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  Resistivity Profiling, Physics tutorial

Introduction:

Resistivity profiling is one of the essential geoelectrical methods. It has a number of variations resulting from possibility of using different configurations of current and estimating electrodes, appropriate from viewpoint of problem being investigated, on the one hand, and from point of view of field operation methods, on the other. Of large number of resistivity configurations deal only with those encountered most often in practice. By electrode array (configuration) we understand precise distribution of electrodes that are used to produce and measure electric field. Electrodes that serve to earth poles of sources are known as current electrodes and are denoted by letters A, B, C, etc. Potential, voltage or other parameters, of electric field are estimated with the help of measuring electrodes, denoted by letters M, N, P, etc. Set of distance; between electrodes determines dimensions of array. As characteristic of depth range we choose one dimension, which is then referred to as length of array L.

Targets:

Ideal traverse target is steeply dipping contact between two rock kinds of very different resistivity, covered under thin and relatively uniform overburden. These targets do exist, particularly in man-modified environments, but changes in evident resistivity because of geological changes of interest are frequently small and should be differentiated from background because of other geological sources. Gravel lenses in clays, ice lenses in Arctic tundra and caves in limestone are all much more resistive than their surroundings but tend to be small and rather difficult to detect. Small bodies which are very good conductors, like (at rather different scales) oil drums and sulphide ore bodies, are generally more easily detected by using electromagnetic methods.

Choice of array:

Preferred arrays for resistivity traversing are those which can be most easily moved. Gradient array, that has only two mobile electrodes separated by the small distance and linked by only moving cable, has much to suggest it. Though, area which can be covered with array is small unless current is supplied by heavy motor generators. Two-electrode array has thus now become array of choice in archaeological work, where target depths are usually small. Care should be taking in handling long cables to electrodes at infinity, but large numbers of readings can be made very rapidly using the rigid frame on which two electrodes, and frequently also instrument and a data logger, are mounted. Several of these frames now include multiple electrodes and give results for number of different electrode combinations. With Wenner array, all four electrodes are moved but as all inter electrode distances are same, mistakes are improbable.

The dipole-dipole array is mostly utilized in IP work where induction effects should be avoided at all costs. Four electrodes have to be moved and observed voltages are generally very small.

Traverse field-notes:

Array parameters remain same along traverse, and array type, spacing and orientation, and very frequently current settings and voltage ranges can be noted on page headers. Technically, only station numbers, remarks and V/I readings are recorded at individual stations, but any changes in current and voltage settings must also be noted as they affect reading reliability. Comments must be made on changes in soil type, vegetation or topography and on cultivated or populated areas where non-geological effects may be faced. These notes will generally be responsibility of instrument operator who will generally be in position to personally inspect every electrode location in course of traverse. As any note about individual field point will tend to explain it in relation to general environment, general description and sketch map must be included. When using frame-mounted electrodes to get rapid, closely spaced readings, results are generally recorded directly in data logger and description and sketch become all-important.

Displaying traverse data:

Results of resistivity traversing are most efficiently shown as profiles that preserve all characteristics of original data. Profiles of resistivity and topography can be offered together, along with abbreviated versions of field notes. Data collected on number of traverses can be illustrated by plotting stacked profiles on base map, but there will generally not then be much room for annotation. Strike directions of resistive or conductive features are more clearly shown by contours than by stacked profiles. Traverse lines and data-point locations must always be display on contour maps. Maps of same area generated using arrays aligned in different directions can be very different.

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