Electricity: measuring and testing – Of geophysical surface or subsurface in situ – Using electrode arrays – circuits – structure – or supports
Patent
1993-10-27
1996-12-24
O'Shea, Sandra L.
Electricity: measuring and testing
Of geophysical surface or subsurface in situ
Using electrode arrays, circuits, structure, or supports
324360, 324347, G01V 306, G01V 3165, G01V 315
Patent
active
055876595
DESCRIPTION:
BRIEF SUMMARY
FIELD OF INVENTION
The invention concerns measuring equipment for mapping the subsurface geology in a terrain by means of electrical profiling, said equipment comprising at least one current circuit for passing a current via current electrodes from a current source through the surface layer to be examined, and at least one voltage circuit with a measuring instrument for measuring, via voltage electrodes, the voltage drop of the current across these.
BACKGROUND OF INVENTION
Electrical profiling of this type has been known for a long time and has been extensively used particularly in recent years as a tool for detailed description of the subsurface geology in connection with e.g. pollution. For this purpose there is usually used an electrode array in a so-called Wenner configuration with four electrodes in the form of earth rods, which are driven down into the earth in a row, with the same mutual spacing and the measuring point defined as the center point of the array. Then a constant electrical current is passed through the earth between the two outermost earth rods, the current electrodes, by means of a current generator, following which the voltage drop of the current across the two central earth rods, the voltage electrodes, is measured. Prior to taking this measurement, the electrochemical surface potentials must have stabilized to a constant value, and this typically takes 20-30 seconds. To balance this residual error contribution two or more measurements are taken in the same electrode array, the current direction being reversed each time, so that the error contribution is substantially balanced owing to the subtraction of the individual measurement results involved by this method. This technique is known e.g. from the patent specifications U.S. Pat. No. 2,640,097 and GB 2 106 653. On the basis of the given current strength, the measured voltage drop across the voltage electrodes and a factor depending solely upon the geometry of the array, the electrical resistance of the earth formation corresponding to a specific focusing depth is finally calculated.
A so-called line profiling is typically used in the taking of the measurements, the measuring configuration being successively moved along a line with the same spacing between the measuring points, and since the electrical resistance is frequently to be registered at several different focusing depths at the same measuring point, the rods are moved symmetrically around this measuring point, and then the measurements are repeated at various rod distances corresponding to the focusing depths to be measured.
When a large number of these line profilings is performed successively, a sum of measurement results can be provided which, in combination, provide a reliable mapping of the electrical resistance of the strata, and thereby give, when interpreted, a satisfactory picture of the geological structure of the area. As will appear, the method is extremely slow and cumbersome, and this applies especially when, as is often the case, it is necessary to examine the formation at several different focusing depths for each individual measuring point, and these are positioned relatively closely to each other for the area to be mapped with a sufficiently high level of detail.
With a view to facilitating this slow and laborious field procedure it has been proposed in U.S. Pat. No. 2,105,247 to use mobile electrodes in the form of e.g. endless tracks on a tractor travelling across the surface of the earth while measurements are continuously taken. In this method it was found necessary to use a relatively great current strength of between 5 and 20 amperes to obtain a voltage signal which was so powerful that it could be distinguished with reasonable certainty from the electrochemical surface voltages which occurred as a consequence of the passage of the voltage electrodes across the varying chemical environment of the surface of the earth. For such a great current strength to be transmitted, the electrode pressure against the earth had to be relatively great to form
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O'Shea Sandra L.
Phillips Roger C.
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