Electromagnetic Imaging in Basalt and Carbonate Covered Basins

George V. Keller, John M. (Jack) Jordan, and M. Tahsin Tasci

Electromagnetic sounding in the time domain, using a long grounded-wire transmitter and an induction loop receiver, was pioneered at the Colorado School of Mines to obtain structural information in areas where near-surface, high-velocity layers such as basalt flows, carbonates, and granite overthrusts or acoustically absorbing layers result in poor seismic data quality. This approach provides better vertical and lateral resolution than any other electrical method. A time-varying magnetic field associated with reversing the current in the transmitting dipole causes induction in any conductor. The amount of induction that takes place depends on the electrical properties of the earth; therefore, measurement of the effects of induction serves as a means for measuring rock con uctivity. The transient received in the vertical-axis receiver loop is due to eddy currents induced in the earth below the loop. These currents diffuse downward and outward with increasing time, eventually decaying into the earth's crust. Once the transmitter is emplaced data can be acquired over tens of square miles even in rugged terrain using a portable receiver. This allows for inexpensive and rapid data acquisition. The soundings are converted to apparent resistivity curves and mathematically inverted to obtain a multi-layered section showing thicknesses and resistivities of geoelectric units. Sounding locations can be arranged either along profiles parallel to the transmitter to make structural cross-sections or in a loose grid to create structural maps on selected horizons.

Field studies from Oregon, Turkey and Northern Ireland will be presented. Implications for marine surveys will be discussed.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California