R. N. Edwards

First results of the MOSES experiment: Sea sediment conductivity and thickness determination, Bute Inlet, British Columbia, by magnetometric offshore electrical sounding

A static electrical method has been developed to determine the electrical resistivity of crustal rock beneath the sea. The transmitter is a vertical, long‐wire bipole, extending from the sea surface to the sea floor. A commutated current, generated on the ship, is fed to two large electrodes: one near the sea surface, the other at the end of a long insulated wire. The return path for the current is through the sea and the subjacent crust. The receiver is a self‐contained, remote, microprocessor‐controlled magnetometer which is deployed from the ship to the sea floor and subsequently recovered. The data are measurements of the azimuthal component of the magnetic field as a function of transmitter‐receiver horizontal separation. The acronym MOSES has been coined for the method. The choice of the name MOSES is appropriate because the system geometry is carefully arranged to remove many of the adverse effects of the relatively conductive sea water. In particular, accurate estimates of sea floor resistivity ar...

A transient electric dipole-dipole method for mapping the conductivity of the sea floor

The electrical conductivity of the sea floor is usually much less than that of sea water, and not all electrical measurements made on the sea floor are particularly sensitive to the electrical conductivity value. The analytic impulse and step‐on transient responses of two conductive, adjoining half‐spaces (with a large conductivity contrast) to an in‐line electric dipole‐dipole electromagnetic system located on the interface are derived. The shape of the transient at relatively early time is seen to be independent of the conductivity of the more conductive half‐space and is indicative of the conductivity of the less conductive haft‐space. Based on this observation, a simple, practical system can be designed to measure sea floor conductivity.

On the theory of magnetometric resistivity (MMR) methods

The Magnetometric Resistivity (MMR) method is based on the measurement of the low-level, low-frequency magnetic fields associated with noninductive current flow in the ground. A component of the magnetic field is measured in the vicinity of one or more grounded electrodes. Recently, the method was tested successfully in the field.The present paper presents the theoretical basis of the method in a unified format. Part of the material is derived from valuable published papers which are difficult to obtain. The remainder of the paper contains original unpublished theoretical results.It is shown that a horizontally layered earth yields no MMR anomaly. The characteristic anomalies for an anisotropic earth, vertical and dipping contacts, thin and thick dikes, and semicylindrical and hemispherical depressions, as well as alpha media are presented in detail.There are two factors which influence the MMR anomaly; geometry and conductivity contrast. For many models, it is possible to separate these two effects. Type curves are presented for very large conductivity contrasts to illustrate the effect of geometry only. Ancillary curves enable finite conductivity contrasts to be deduced from field data.

Offshore electrical exploration of sedimentary basins: The effects of anisotropy in horizontally isotropic, layered media

Our electrical method for exploring beneath the sea consists of a long, vertical, bipolar ac source, extending downward from the sea surface to the bottom of the sea and a remote, encapsulated, microprocessor‐controlled magnetometer located on the sea floor. The amplitude and phase of the magnetic field are measured over a range of suitable frequencies and transmitter‐magnetometer separations. At the low‐frequency static limit, apparent resistivity curves, similar to standard Schlumberger resistivity sounding curves, are constructed as an aid in the direct interpretation of isotropic crustal resistivity. An intermediate relatively resistive or relatively conductive zone is detectable when the transmitter‐receiver separation exceeds the order of twice the depth to the zone. The physical property resolved by the method in an anisotropic crust, which has different horizontal and vertical resistivities, is the geometric mean of the two independent resistivities. The thickness of a layer is indeterminate. A la...

Electromagnetic soundings in the sedimentary basin of southern Ontario—A case history

The results of electrical and electromagnetic (EM) soundings conducted in the sedimentary basin of southern Ontario are presented. The sounding sites are located strategically to take advantage of the dipping nature of the sediments, a progressive study being undertaken from shallow to deep sections. The vertical magnetic field transfer function, estimated with a pseudonoise source EM system, and conventional Schlumberger apparent resistivity are jointly inverted. For each site, the layered earth model containing the minimum number of layers is fitted to the data. The joint inversion enables up to eight distinct layers to be identified at some sites. The nonuniquenesses or ambiguities in each model, for example S (conductivity-thickness product) equivalence of a deep thin conductive layer, are revealed through an eigensolution analysis. The intrinsic ambiguities of the models of the deeper sections are resolved by a systematic, progressive site-to-site correlation of electrical units across the basin. For example, the conductivity of a conductive unit is measurable when it is at a shallow depth. In a deeper section, if the unit appears as a thin layer, it is assigned this value of conductivity removing the S-equivalence ambiguity. The individual inversions combined with the site-to-site correlation produce an overall electrical model of the basin consistent with the known geologic section.

An automatic technique for presentation of coincident-loop, impulse-response, transient, electromagnetic data

Coincident‐loop TEM sounding data are often presented by plotting the half‐space apparent conductivity as a function of delay time. A new algorithm generates an improved presentation that plots the apparent conductivity as a function of depth. The resulting data may be further processed to sharpen or “spike” the smoothly varying apparent‐conductivity/depth curves in an attempt to better represent the rapid changes in conductivity that often exist in the earth. The algorithm described involves an approximation, but is simple, easy to use, and computationally efficient. A layered conductivity structure is assumed, so the algorithm is best for areas where the geology is approximately horizontal. However, the algorithm can also be used to identify anomalous features that are not infinite horizontal layers. The spiked conductivity models derived from synthetic data are consistent with the original layered‐earth models and show a greater resolution than the apparent‐conductivity/depth curves, and sometimes ampl...

A simple parametric model for the electromagnetic response of an anomalous body in a host medium

A simple approximate representation of the spectral response of an arbitrary kind of electromagnetic (EM) prospecting system to a small conductive target in a conductive environment has been derived. The representation contains the direct response from the layered host medium and the first‐order effects of eddy current induction, current channeling, magnetic induction, and the coupling between eddy current and magnetic inductions in the anomalous body, as modified by the host medium. The only significant computational task in the representation is evaluation of a few Green’s functions for the host medium. As a guide to establishing proper approximations, a fundamental study of integral equations is presented. Very simple solutions for the secondary or scattering sources which represent the EM effect of the body are obtained for a few basic cases. Equations for more general cases are complicated by additional terms in the Green’s functions which represent ac interaction between scattering sources and the h...

A FIELD TEST OF THE MAGNETOMETRIC RESISTIVITY (MMR) METHOD

The electrical prospecting method, known as the Magnetometric Resistivity (MMR) method, is based on the measurement of the low level (about 100 milligamma), low‐frequency (1–5 Hz) magnetic fields associated with noninductive current flow in the ground. The horizontal component of the magnetic field is measured along profiles which are at right angles to a baseline joining two widely separated current electrodes.The field test was conducted on a plateau in the western cordillera, where the topography is characterized by steep hills, bold ridges, gullies and narrow canyons. A steep faulted contact between basement rocks of differing resistivity is exposed on one flank of the plateau, beneath over 500 m of Tertiary volcanics and sediments.The object of the test was to determine if the basement contact could be mapped by the MMR method, working entirely on top of the plateau. The plan position of the contact could be inferred approximately from measurements at the outcrop.The object was achieved with a minimu...

Polymetallic sulfide exploration on the deep sea floor: The feasibility of the MINI-MOSES experiment

The recent discoveries of polymetallic sulfide deposits on the deep sea floor have created an interest in geophysical techniques for mapping them. A magnetometric resistivity method (MINI‐MOSES) has been developed for detecting the distribution of sub‐sea‐floor electrical resistivity. A vertical, long‐wire bipole feeds electric current into the ocean. Some of the current enters the sea floor, and its magnetic field is measured at various distances by a low‐frequency induction coil magnetometer. The method is sensitive to conductive and resistive zones at depth, and information about three‐dimensional resistivity structures can be obtained. The small load resistance of an electric transmitter grounded in the ocean allows high current to be driven by a battery‐powered instrument. The induction coil receiver is more sensitive and less noisy than conventional flux‐gate magnetometers over the selected frequency band. This receiver is therefore well‐adapted to the almost noise‐free deep sea‐floor environment. B...

Magnetometric resistivity (MMR) anomalies of two‐dimensional structures

An inexpensive, rapid method has been developed for computing all three components of the magnetic field due to galvanic current flow from a point electrode in the vicinity of a conductive subsurface structure of infinite strike‐length and arbitrary cross‐section. For any three‐dimensional (3-D) structure, the magnetic field may be written as a sum of surface integrals over boundaries defining changes in conductivity by a direct modification of the Biot‐Savart law. The integrand of each surface integral includes the components of the electric field tangential to the boundary, which may be evaluated on the boundary using a standard integral equation technique. In the case of a two‐dimensional (2-D) structure, a reformulation of the theory by taking a one‐dimensional Fourier transform along the strike results in the reduction of both the surface integrals necessary to solve the integral equation for the electric field, and the integrals used in computing the magnetic field, to line integrals in wavenumber d...