We investigate the feasibility of imaging the electrical conductivity in a cross‐section of an object (such as a core sample) by numerical inversion of low‐frequency, electromagnetic (EM) boundary data. Current flow is assumed to be confined to the cross‐section, which is modeled as a network of resistors. The network serves as a discrete approximation of the distributed‐parameter system that is described mathematically by Maxwell’s equations for steady current flow in a nonhomogeneous medium. A complete set of linearly independent voltage vectors is applied to the peripheral nodes, and the resulting node currents serve as the measured data for estimating the internal conductivity pattern (image). We generate estimates of this conductivity image by using an iterative process on network equations that are linearized in the unknown conductance variables. The mathematical feasibility of this approach is demonstrated by computer simulation studies using data generated from the network model. Reconstructed images are presented for sample conductance patterns under both ideal and noisy data conditions. An error analysis is performed to relate data noise to image‐estimation error.

Fracture-scale heterogeneity plays an important role in driving dispersion, mixing and heat transfer in fractured rocks. Current approaches to characterize fracture scale flow and transport processes largely rely on indirect information based on the interpretation of tracer tests. Geophysical techniques used in parallel with tracer tests can offer time-lapse images indicative of the migration of electrically-conductive tracers away from the injection location. In this study, we present a methodology to invert time-lapse ground penetrating radar reflection monitoring data acquired during a push-pull tracer test to infer fracture-scale transport patterns and aperture distribution. We do this by using a probabilistic inversion based on a Markov chain Monte Carlo algorithm. After demonstration on a synthetic dataset, we apply the new inversion method to field data. Our main findings are that the marginal distribution of local fracture apertures is well resolved and that the field site is characterized by strong flow channeling, which is consistent with interpretations of heat tracer tests in the same injection fracture.

We use computer models and experiments to explore the feasibility of communication between points underground and on the Earth's surface. Emphasis is placed on ELF-VLF electromagnetic propagation through the Earth; nominally, we investigated propagation in the 200 Hz-30 kHz frequency range. The computer modeling included calculations of the fields of a point electric or magnetic source in a homogeneous half space or a stratified earth. Initial results for an insulated antenna of finite length are also considered. The experiments involved through-the-Earth transmissions at two locations in Pennsylvania, both of which had large formations of limestone. Initial results indicate that information rates as high as kbits/s may be possible for subsurface depths of 300 m or less. Accuracy of these estimates depends on the electromagnetic propagation constants of the rock, the noise characteristics, and modulation scheme. Although a nuisance for evaluating through-the-Earth propagation, the existence of subsurface metal conductors can improve the transmission character of the site.

Electromagnetic geophysical tomography, an adaptation of the CAT (computerized axial tomography) concept, was developed at the Lawrence Livermore National Laboratory to investigate rock below the ground surface. Electromagnetic geophysical tomography (EGT) can be used to characterize prospective urban subways, locating tunnels and natural caverns, mapping fractures in rock, and delineating groundwater pathways. In most applications where EGT systems are used, the sensors (antennas) are placed within two boreholes. The transmission characteristics of radio waves are measured between numerous transmitter and receiver locations. Since the electrical properties of rock mass affect radio waves passing through it, it is possible to calculate the electromagnetic properties of the rock sampled by the waves. By moving the antennas up and down the boreholes, multiple views of the rock are obtained. The EGT image is reconstructed using a computer and is then displayed on a CRT and analyzed. The image provides a detailed map of the electrical properties of the rock. These images enable an interpreter to understand the geologic structures and materials present.

A question of great significance is, “What are the properties of the ground?” This is of importance in locating natural resources (such as mineral and oil deposits) and in fundamental studies (such as scientific continental drilling interests). Much prior work has addressed surface and borehole measurements for economically driven natural resource applications. Geophysical measurements to ascertain fundamental properties of the earth have seen less attention. The availability of one or more deep boreholes thus opens new scientific vistas.

The mathematical formulae governing the interaction of a low-frequency source of electric current with a spherical anomaly are given. These formulae are used to determine the apparent resistivity and induced-polarization response between electrodes located within boreholes in the medium. Numerical results are given illustrating the spatial variation of these parameters.

Geophysical methods are commonly used for remotely inferring the character of a construction or excavation site. To acquire this inferential data, geophysical equipment has undergone extensive development. However, the data processing procedures by which this data is interpreted has undergone much less development. This report describes a number of procedures that have been developed to aid the interpretation process. Examples are given of how one can appropriately couple equipment-generated squiggly-line data with a mathematical description of the physical interaction. Under certain circumstances it is possible to generate an image (a picture) of the geophysical properties of the site. This picture can be more meaningful to non-geophysicists than a squiggly-line presentation. Such pictures have the potential promise of providing detailed data of the site character.

The Lawrence Livermore National Laboratory is currently assessing the capabilities of high resolution geophysical methods to characterize geologic sites for the disposal of high level nuclear waste. A successful experiment has recently been performed in which salt water tracers and high frequency electromagnetic waves were utilized to map rock mass fracture zones in-situ. Multiple cross-borehole EM transmissions were used to generate a tomographic image of the fractured rock region between two boreholes. The tomographs obtained correlate well with conventional wireline geophysical logs which can be used to infer the location of fractured zones in the rock mass. This indirect data suggests that the geotomography and brine tracer technique may have merit in mapping fractured zones between boreholes.

Prior theoretical results to describe the magnetic field caused by a vertical steel well casing are confirmed by field tests. A proton magnetometer was used to survey a gridded area about a known well casing. The measured magnetic fields are plotted in isometric views to show the steel well casing with and without interfering magnetic fields.

Geophysical exploration for engineering purposes is conducted to decrease the risk in encountering site uncertainties. Such studies are needed in construction of underground facilities. Current responsibilities, opportunities and challenges for those with geophysical expertise are defined. These include: replacing the squiggly line format, developing verification sites for method evaluations, applying knowledge engineering and assuming responsibility for crucial national problems involving rock mechanics expertise. For the covering abstract of the symposium see TRIS 452576. (TRRL)

The Lawrence Livermore National Laboratory is evaluating high resolution geophysical techniques for characterization of nuclear waste repository sites. This report presents the results of the first phase of this project. We describe the evaluation of a new geophysical technique used to map fractures remotely between boreholes: electromagnetic geotomography used in conjunction with water tracers. Salt water is forced into the fractured rock mass, attenuating the high frequency electromagnetic waves used for probing. The locations in the rock where the salt water has induced signal losses are then mapped by geotomography. An experiment using this technique has been performed near Oracle, Arizona, in a granitic rock mass. The data obtained were reduced to gray level images, which show the calculated signal transmission properties of the rock mass. We analyzed these images and compared them with borehole geophysical data: neutron logs, acoustic velocity logs, caliper logs, and acoustic televiewer logs. Comparisons between the images and the borehole geophysical data suggest that geotomography has merit when used to map fracture in granite. Image anomalies, which can be indicative of fracturing along the borehole walls, usually coincide with geophysical log anomalies. Under the conditions of the Oracle experiment, available data indicate that clusters of fracture zones were detected. Single fractures were not detected. The thickness of the smallest recognizable fractured zone was 0.6 m (2 ft).