H. Frank Morrison

Electromagnetic precursors to earthquakes in the Ulf band: A review of observations and mechanisms

Despite over 2 decades of international and national monitoring of electrical signals with the hope of detecting precursors to earthquakes, the scientific community is no closer to understanding why precursors are observed only in some cases. Laboratory measurements have demonstrated conclusively that self potentials develop owing to fluid flow and that both resistivity and magnetization change when rocks are stressed. However, field experiments have had much less success. Many purported observations of low-frequency electrical precursors are much larger than expectations based on laboratory results. In some cases, no precursors occurred prior to earthquakes, or precursory signals were reported with no corresponding coseismic signals. Nonetheless, the field experiments are in approximate agreement with laboratory measurements. Maximum resistivity changes of a few percent have been observed prior to some earthquakes in China, but the mechanism causing those changes is still unknown. Anomalous electric and magnetic fields associated with fluid flow prior to earthquakes may have been observed. Finally, piezomagnetic signals associated with stress release in earthquakes have been documented in measurements of magnetic fields.

Marine magnetotellurics for petroleum exploration. Part I: A sea-floor equipment system

Induction in electrically conductive seawater attenuates the magnetotelluric (MT) fields and, coupled with a minimum around 1 Hz in the natural magnetic field spectrum, leads to a dramatic loss of electric and magnetic field power on the sea floor at periods shorter than 1000 s. For this reason the marine MT method traditionally has been used only at periods of 103 to 105 s to probe deep mantle structure; rarely does a sea‐floor MT response extend to a 100-s period. To be useful for mapping continental shelf structure at depths relevant to petroleum exploration, however, MT measurements need to be made at periods between 1 and 1000 s. This can be accomplished using ac-coupled sensors, induction coils for the magnetic field, and an electric field amplifier developed for marine controlled‐source applications. The electrically quiet sea floor allows the attenuated electric field to be amplified greatly before recording; in deep (1-km) water, motional noise in magnetic field sensors appears not to be a proble...

ELECTROMAGNETIC FIELDS ABOUT A LOOP SOURCE OF CURRENT

Integral expressions for the electromagnetic field components produced by a horizontal loop, carrying a current Ieiωt and placed on or above the surface of an n‐layered half‐space, are deduced in a form such that numerical integration can be performed easily. The expressions are free of approximations and completely general for all frequencies. They are constrained only to the uniformity of current around the transmitting loop. The resulting computed electromagnetic fields are valid for arbitrary values of the electrical parameters σ, μ, and e. The quasi‐static approximation for the region above the half‐space, wherein the wave equation is replaced by the Laplace equation, can be avoided. Measurements outside the loop constitute induction depth sounding. Induction depth sounding curves of field components and magnetic polarization parameters show good resolution of subsurface layering. In particular, it is suggested that the measurements of tilt angle and/or ellipticity of the magnetic polarization ellips...

Marine magnetotellurics for base-of-salt mapping: Gulf of Mexico field test at the Gemini structure

A sea‐floor magnetotelluric (MT) survey was conducted over the Gemini subsalt petroleum prospect in the Gulf of Mexico (GOM) to demonstrate that the base of salt can be mapped using marine magnetotelluric (MMT) methods. The high contrast in electrical resistivity between the salt and the surrounding sediments provides an excellent target for MMT. The Gemini salt body, located at 28° 46′ N 88° 36′ W, is a relatively complex shape buried 2–5 km below the sea floor in 1-km-deep water. Its geometry has been previously determined using 3-D seismic prestack depth migration with well log control. In order not to confuse limitations in interpretation technique with limitations in data acquisition, numerical forward and inverse modeling guided the survey design to locate a profile that would be amenable to 2-D inversion, even though the body was clearly 3-D. The seismic imaging of the base of salt along the chosen profile is considered good, thus providing a good control for testing the MT method. In many other ar...

ELECTROMAGNETIC COUPLING IN FREQUENCY AND TIME-DOMAIN INDUCED-POLARIZATION SURVEYS OVER A MULTILAYERED EARTH

Electromagnetic coupling responses in frequency and time‐domain induced‐polarization measurements over a multilayered earth are evaluated. For collinear dipole‐dipole and pole‐dipole configurations over a dissipative layered subsurface, the percent frequency effects of electromagnetic coupling are seen to be as high as 60 percent for large L2(σ1f) values, where L is the length of the receiving dipole, σ1 is the conductivity of the top layer of the half‐space, and f is the higher frequency of excitation used. In both frequency and time‐domain analyses, the distinctive effects of layering compared to that of a homogeneous half‐space response are shown for different electrode configurations, layer geometry, and electrical parameters of the subsurface. The pole‐dipole configuration of electrodes, in general, exhibits higher coupling compared to the dipole‐dipole configuration. In time‐domain measurements, the late off‐time transient decays reflect almost entirely the normal polarizability of the layered subsu...

Marine magnetotellurics for petroleum exploration, Part II: Numerical analysis of subsalt resolution

In areas where seismic imaging of the base of salt structures is difficult, seaborne electromagnetic techniques offer complementary as well as independent structural information. Numerical models of 2-D and 3-D salt structures demonstrate the capability of the marine magnetotelluric (MT) technique to map the base of the salt structures with an average depth accuracy of better than 10%. The mapping of the base of the salt with marine MT is virtually unaffected by internal variation within the salt. Three-dimensional anticlinal structures with a horizontal aspect ratio greater than two can be interpreted adequately via two-dimensional inversions. Marine MT can distinguish between salt structures which possess deep vertical roots and those which do not. One measure of the relative accuracy of MT and seismic methods can be made by considering the vertical and lateral position errors in the locations of interfaces caused by neglecting velocity anisotropy in migration. For the shallow part of the section where two-way travel times are on the order of 1 s, the vertical and lateral position errors in the locations of salt-sediment interfaces from 2-D MT inversion is more than twice the expected migration error in reflectors in transversely isotropic sediments, such as those in the Gulf of Mexico. Deeper in the section where two-way times are on the order of 4 s, lateral position errors in migration become comparable to those of the MT inverse, whereas seismic vertical position errors remain more than a factor of two smaller than MT errors. This analysis shows that structural mapping accuracy would be improved using MT and seismic together.

Theoretical and practical considerations for crosswell electromagnetic tomography assuming a cylindrical geometry

An iterative Born imaging scheme is employed to analyze the resolution properties of crosswell electromagnetic tomography. The imaging scheme assumes a cylindrical symmetry about a vertical magnetic dipole source and employs approximate forward modeling at each iteration to update the internal electric fields. Estimation of the anomalous conductivity is accomplished through least-squares inversion. Much of the mathematical formulation of this diffusion process appears similar to the analysis of wavefield solutions, but the attenuation implicit in the complex propagation constant invalidates many of the accepted wavefield criteria for resolution.Images of illustrative models show that vertical resolution improves with increasing frequency and with increased spatial sampling density. In addition, greater conductivity contrasts between the target and the background can result in better resolution. The horizontal resolution depends on the maximum aperture that is employed and with increasing frequency, larger apertures are needed to obtain optimal results. However, the maximum aperture that can be employed, and thus the horizontal resolution, is limited by the rate of attenuation and the noise present in the measurements. Weighting the long-offset data equally with the zero-offset data can improve the resolution if the noise is not a function of the dynamic range of the measurement system. At lower frequencies, the resolution can be improved by measuring both the horizontal and vertical components of the magnetic fields. In addition, multiple frequencies can be employed to improve the resolution for limited aperture measurements.The general applicability of the cylindrically symmetric geometry is examined by comparing the 2-D sensitivity functions to those produced by a 2.5-D model, and by imaging a 3-D body with the 2-D iterative Born scheme. For borehole separations greater than five skin depths it is demonstrated that the measurements, and thus the images, are not affected by the geometry of the conductive zone outside of the interwell plane. Thus the 2-D imaging scheme can be employed in these situations. For borehole separations less than five skin depths, artifacts are produced in the images which will lead to faulty interpretations.

VECTOR MAGNETIC ANOMALIES DERIVED FROM MEASUREMENTS OF A SINGLE COMPONENT OF THE FIELD

Three‐component magnetic data are derivable from measurements of one single component of the magnetic field over a plane. The technique involves computation of the double‐Fourier‐series coefficients of the measured magnetic anomaly, multiplication of the coefficients by a filter operator, and, finally, evaluation of the magnetic components by taking the inverse Fourier transform. The desired filter operator is obtained from a simple relationship between the components of a potential field. The scheme has been tested with excellent results on the fields of a vertical prismatic model.

Mapping and monitoring electrical resistivity with surface and subsurface electrode arrays

Electrical resistivity measurements using combinations of subsurface and surface electrodes are more sensitive to subsurface inhomogeneities than arrays confined to the surface. A further advantage of the subsurface configuration is that the strong influence of near‐surface inhomogeneities can be reduced by differencing the measured apparent resistivities with a reference set of values obtained with the subsurface electrode(s) at a particular depth. This process accentuates the response of features near the downhole electrode while canceling the response of the near‐surface features. An idealized two‐dimensional model of a nuclear waste repository has been used to demonstrate the effectiveness of this differencing scheme. It is shown that resistivity measurements using borehole electrodes well away from the repository and on the surface are sensitive to changes in the repository that could not be practically observed from surface measurements. This sensitivity is preserved in the presence of a conducting ...

Electrical resistivity measurement through metal casing

Methods using dc electrical arrays to measure formation resistivity through casing have relied on approximate forms for the current and potential distributions to derive a simple relationship between the formation resistivity and the transverse resistance calculated from measurements of the potential and its second derivative inside the casing. We have derived a numerical solution for the potentials and their derivatives to examine the accuracy of the approximate forms for casing of finite-length, annular zones of varying radius, and for vertical discontinuities such as layers or abrupt changes in annular zone radius. For typical conductivity contrasts between the casing and formation, the approximate relationships may be off by as much as 60 percent for long casing and may show variations of 20 to 30 percent as the electrode array moves along the casing. In principle an iterative scheme could be devised to correct the readings if high accuracy was required. The numerical results show that to first order the current flow from the casing is radial, and that all the analytic expressions based on this assumption for evaluating layer resolution and the effects of annular layers are valid. An interesting byproduct of this study has been the discovery that the distortion of the potentials in a nearby well by an annular disk (e.g., an injected steam zone) surrounding the current injection well is greater if the injection well is cased. Crosswell resistivity surveys appear feasible if one of the wells is cased.