A Wenner array routine for optimum electrode spacing selection for resistivity measurements over thin subsurface media

This study was carried out with a view to establishing a routine for the selection of the optimum electrode spacing for the accurate measurement of resistivity within unusually thin (5-100 cm) subsurface layer in agro geophysics. The Hummel's image theory for 2-Layer Wenner Vertical Electrical Sounding (VES) formed the theoretical basis for the study. For the purpose of simulation, a computer program was developed using the theory. The simulation process involved varied upper and lower layer resistivities in conjunction with a spectrum of electrode spacings (a) and varied upper layer thicknesses (h). The relationship between electrode spacing and upper layer thickness was determined using the asymptotic resistivity characteristics of model curves and layer sensitivity models obtained from simulation. The deduced relationships were validated using the same approach on field data. It was observed from the model curves that with respect to electrode spacing and thickness, asymptotic limits remained the same irrespective of the nature of the upper layer resistivity (resistive or conductive). On the other hand, the nature of the resistivity of the lower layer was a factor in the attainment of its asymptotic resistivity. The observations from field data were congruous with those from simulation. Very strong (R = 0.99) direct linear relationships existed between electrode spacing and upper layer thickness with respect to the nature of upper layer resistivity. The study identified that the electrode spacing that will keep resistivity measurements within the upper layer with little or no interference from the underlying layer is less than half of its thickness. Also, electrode spacing not less than six times the upper layer thickness is required to accurately measure the resistivity of the lower layer when the upper layer is resistive. However, for a conductive upper layer, an electrode spacing of about twelve times the thickness of the upper layer is required to measure the true resistivity of the lower layer.