Discussion: On ‘Negative apparent resistivity in dipole–dipole electrical surveys’ (Jung, H.K., Min, D.J., Lee, H.S., Oh, S.H., and Chung, H., Exploration Geophysics, 40, 33–40)

Abstract

Corresponding author. Email: choik@kangwon.ac.krThe DC resistivity method is a well known geophysicalexploration technique and is widely applied in miningexploration, groundwater surveys, and engineering andenvironmental investigations. Negative apparent resistivities,especially in 2D surface dipole–dipole surveys, are oftenobserved and generally have been regarded as measurementerrors or the effect of noise. Cho et al. (2002) insisted that ifthe earth is isotropic and flat, and the survey line is straight,apparent resistivity is always positive regardless of subsurfacestructures. On the contrary, Jung et al. (2009) insisted thatnegative apparent resistivity could be caused by a geologicalstructure. In this discussion, we re-examine the possibility ofnegative apparent resistivity since the conclusions of these twopapers are entirely contrary. We assume that a 2D resistivitysurvey is performed alongastraight lineonthe flat surface of anisotropicearth,asJungetal.similarlyadopted.Wefirstdescribethe behaviour of surface charge, electric field, potential, andcurrent inside and outside an inhomogeneity. We carry out 3Dfinite-elementmodellingforthesameU-shapedmodelpresentedby Jung et al. as a typical model that yields negative apparentresistivity.Then,wecompareourresultwiththatofJungetal.anddiscuss whether negative apparent resistivity is really possibleunder the stated conditions.First,letusexplainthebehaviourofelectricfieldandpotentialdifference. Suppose that an inhomogeneity with resistivity ris surrounded by a uniform medium having resistivity r