A vector EM system and its field applications

Abstract

At Kennecott Exploration Services we built and applied widely a unique vector electromagnetic (VEM) system that measures magnetic field amplitude and phase at four frequencies: 26, 77, 232, and 695 Hz. Stable crystal oscillators enable us to measure phase without a wire link for any transmitter‐receiver configuration. A square‐wave is transmitted into a loop, and the signal is read at two frequencies: the fundamental and the third harmonic. The amplitude readout is logarithmic with switched 10-dB gain increments. Amplitude resolution is 0.1 dB and phase resolution is 0.5 degree. The VEM system has been applied successfully in a wide variety of exploration terranes and in several field geometries. For reconnaissance work, we utilize a large rectangular source loop similar to the Turam geometry, but with only one receiver coil. Amplitude and phase data can be reduced to field strength ratio and phase difference as in Turam or to in‐phase and quadrature components. In‐phase and quadrature are better for deep targets, because the standard Turam reduction discriminates against deep conductors. For investigating narrow zones or for defining a conductor, we use a fixed vertical loop source; in that case, the receiver measures the amplitude and phase of the vertical field, rather than the conventional tilt angle. Borehole EM surveys (surface transmitter‐downhole receiver and downhole transmitter‐surface receiver) are useful both for determining whether an anomaly has been tested by drilling, and for mapping conductors.