. Early geophysical papers of the Society of Exploration Geophysicists. Petroleum; Prospecting. ELECTRICAL PROSPECTING 3 marker bed which is below the unconformity and is conformable to the oil bearing horizon. The depth to which it is possible to work with any geo- physical scheme is, therefore, an important consideration. The depth which can be obtained reliably in electrical prospecting is a much debated question. Up to the present time the depth at which structures have been reliably mapped by electrical surveys has not exceeded 1500 feet. Electrical surveying has gone but a little deeper
. Early geophysical papers of the Society of Exploration Geophysicists. Petroleum; Prospecting. ELECTRICAL PROSPECTING 3 marker bed which is below the unconformity and is conformable to the oil bearing horizon. The depth to which it is possible to work with any geo- physical scheme is, therefore, an important consideration. The depth which can be obtained reliably in electrical prospecting is a much debated question. Up to the present time the depth at which structures have been reliably mapped by electrical surveys has not exceeded 1500 feet. Electrical surveying has gone but a little deeper than the core drill. While electrical prospecting is thus a competitor of the core drill, it may be used as an aid in making correlations between core drill holes and also between wells in cases where geological correlation is difficult. This is ac- complished by making resistivity measurements in the holes at a series of depths and correlating on a resistivity basis. Electrical methods of geophysical exploration may be divided into two classes, direct current methods and alternating current methods. The first of these also includes the use of very low frequency alternating currents which are commonly used to eliminate the effects of polarization. The frequency, however, is low enough so that the effect of currents induced by the varying magnetic field may be neglected. Induced currents play the major role in the second, or alternating current method of prospecting. II. Direct Current Methods of Prospecting There are a great many direct current methods available for the determin- ation of the changes in resistivity with depth, but the same fundamental. Fig. 1. Lines of flow in a homogeneous earth. principles apply to all of them. A common practice is to pass the current into the earth by means of two electrodes on the surface, as shown in Fig. 1. If the earth is homogeneous, the lines of flow are arcs of circles connecting the elec- trodes. The potential on the surface of the e
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