The Magnetometric Resistivity Method and its Application to the Mapping of a Fault

Abstract

An electrical prospecting method, known as the Magnetometric Resistivity (MMR) method, is described both in theory and in practice. It is based on the measurement of the low level (<5 gamma), low-frequency (1–5 Hz) magnetic fields associated with noninductive current flow in the earth. The field techniques are similar to those employed in the gradient array resistivity method, except the potential electrodes are replaced by a highly sensitive component magnetometer. The horizontal component of the magnetic field, which is at right angles to the line joining the current electrodes, is usually measured. The MMR anomaly is the difference between the measured values and the 'normal' values, which are the values of the component expected at the surface of a uniform earth.It is shown that an earth composed of uniform, horizontal layers gives no MMR anomaly. But the MMR anomalies due to a current dipole, a sphere and a fault are calculated. The anomaly due to a sphere is compared with anomalies for the sphere obtained by other geophysical methods. The principal profile of the anomaly is similar to that of the vertical field anomaly of a vertically magnetised sphere.It is argued that the MMR method may be superior to the gradient-array resistivity method in mapping inhomogeneities that are embedded at depth in a conductive host or are located beneath conductive overburden. The latter is demonstrated using an analogue model.The field apparatus and the field techniques of the method are described. The data taken during a field test of the method over the Gloucester Fault, at Leitrim, Ontario, are presented and are interpreted in terms of a conductivity contrast across the fault. The contrast determined agrees quantitatively with estimates obtained using other electrical methods.