CSORGEI, J., L. NEMESI, A. PAPA, and G. VARGA, Eotvos Lorand Geophysical Institute of Hungary, Budapest, Hungary
One of the basic tasks of hydrocarbon prospecting is to investigate the morphology and internal structure of the basement underlying the sediments. Gravity is applied world wide to solve this task; the depth to the basement, however, cannot always be determined. With increasing depth in basins of 3-8 km depth, basement structures are poorly reflected; anticorrelation was found in some cases.
Telluric and magnetotelluric apply a natural electromagnetic field around the Earth. This field shows variations in a very wide frequency range. Electromagnetic waves of different frequencies can penetrate into different depths, providing information on the internal structure of the Earth.
A survey begins with a limited number of magnetotelluric measurements to learn the basic geologic model and to tune up the frequency range, which provides data about the depth interval we are interested in.
In the second stage, telluric measurements are done in a dense grid (1-2 km station spacing), using the frequencies where penetration depth exceeds the depth to the basement.
In the third stage, magnetotelluric soundings are performed on the telluric anomalies. When interpreting these soundings, we can decide whether the anomalies are caused by changes in the depth to the basement or by changes in the resistivity of the overlying sedimentary sequence. As a result, resistivity maps of the sediments and basement depth maps can be constructed. Both maps are useful in finding potential hydrocarbon traps and can be used to design a cost-effective detailed seismic survey.
The case histories presented in this paper are from the Carpathian basin in Hungary.
AAPG Search and
Discovery Article #90990©1993 AAPG International Conference and Exhibition, The
Hague, Netherlands, October 17-20, 1993.