the horizontal dimension when other information is lacking. A field example using borehole data collected at the Gloucester special waste site near Ottawa, Canada, as well as synthetic examples, demonstrate the technique and performance issues.

Compensating for temperature variations in time-lapse electrical resistivity difference imaging

Variations in temperature during time-lapse electrical resistivity imaging (ERI) surveys introduce changes in electrical conductivity (EC). When the goal of the time-lapse ERI survey is to image changes in EC due to changes in saturation or pore water salinity, compensation must be made for the effect of temperature variations. A temperature-compensation method can approximate time-lapse ERI data with the effect of temperature variations removed. First uncompensated ERI data are inverted. The inversion model then is adjusted to a standard temperature image. Forward simulations are performed using the uncompensated inversion and the standard temperature equivalent model. The temperature-compensated simulated resistance data are subtracted from the uncompensated simulated resistance data, forming data correction terms. The data correction terms then are subtracted from the measured data to yield temperature-compensated data. Using the temperature-compensated data, inversions have been carried out on two syn...

Eulerian‐Lagrangian solution of the vertically averaged groundwater transport equation

Eulerian-Lagrangian methods (ELMs) have been developed to reduce smearing and oscillations in numerically generated advection-dominated transport solutions. Improvements in front propagation characteristics are gained, but new sources of errors are introduced in the tracking step of ELMs. The Eulerian-Lagrangian least squares collocation method (ELLESCO) is applied to the vertically averaged transport equation, and the ELLESCO method is used to demonstrate how different types of discretization errors affect ELM solutions. The accuracy of ELMs is partially dependent on the accuracy of a tracking step. The tracking integration can be made as accurate as desired by dynamically adjusting the number of second-order Runge-Kutta integretion steps per finite difference transport step. However, the accuracy of the final position is dependent on accurate velocities. Inaccurate velocities can lead to poorly located concentration fronts and mass balance errors. Although ELLESCO can extend the spatial and temporal discretization limits of classical methods, concentration fronts must be spread over one and a half elements to avoid oscillations in the solution. Time truncation errors associated with concentration dependent source terms lead to mass balance errors, and source terms should be computed with the Crank-Nicolson approximation to avoid excessive truncation error. The size of the finite difference transport time step and the magnitude of the local velocity determine the distance between the head and foot of the particle back tracks. This distance, referred to as the Lagrangian spatial resolution, must be less than the characteristic length of areal sources, or distortions will occur in the concentration front. The limit on the Lagrangian spatial resolution is an effective limit on the transport finite difference time step size.

A Comparison of Electrical Resistivity, Ground Penetrating Radar and Seismic Refraction Results at a River Terrace Site

Electrical resistivity imaging (ERI), ground penetrating radar (GPR) and seismic refraction (SRF) profiles were repeated over three lines on a terrace of the Bow River. The site had a resistive gravel layer overlying mudstone bedrock with horizontal transitions to lacustrine and overbank deposits. Electrical resistivity results were best for determining changes in sediment types and detecting boundaries, but the ERI smoothness constraint blurred the location of the boundaries. The GPR gave the most resolution and showed internal structures that the other methods did not image. The GPR signal was severely attenuated in several areas where the surficial sediments became too conductive because of a fine grained component. The seismic refraction inversion provided good reproduction of the bedrock interface, but it did not detect changes in the composition of the surficial sediments. It also required the introduction of a low velocity surficial layer not indicated by the other methods that may be related to the increase in effective stress with depth. Jointly interpreting the three data sets gives a more reliable and less ambiguous interpretation than any single method. The data may be useful to test joint inversion algorithms and are available for download.

Integration of seismic methods with reservoir simulation, Pikes Peak Heavy Oil Field, Saskatchewan

Reservoir characterization is essential for providing optimal recovery from heavy-oil fields. The process of reservoir characterization is demonstrated for a steam injection project at Pikes Peak heavy oil field near Lloydminster, Saskatchewan, Canada. Geologic, geophysical and reservoir engineering data are used to improve the interpretation of reservoir conditions. There is ambiguity in modeling any of these data. However, in modeling all data sets in “cooperative inversion,” ambiguity is decreased, thereby enhancing our knowledge of the reservoir. One of the main benefits of this oil-field modeling is that bypassed oil and steam fronts are effectively mapped.