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 minimum of data processing. Using a theoretical model which resembles a thick, outcropping vertical dike of infinite vertical extent, the contrast in resistivity across the contact is estimated. A further model, that of an exponential “alpha” center, is also fitted to the data in an effort to pin–point an anomalous region which may have unusually high conductivity.