Total Longitudinal Conductance Research Articles

Electrical resistivity survey of the Kerri-Kerri Formation, Darazo, Nigeria

Results from existing borehole data show the erratic nature of the Paleocene age Kerri-Kerri Formation as an aquifer in the Darazo area. In order to understand the subsurface complexity (variation in lithology and structures) of this formation vis-a-vis aquifer characteristics, a detailed geoelectrical survey was carried out. Results of the Schlumberger soundings revealed a direct correlation between the complexity of the curves with the thickness of the sediments. In addition to delineating the broad lithological units, it is now found that the area is highly faulted. Although the major productive aquifer is found to lie at greater than 100 m in depth, an encouraging relationship exists between the surface measured Dar-Zarrouk parameters (total longitudinal unit conductance ( S) and transverse unit resistance ( T) and the borehole production characteristics). Areas with T values generally greater than 1.0 × 10 6 Ohm-m 2 are found to correspond to zones with the highest transmissibility and storage coefficients. Consequently, high priority zones are delineated for future groundwater development.

Asymptotic formula of the transform function for the layered‐earth potential and its applications to interpretation of resistivity sounding data

With the development of computation techniques based on the digital linear filter method and the wide use of computers, automatic interpretation of vertical electric sounding (VES) data has become practical and common. The digital linear filter method developed in Ghosh (1971a, b) makes solving the inverse problem of VES easier and more practical, because it requires less computer time and storage space, is more accurate than other methods, and does not restrict the number of layers, electrode spacings, and layer depths. However, considerable ambiguity in the interpretation may result when many layers are involved. This means that strongly differing layer distributions may yield apparent resistivity sounding curves that, although not equal, differ so slightly that they cannot be accurately distinguished. In other words, the interpretation provided is only one of many acceptable interpretations, all of which are consistent with the field curves within the accuracy of measurements. Some combination parameters obtained may be useful in this case, and they are needed sometimes. I present a new method to interpret resistivity sounding data, by which some combination parameters such as total longitudinal conductance, total transverse resistance, and the resistivity of the substratum can be obtained.

GEOPHYSICAL INVESTIGATIONS OF GROUNDWATER RESOURCES IN THE KALAHARI BASIN

A regional geoelectrical survey of a semiarid development area of about [Formula: see text], approximately 300 km NNE of Windhoek, has indicated a quadripartite subdivision of the Kalahari sediments of late Cretaceous to Tertiary age, with a variable development due to considerable undulations of the Damara basement. The Karroo formations are apparently less well represented. A two‐fold interpretative procedure, in which an in itial semiqualitative interpretation involving total longitudinal conductance forms the basis for a subsequent quantitative interpretation, has made it possible to map confidently the major lithological units despite the dearth of reliable stratigraphic control. By relating the geoelectrical data to the groundwater head levels, the degree of penetration and production characteristics of boreholes, and the chemical analyses of water samples, the calcareous sands and sandstones of presumed Middle Kalahari age have been seen to constitute the major aquifer in terms of higher sustained yields of good quality water. Encouraging correlations have been noted between these same production characteristics and the geoelectrically derived thickness, resistivity and transverse resistance of this aquifer. It has therefore been possible to delineate the most promising areas for future groundwater development. The Dar Zarrouk parameters form the basis of a suggested procedure for optimizing the initial stages of groundwater exploration in similar arid and semiarid regions where there is a general absence of lithostratigraphic information.

AN EFFECTIVE ARRAY SPACING FOR ELECTRICAL SOUNDING CURVES

The translation of the right asymptotes of electrical sounding curves with respect to the line of the total longitudinal conductance S is considered. The condition is given which the abscissa of sounding curves must satisfy in order to make the asymptote and S line coincide. The coordinate determinated from this condition is an effective array spacing.

Some geoelectrical investigations for potential groundwater zones in part of Azamgarh area of U.P.

The results of some electrical sounding measurements have been used for defining potential groundwater zones in part of Azamgarh district of Uttar Pradesh, India. The area covered is a part of gangetic alluvial tract. Aquifers consist of lenticular sand masses distributed within the alluvium. The ideal geoelectrical conditions, generally desired for the interpretation of resistivity curves, lack in the area. Among the number of layers encountered in lithologs, only a few are delineated by quantitative interpretation of the sounding data. A study of the total transverse resistance and longitudinal conductance for a particular depth of the alluvial column has helped in defining potential groundwater zones.

A RAPID GRAPHICAL METHOD FOR THE INTERPRETATION OF A‐ AND H‐TYPE ELECTRICAL SOUNDINGS

Three‐layer vertical electrical sounding (VES) curves that register the effect of a highly resistive electric basement are of either the minimum (H) or the double‐ascending (A) type. The correct interpretation of these two important types of VES curves depends largely on the successful determination of the resistivity of the second layer. This resistivity can sometimes be evaluated from electric logs, from a comparative study of field VES curves, from electrical measurements on outcrops, or from parametric VES curves. A new single auxiliary curve has been devised to help interpret both these types of VES curves, provided that the resistivity of the second layer is known. The thickness of the middle layer is determined accurately and rapidly by the value of the abscissa at the intersection of the total longitudinal conductance line S with the new auxiliary curve. The auxiliary curve is used to interpret an H‐type curve, an A‐type curve, and a five‐layer (KQH‐type) parametric VES curve. Analysis of the KQH parametric curve shows that the resistivity of the third layer may vary by more than a thousandfold without affecting the shape of the curve, whereas the resistivities of the second and fourth layers may correspondingly vary by less than one order of magnitude.

CALCULATING THE PARAMETERS USED IN TELLURIC CURRENT PROSPECTING

In telluric current prospecting, the amplitudes of variations of the telluric‐electric field are measured simultaneously at a fixed base station and a roving field station. In sedimentary basins, variations in electrical conductance through the sedimentary column may be studied by comparing the amplitude of electric field variations at the roving station with the amplitude of the same electric field variations at the base station. The parameter ordinarily used in interpreting telluric current data is μ, the ratio of electric field intensity at a roving station location to the electric field intensity at the base station. Under favorable conditions, the ratio μ is directly proportional to the total longitudinal conductance of the rock layers above insulating basement rock.