Hung-Wen Tseng

Crosshole electromagnetic tomography: A new technology for oil field characterization

With the advent of crosshole seismic technology in the 1980s, a new generation of high resolution geophysical tools has become available for reservoir characterization. The chief improvement is simply that the tools are deployed in boreholes so measurements take place much closer to the region of interest.

3D interpretation of electromagnetic data using a modified extended Born approximation

We present a new method, dubbed the modified extended Born approximation (MEBA), for efficient three-dimensional (3D) simulation and inversion of geophysical frequency-domain electromagnetic (EM) data for a targeted object lodged in a layered half-space. Based on the integral equation method and modified from an extended Born approximation technique, the MEBA method calculates the total electric field in an electrical conductivity inhomogeneity without any need for solving a huge matrix equation. This is done by multiplying the background electric field by a depolarization tensor. The Fourier transform and the convolution theorem are used to dramatically increase the computational efficiency. Comparisons of MEBA-generated numerical data for tabular targets with data generated by other means are used to verify the scheme and check its range of validity. The results indicate that the MEBA technique yields better accuracy when current channeling in the conductivity anomaly dominates over the induction process. The MEBA algorithm has been incorporated into a least-squares inversion scheme which is used to interpret borehole-to-surface EM tomography field data. The survey served to monitor the subsurface conductivity change associated with the extraction of a volume of saltwater previously injected into a known aquifer.

A borehole-to-surface electromagnetic survey

The results of a limited field trial confirm the usefulness of borehole‐to‐surface electromagnetic (EM) measurements for monitoring fluid extraction. A vertical EM profiling experiment was done at the University of California Richmond Field Station, where we simulated a brine spill plume by creating a saline water injection zone at the depth of 30 m. The data acquisition mode was analogous to the reverse vertical seismic profiling (VSP) configuration used for seismic measurements in that the EM transmitter traversed the PVC-cased borehole used for fluid injection and extraction while the receivers were deployed on the surface. The EM measurements were made at 9.6 kHz with an accuracy of 1% in signal amplitude and 1° in signal phase. Observations were taken at 5-m intervals along two intersecting profiles that were centered on the injection well and extended for 60 m on either side of it. The presence of the injected salt water, at the expected 30 m depth, was indicated clearly by differences between the p...

Topographic responses in magnetometric resistivity modeling

The magnetometric resistivity (MMR) topographic responses due to earth topography were simulated using a finite‐element method. An algorithm was developed and the computer program was verified by comparison with analytic responses for half‐space and contact models. The topographic responses for different rugged surfaces were computed, and the model results indicate topographic effects can affect MMR sounding interpretation. In general, MMR topographic responses do depend on surface form; the more rugged the ground surface is, the larger the MMR topographic anomaly will be. These topographic effects will decrease as the distance between the source (and/or receiver) position and the high relief area is increased. We only address the problem of determining MMR anomalies over a two‐dimensional (2-D) topography. A numerical example illustrates an effective means of reducing the terrain effects for a 45‐degree dipping fault model incorporating a 45‐degree ramp surface, suggesting that the finite‐element modelin...

A high frequency electromagnetic impedance imaging system

Non-invasive, high resolution geophysical mapping of the shallow subsurface is necessary for delineation of buried hazardous wastes, detecting unexploded ordinance, verifying and monitoring of containment or moisture contents, and other environmental applications. Electromagnetic (EM) techniques can be used for this purpose since electrical conductivity and dielectric permittivity are representative of the subsurface media. Measurements in the EM frequency band between 1 and 100 MHz are very important for such applications, because the induction number of many targets is small and the ability to determine the subsurface distribution of both electrical properties is required. Earlier workers were successful in developing systems for detecting anomalous areas, but quantitative interpretation of the data was difficult. Accurate measurements are necessary, but difficult to achieve for high-resolution imaging of the subsurface. We are developing a broadband non-invasive method for accurately mapping the electrical conductivity and dielectric permittivity of the shallow subsurface using an EM impedance approach similar to the MT exploration technique. Electric and magnetic sensors were tested to ensure that stray EM scattering is minimized and the quality of the data collected with the high-frequency impedance (HFI) system is good enough to allow high-resolution, multi-dimensional imaging of hidden targets. Additional efforts are being made to modify and further develop existing sensors and transmitters to improve the imaging capability and data acquisition efficiency.

A Wide‐Band Electromagnetic Impedance Profiling System for Non‐Invasive Subsurface Characterization

A non-invasive, wide-band electromagnetic (EM) impedance difference system for shallow subsurface electrical structure characterization in environmental and engineering problems has been developed at the Lawrence Berkeley National Laboratory (LBNL). Electrical parameters of interest are electrical conductivity and dielectric permittivity that are deduced from the impedance difference data. The prototype system includes a magnetic loop transmitter, which operates between 0.1 MHz and 100 MHz, an electrical dipole antenna for observing the electric field, and a loop antenna for measuring the magnetic field.All antennas are mounted on a cart made of non-metallic material for easy movement of the whole array for profiling. Surface EM impedance difference is obtained by taking the difference of the ratios of the electric fields to the magnetic fields at selected frequencies at two different levels. Numerical simulations will be presented to verify this new approach. A set of the impedance difference data acquired at the University of Californias Richmond Field Station compares reasonably well with simulation results based on a model obtained with the resistivity method and in situ TDR (time domain reflectometry)measurements.

High-Frequency Electromagnetic Impedance Measurements for Characterization, Monitoring and Verification Efforts

Non-invasive, high-resolution imaging of the shallow subsurface is needed for delineation of buried waste, detection of unexploded ordinance, verification and monitoring of containment structures, and other environmental applications. Electromagnetic (EM) measurements at frequencies between 1 and 100 MHz are important for such applications, because the induction number of many targets is small and the ability to determine the dielectric permittivity in addition to electrical conductivity of the subsurface is possible. Earlier workers were successful in developing systems for detecting anomalous areas, but no quantifiable information was accurately determined. For high-resolution imaging, accurate measurements are necessary so the field data can be mapped into the space of the subsurface parameters. We are developing a non-invasive method for accurately mapping the electrical conductivity and dielectric permittivity of the shallow subsurface using the EM impedance approach (Frangos, 2001; Lee and Becker, 2001; Song et al., 2002). Electric and magnetic sensors are being tested in a known area against theoretical predictions, thereby insuring that the data collected with the high-frequency impedance (HFI) system will support high-resolution, multi-dimensional imaging techniques.