Alex Becker

Two-dimensional mapping of sea-ice keels with airborne electromagnetics

Airborne electromagnetic (AEM) equipment can be used to sense sea ice thickness by interpreting the AEM data to obtain the distance from the towed bird that holds the EM system to the ice/seawater interface. The ice thickness itself is obtained by subtracting from that quantity the distance from the bird to the upper ice surface, as determined by a laser altimeter. To interpret AEM data acquired over sea‐ice keels, we first solve the forward problem using an integral equation approach to the Neumann boundary‐value problem. In this approach, we assume that sea ice is an insulator and that seawater is a perfect conductor. When the ice keel is two‐dimensional, the pertinent equations can be transformed into the wavenumber domain along the strike direction, resulting in the rapid numerical computation of the AEM response. By compiling numerical‐model results, we constructed an interpretation chart that relates the parameters of the observed AEM response anomaly to the geometric variables of the ice keel. The ...

Geophysical exploration with audiofrequency natural magnetic fields

Experience with the AFMAG method has demonstrated that an electromagnetic exploration system using the Earth’s natural audiofrequency magnetic fields as an energy source is capable of mapping subsurface electrical structure in the upper kilometer of the Earth’s crust. We resolved the limitations of this method by adapting the tensor analysis and remote reference noise bias removal techniques from the geomagnetic induction and magnetotelluric methods to computation of the tippers. After a thorough spectral study of the natural magnetic fields, we designed lightweight magnetic field sensors capable of measuring the magnetic field throughout the year. We also built a digital acquisition and processing system with the ability to provide audiofrequency tipper results in the field. This new instrumentation was used in a study of the Mariposa, California site previously mapped with AFMAG. This study once again demonstrates the usefulness of natural magnetic field data in mapping an electrically conductive body. ...

Audio‐frequency electromagnetic tomography in 2-D

The mathematical formulation of acoustic diffraction tomography is applied to the problem of low frequency, diffusive electromagnetic (EM) fields. EM tomographic inversion, in two-dimensional (2-D) Cartesian geometry, is illustrated for a crosshole source-receiver configuration. The object function of the conductivity distribution is related to the transformed and filtered data by an inverse Fourier transform in the vertical direction and an inverse Laplace transform in the lateral direction. The reconstructed conductivity image is found to be a band-limited version of the actual conductivity distribution. To stabilize the inversion, a regularized least-squares method is used for image reconstruction. As in the seismic case, the inversion quality can be understood by inspecting the wavenumber domain coverage of the object function. Numerical experiments show that the resolution is better in the vertical direction than in the horizontal and it is also a function of source operating frequency. The position and attitude of the target are recovered well.

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.

Airborne electromagnetic bathymetry

Airborne electromagnetic techniques provide an efficient means for determining the bathymetry of shallow seas and coastal regions. Offshore time‐domain data collected with the INPUT® system may be interpreted quickly and inexpensively using the simple table look‐up algorithm described here. This interpretation algorithm is based on two dimensionless parameters which depend on the six channel amplitudes of the recorded INPUT data. These amplitudes are computed and tabulated for a suite of simple two‐layer models in which seawater depth is the only variable quantity. The experimental data can then be interpreted by comparison with the tables of theoretical values to obtain the bathymetry beneath the flight path. The inversion algorithm has proven accurate and stable. In the range of 0–40 m of seawater, the representative difference between the interpreted depths and the charted depths is about 2 m. Bottom gradients lower than 5 percent were resolved accurately beneath 0–25 m of water. Independent interpreta...

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.

UXO detection and identification based on intrinsic target polarizabilities: A case history

Electromagnetic induction data parameterized in time dependent object intrinsic polarizabilities allow discrimination of unexploded ordnance (UXO) from false targets (scrap metal). Data from a cart-mounted system designed for discrimination of UXO with 20 mm to 155 mm diameters are used. Discrimination of UXO from irregular scrap metal is based on the principal dipole polarizabilities of a target. A near-intact UXO displays a single major polarizability coincident with the long axis of the object and two equal smaller transverse polarizabilities, whereas metal scraps have distinct polarizability signatures that rarely mimic those of elongated symmetric bodies. Based on a training data set of known targets, object identification was made by estimating the probability that an object is a single UXO. Our test survey took place on a military base where both 4.2-inch mortar shells and scrap metal were present. The results show that we detected and discriminated correctly all 4.2-inch mortars, and in that process we added 7%, and 17%, respectively, of dry holes (digging scrap) to the total number of excavations in two different survey modes. We also demonstrated a mode of operation that might be more cost effective than the current practice.

Automated interpretation of airborne electromagnetic data

Recent improvements in equipment quality make it possible to increase the usefulness of airborne electromagnetic (EM) systems in areas of moderate electrical conductivity for the purpose of constructing simple electrical property maps which can be related to surficial geology. This application of airborne electromagnetics may be demonstrated and evaluated using Barringer/Questor Mark VI Input® survey results in places where independent verifications of the airborne data interpretation are available. For this purpose we have developed a set of computer algorithms which read digitally recorded Input data and interpret them automatically in terms of a simple electrical section that is defined by a single conductive layer whose thickness, conductivity, and subsurface depth are determined from the data. Because this technique is formally based on a one‐dimensional, three‐layer, three‐parameter, horizontally stratified earth model, it is only applicable in regions where the surficial formations are mildly dippi...

Evaluation of terrain effects in AEM surveys using the boundary element method

In mountainous areas, electromagnetic terrain effects are readily observed in the course of VLF (14-20 kHz) measurements made on the surface and constitute a serious source of geological noise that affects the collected data. One may, therefore, inquire whether similar effects will be observed during the course of conventional helicopter-towed electromagnetic (HEM) surveys as the frequency of the newer systems is increased beyond the lower regions of the audio range. To answer the question, we have evaluated the terrain effects that would be observed with a conventional HEM system in a number of simple cases. The operating frequency chosen for most of the numerical simulations was 8 kHz, while the topographic features investigated were taken to be two-dimensional. The calculations were done using the boundary element method of solving the appropriate integral equations. Accuracy of the numerical solutions was shown to vary from 1 percent for a half space to 10 percent for a shallow valley where the verification was done on a laboratory scale model. For the models investigated, the amplitude of the computed secondary fields shows a distinct correlation with the overflown topography. Surprisingly, however, the phase of the secondary field remains invariant and so may be reliably used to compute the resistivity of the terrain below the aircraft.

Airborne resistivity mapping

A brief overview is presented of the theory and techniques for mapping the electrical resistivity of surficial formations by interpreting airborne electromagnetic induction data. Although the method has not yet reached its full potential, useful information can be obtained on the thickness, the conductivity, and the depth below the surface of any conductive unconsolidated material. Practical applications of this geophysical method include the mapping of aquifers, the delineation of salt-water intrusions, and the estimation of the depth of valley fill material. The instrumentation is described and both time- and frequency-domain systems are discussed. Methods and examples of automated data interpretation are given. On the basis of the results available it is concluded that with the available equipment and using the limited methods of interpretation described, one can map the thickness of the uppermost layer (be it conductive or resistive) to a depth somewhat greater than 50 m with an accuracy of about 10-20 m. >