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Electrical methods can be used directly as a resistivity tool which not only defines structure but also provides information, such as rock type, fluid content, and porosity, necessary to resolve lithologic and stratigraphic problems.
To help meet the challenge of today's petroleum exploration problems, a multi-methodology electrical resistivity system has been developed. This system uses various source-receiver arrays and multiple source and receiver types.
Seismic roll-along cables and an eight-channel digital recording system are used to achieve rapid field coverage. Measurements are made in profile every 220 ft (67 m).
Five decades of frequency can be covered to produce soundings from the surface to a maximum of 30,000 ft (9,100 m) in depth. A powerful transmitter is used to obtain near-zone (late time) and far-zone (early time) electromagnetic soundings as well as DC soundings. Magnetotelluric measurements are used to reach depths below 20,000 ft (6,100 m) if necessary.
The data are displayed as profiles of closely spaced pseudo E-logs and induction logs that are correlatable to existing well logs.
Two case histories of exploration for subtle sand traps include one located in the D J basin of Colorado. The other describes the detection of a 30 ft (9 m) sand sequence at a depth of 2,500 ft (760 m) on the eastern shelf of the Permian basin in west Texas. The approaches used in these case histories have direct application to many problems of the central valleys of California.
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