The engineering geophysical sounding method has been applied for in-situ investigation of unconsolidated near-surface formations since the 1990's. In this study, we offer an alternative to geophysical inversion methods for data processing. Factor analysis is applied to engineering geophysical sounding and cone penetration test data to extract information on water saturation of the shallow region. A linear correlation is indicated between water saturation and one of the new variables derived by factor analysis. We suggest a general formula for estimating water saturation and apply it on data measured in twelve shallow boreholes situated over a nuclear waste disposal site. We extend the statistical procedure to estimate the spatial distribution of water (air) saturation between the boreholes. We compare the statistical results to inversion estimations. The study shows that factor analysis gives a quick and reliable solution for the assessment of hydrogeological conditions in shallow prospecting sites.
The ideal rock model in electrical well logging for prospecting hydrocarbon consists of three cylindrical layers characterized by homogeneous resistivities. The second layer of the model respresents the zone of invasion, where under real circumstances the resistivity is not constant but changes with the distance from the borehole. This condition could be taken into consideration, but the solution of the electrical direct problem for such case is very complicated. Any kind of invasion resistivity profile can be approximated by many cylindrical layers of homogeneous resistivities. A recursive formula is derived by which the many‐layer problem can be solved simply. Numerical calculations were made to study the effect of the inhomogeneity of the invaded zone. Apparent resistivities of different Laterolog and normal arrangements were calculated for several models having linearly increasing resistivity profile in the invaded zone. These apparent resistivity values were evaluated by least‐squares fitting to determine the equivalent electrical parameters of the usual model of three homogeneous layers. The results show that there is practically no error in determination of the true resistivity, but the depth of invasion may be significantly smaller than that of the linear resistivity profile.
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