Anthony L. Endres

A new concept in modeling the dielectric response of sandstones; defining a wetted rock and bulk water system

Experimental data for the real part of the dielectric constant (K′) of three sandstone samples are considered as a function of the level of water saturation (Sw) in the frequency range 60 kHz to 4 MHz. Existing theoretical models have previously shown poor agreement with K′ versus Sw data for rock samples, undoubtedly due to the complexity involved in adequately accounting for geometrical and electrochemical effects. In analyzing the data presented here, we find a pronounced increase in K′ in the low saturation region which in all cases can be attributed to the establishment of geometrical and surface effects associated with the rock‐water interface. When this increase in K′ is accounted for by defining wetted matrix parameters, the data show excellent agreement with existing theoretical models.

The effects of pore‐scale fluid distribution on the physical properties of partially saturated tight sandstones

Pore‐scale fluid distribution has a significant effect on the physical properties of a partially saturated porous medium. Experimental data for the dielectric response and elastic wave velocities for a tight gas sandstone undergoing a cycle of water saturation change through imbibition and drainage are analyzed. Mathematical formulations describing the internal geometrical configuration of a porous medium in terms of a rock matrix background with embedded oblate spheroidal inclusions representing the porosity are used to theoretically predict the dielectric constant and elastic wave velocities of the partially saturated sandstone. Simple geometrical models, incorporating homogeneous and heterogeneous inclusions, are used to simulate the pore‐scale fluid distribution which should result from the two saturation methods employed. It is found that these simple scenarios accurately predict the functional form and magnitude of the observed saturation‐induced hysteresis in the experimental data for both the dielectric constant and the elastic wave velocities.

Incorporating pore geometry and fluid pressure communication into modeling the elastic behavior of porous rocks

Inclusion‐based formulations allow an explicit description of pore geometry by viewing porous rocks as a solid matrix with embedded inclusions representing individual pores. The assumption commonly used in these formulations that there is no fluid pressure communication between pores is reasonable for liquid‐filled rocks measured at high frequencies; however, complete fluid pressure communication should occur throughout the pore space at low frequencies. A generalized framework is presented for incorporating complete fluid pressure communication into inclusion‐based formulations, permitting elastic behavior of porous rocks at high and low frequencies to be described in terms of a single model. This study extends previous work by describing the pore space in terms of a continuous distribution of shapes and allowing different forms of inclusion interactions to be specified. The effects of fluid pressure communication on the elastic moduli of porous media are explored by using simple models and are found to ...

The non-invasive characterization of pumping-induced dewatering using ground penetrating radar

Abstract Ground penetrating radar (GPR) profiling is a non-invasive geophysical technique that has been used by Endres et al. [Ground Water 38 (2000) 566] to successfully image pumping-induced drainage in an unconfined aquifer. However, the drained water volume calculated from the GPR data was significantly less than the actual pumped volume. To investigate the reasons for this discrepancy, a seven-day pumping test and five-day recovery test was performed at Canadian Forces Base Borden in Ontario, Canada. A dense spatial coverage of GPR profiles was used to better quantify variations in drainage due to small-scale aquifer heterogeneity. In addition, a neutron moisture content probe was used to directly observe drainage and the soil moisture profile at a sealed well near the pumping well. Neutron logging indicated that the transition zone translated downward during pumping without undergoing significant extension. Comparison of the GPR- and neutron-derived transition zone drawdowns show nearly equal responses. Both of these observations support the hypothesis that the behaviour of the GPR reflection is an accurate measure of the transition zone response. In contrast, transition zone drawdown obtained from both GPR and neutron logging are significantly delayed relative to potentiometric drawdown, resulting in an extended capillary fringe. The drained water volume was determined from the GPR-derived transition zone drawdown data using a number of different approaches. Methods that incorporated information about spatial variations in drainage gave larger estimates of drained water volume; however, those estimates were still lower than the actual pumped volume. The unaccountable volume of water could be a result of several factors—aquifer heterogeneity may still not be adequately represented by the increased GPR coverage, and/or leakage from the underlying aquitard may be providing a significant volume of water.

Long-term ground penetrating radar monitoring of a small volume DNAPL release in a natural groundwater flow field

An earlier field experiment at Canadian Forces Base Borden by Brewster and Annan [Geophysics 59 (1994) 1211] clearly demonstrated the capability of ground penetrating radar (GPR) reflection profiling to detect and monitor the formation of DNAPL layers in the subsurface. Their experiment involved a large volume release (770 L) of tetrachloroethylene into a portion of the sand aquifer that was hydraulically isolated from groundwater flow by sheet pile walls. In this study, we evaluated the ability of GPR profiling to detect and monitor much smaller volume releases (50 L). No subsurface confining structure was used in this experiment; hence, the DNAPL impacted zone was subjected to the natural groundwater flow regime. This condition allowed us to geophysically monitor the DNAPL mass loss over a 66 month period. Reflectivity variations on the GPR profiles were used to infer the presence and evolution of the solvent layers. GPR imaging found significant reflectivity increases due to solvent layer formation during the two week period immediately after the release. These results demonstrated the capacity of GPR profiling for the detection and monitoring of lesser volume DNAPL releases that are more representative of small-scale industrial spills. The GPR imaged solvent layers subsequently reduced in both areal extent and reflectivity after 29 months and almost completely disappeared by the end of the 66 month monitoring period. Total DNAPL mass estimates based on GPR profiling data indicated that the solvent mass was reduced to 34%-36% of its maximum value after 29 months; only 4%-9% of the solvent mass remained in the study area after 66 months. These results are consistent with independent hydrogeological estimates of remaining DNAPL mass based on the downgradient monitoring of the dissolved solvent phase. Hence, we have concluded that the long-term GPR reflectivity changes of the DNAPL layers are likely the result from the dissolution of chlorinated solvents residing in those layers. The long-term monitoring results demonstrated that GPR profiling is a promising non-invasive method for use at DNAPL contaminated sites in sandy aquifers where temporal information about immiscible contaminant mass depletion due to either natural flow or remediation is needed. However, our results also indicated that the GPR signature of older DNAPL impacted zones may not differ greatly from the uncontaminated background if significant mass reduction due to dissolution has occurred.

Geophysics at the interface: Response of geophysical properties to solid‐fluid, fluid‐fluid, and solid‐solid interfaces

Laboratory studies reveal the sensitivity of measured geophysical properties to solid-fluid, fluid-fluid, and solid-solid interfaces in granular and fractured materials. In granular materials, electrical properties and nuclear magnetic resonance relaxation times exhibit a strong dependence on the size and properties of the solid-fluid interface. The electrical and seismic properties of granular materials and the seismic properties of fractured materials reveal a dependence on the size or geometry of fluid-fluid interfaces. Seismic properties of granular and fractured materials are affected by the effective stress and cementing material at solid-solid interfaces. There have been some recent studies demonstrating the use of field-scale measurements to obtain information about pore-scale interfaces. In addition, a new approach to geophysical field measurements focuses on the geophysical response of the field-scale interface itself, with successful applications in imaging the water table and a redox front. The observed sensitivity of geophysical data to interfaces highlights new ways in which geophysical measurements could be used to obtain information about subsurface properties and processes.

A new framework for estimating englacial water content and pore geometry using combined radar and seismic wave velocities

[1] Ice mechanical properties, and hence the response of glaciers to climate change, depend strongly on the presence of liquid water at ice-grain boundaries. The propagation velocities of radar and seismic waves are also highly sensitive to this water. Mixing laws, typically the Looyenga and Riznichenko formulae, have traditionally been used to quantify liquid water content within glaciers from such velocity data; however, it has become apparent that these mixing laws are geometrically inconsistent. We present an inclusion-based effective medium approximation in which we model water inclusions within solid ice. Two types of inclusions are used: spherical inclusions to represent water in the grain junction nodes, and high-aspect ratio spheroidal inclusions to represent water in the grain boundary veins. We apply this model to radar and seismic data from a polythermal glacier in Svalbard to quantify both inclusion geometry and the unfrozen water content within the warm ice.

Quantifying Lake Athabasca (Canada) water level during the 'Little Ice Age' highstand from palaeolimnological and geophysical analyses of a transgressive barrier-beach complex

We combine multiproxy palaeolimnological and geophysical analyses of a barrier-beach complex to estimate the water level of a sustained Lake Athabasca (Canada) highstand during the ‘Little Ice Age’ (LIA; 1600—1900 CE). Palaeolimnological analyses on sediment cores from the lagoon behind the barrier indicate high water levels during the LIA, controlled by subsurface hydrological connection with Lake Athabasca. Key features in the LIA stratigraphic interval are sand laminations deposited by overwash events and low C/N ratios reflecting deposition of predominantly aquatic organic matter. Ground penetrating radar profiles of the barrier reveal a depositional transgression sequence composed of waterlain landward-dipping foreset beds and horizontal topset beds, overlain by aeolian deposits. Stratigraphic relations suggest that the LIA washover deposits in the lagoon formed as the barrier was actively translating landward, and were generated by high-water events on Lake Athabasca that overtopped the barrier. This indicates Lake Athabasca rose to at least the elevation defined by the contact between the waterlain and aeolian sediments in the barrier, which is >4 m above the historical daily average from gauged records available since 1930 and likely represents storm events during the highstand. Assuming a similar relation between daily average and maximum lake level as in the historical gauge record, our findings suggest that Lake Athabasca was on average 2.3 m higher during the LIA than present day. Extrapolation of this high-water plane into the adjacent Peace-Athabasca Delta indicates that 70% of the modern landscape was frequently and perennially flooded until very recently, consistent with palaeolimnological evidence from several lakes in the delta.

A pore-size scale model for the dielectric properties of water-saturated clean rocks and soils

The dielectric properties of water-saturated rock and soils are strongly dependent on the amount and nature of their porosity; interpretation of these geophysical data requires petrophysical models that incorporate both of these elements. The differential effective medium approximation (DEMA) is used to develop a dielectric permittivity model for clean (i.e., clay free) media that divides the pore spaces into elements corresponding to three categories of relative size scale: microscopic porosity (e.g., intergranular cracks), mesoscopic porosity (e.g., main pore volumes), and macroscopic porosity (e.g., vugs and fractures). The hierarchical size-scale structure imposed by the DEMA iterative embedding process is used to assign each pore space category its role in model construction. Use of this model demonstrates that the relationship between dielectric permittivity and porosity is significantly affected by the size scales of pores present in the rock models. A region of realizable permittivity-porosity val...

Site characterization at the Groundwater Remediation Field Laboratory

Environmental site assessment usually includes determining the geologic character of the subsurface after contamination has occurred. However, at the Groundwater Remediation Field Laboratory (GRFL), we acquired extensive geophysical and geotechnical data before planned contaminant releases happened. These prespill geophysical images provide background data for comparison with data acquired in the future. Additionally, a large amount of cone penetrometer (CPT) data provide in situ geotechnical measurements to compare with surface geophysics.