Stewart Sandberg

A 3D ERT study of solute transport in a large experimental tank

A high resolution, cross-borehole, 3D electrical resistivity tomography (ERT) study of solute transport was conducted in a large experimental tank. ERT voxels comprising the time sequence of electrical images were converted into a 3D array of ERT estimated fluid conductivity breakthrough curves and compared with direct measurements of fluid conductivity breakthrough made in wells. The 3D ERT images of solute transport behaviour were also compared with predictions based on a 3D finite-element, coupled flow and transport model, accounting for gravity induced flow caused by concentration differences. The tank (dimensions 185×245×186 cm) was filled with medium sand, with a gravel channel and a fine sand layer installed. This heterogeneous system was designed to complicate solute transport behaviour relative to a homogeneous sand tank, and to thus provide a challenging but insightful analysis of the ability of 3D ERT to resolve transport phenomena. Four ERT arrays and 20 piezometers were installed during filling. A NaCl tracer (conductivity 1.34 S/m) was injected and intensively monitored with 3D ERT and direct sampling of fluid chemistry in piezometers. We converted the bulk conductivity estimate for 250 voxels in the ERT imaged volume into ERT estimated voxel fluid conductivity by assuming that matrix conduction in the tank is negligible. In general, the ERT voxel response is in reasonable agreement with the shape of fluid conductivity breakthrough observed in six wells in which direct measurements of fluid conductivity were made. However, discrepancies occur, particularly at early times, which we attribute to differences between the scale of the image voxels and the fluid conductivity measurement, measurement errors mapped into the electrical inversion and artificial image roughness resulting from the inversion. ERT images revealed the 3D tracer distribution at 15 times after tracer injection. The general pattern and timing of solute breakthrough observed with ERT agreed with that predicted from the flow/transport modelling. However, the ERT images indicate a vertical component of tracer transport and preferential flow paths in the medium sand. We attribute this to transient vertical gradients established during tracer injection, and heterogeneity caused by sorting of the sand resulting from the filling procedure. In this study, ERT provided a unique dataset of 250 voxel breakthrough curves in 1.04 m3. The use of 3D ERT to generate an array of densely sampled estimated fluid conductivity breakthrough curves is a potentially powerful tool for quantifying solute transport processes.

Resistivity and induced polarization monitoring of salt transport under natural hydraulic gradients

We demonstrate the use of resistivity/induced polarization (IP) monitoring of salt transport under natural hydraulic loads. Electrical monitoring of saline tracer transport during forced injection has been demonstrated previously. Detection of tracer transport under natural hydraulic loading is difficult because neither the hydraulic load nor the tracer resistivity can be controlled. In one study, we identify the electrical response to salt transport in a dynamic beach environment. Resistivity/IP imaging resolved the structure of the saltwater‐freshwater interface and evidence for tide‐induced groundwater transport. Resistivity increases in the near surface and at depth, upbeach of the high‐tide mark, accompanied by tidal transgression. We attribute this to desaturation and decreasing salinity in the near surface and to decreasing salinity at depth, despite tidal transgression. Monitoring of groundwater levels indicates a phase lag between the tide level and groundwater level, supporting the electrical da...

Controlled-source audiomagnetotellurics in geothermal exploration

Theoretical and field tests indicate that the controlled‐source audiomagnetotelluric (CSAMT) method provides an efficient means of delineating the shallow resistivity pattern above a hydrothermal system. Utilizing a transmitter overcomes the main limitation of conventional audiomagnetotellurics—variable and unreliable natural source fields. Reliable CSAMT measurements can be made with a simple scalar receiver. Our calculations for a half‐space show that the plane‐wave assumption is valid when the transmitter is more than 3 skin depths away in the broadside configuration and more than 5 skin depths away in the collinear configuration. Three‐dimensional (3-D) numerical modeling results for a bipole source 5 skin depths away compare well with those for a plane‐wave source, showing that the method is valid. A CSAMT survey at the Roosevelt Hot Springs geothermal area in Utah produced apparent resistivity contour maps at four frequencies: 32, 98, 977, and 5208 Hz. These maps show the same features as those of a...

Survey Design Procedures and Data Processing Techniques Applied to the EM Azimuthal Resistivity Method

The electromagnetic (EM) azimuthal resistivity method is an alternative to the galvanic azimuthal resistivity survey. The advantages of the EM approach include (1) reduced acquisition time, (2) simple field procedure, and (3) a reduced data acquisition area. The primary disadvantage of the EM method is the magnitude of the data noise.Signal processing was applied to assist quantification of noise in EM azimuthal resistivity datasets and to enable noise reduction. Comparison of the energy in the even and odd coefficients of the power spectra allowed the signal‐to‐noise ratio to be identified. Linear phase filters were used to suppress high frequency noise. These techniques were applied to EM data collected at three study sites. Following data processing, the EM azimuthal resistivity datasets revealed apparent resistivity lobes consistent with the orientation of fracture strike mapped at two of these sites. At one site the processed EM dataset correlated closely with an azimuthal resistivity dataset collect...

Magnetic prospecting at a prehistoric and historic settlement in Maine

Magnetic surveys at a prehistoric and historic site in Maine were performed in order to improve understanding of cultural features identified from excavations that detailed cultural characteristics at discrete locations, but failed to define the geometry and extent of subsurface features. The objective of the magnetic work was to non-invasively define this spatial distribution of features. Such information is required to nominate both sites to the US National Register of Historic Places. Environmental and cultural factors in Maine favour the application of the magnetic method in archaeological prospecting. At the Spiller Farm palaeoamerican site, excavations had revealed a hearth and over 130 fragments of stone tools. The objective of the magnetic survey was to define additional hearths and cultural material within the vicinity. Measurements were made on a 45 by 25 m grid at 1 m intervals. Signal processing improved definition of a +50 nT anomaly that, upon partial excavation, was found to be associated with a sizeable burnt feature. Laminations in the excavated feature, combined with an increase in palaeo-artefact counts around it, indicate that the feature is a palaeo-hearth, rather than a natural burnt feature. At the Fort Hill historic site, previous excavations revealed a hearth, stone debris related to the fort foundations and numerous historic artefacts but did not resolve the fort geometry. Magnetic measurements were performed on a 55 by 45 m grid at 1 m intervals. Linear anomalies (+20 nT) were interpreted primarily as the response to residual metal in the fort foundations. The geometry of the fort and the meeting house, known to exist from historic accounts, were directly inferred from the survey. The hearth location and artefact distribution correlate well with the magnetic data. The dimensions of the fort (41 by 19 m) were found to be considerably larger than that expected from historic references. Copyright

Geophysical Monitoring Through a Tidal Cycle at Crescent Beach State Park, Maine

This paper presents results of an experiment in which data from a combination of geophysical methods (resistivity, induced polarization, electromagnetic profiling, and magnetic profiling) were used to identify shallow geologic and hydrogeologic structure, and to enhance understanding of groundwater flow in a dynamic beach environment. Based on this study, well sorted beach sand is underlain by laterally variable glacial till, which overlies bedrock. An extensive patch of glacial till was mapped toward the ocean side of the beach. Bedrock has lithologic variability, as evidenced by a non-uniform magnetic response. In particular, a northeast-southwest trending magnetic body, believed to be a basalt dyke, was detected by its magnetic signature. During the transgressive phase of the tidal cycle, saline groundwater flow appeared to be quite rapid, extending ahead of the surfical water by at least 4m, as evidenced by the electromagnetic response. During the regressive phase, a slug of saline water remained up b...