An apparent-resistivity concept for low-frequency electromagnetic soundingtechniques

Apparent resistivity is a useful concept for initial quickscan interpretati on and quality checks in the field, because it represents the resistivity p roperties of the subsurface better than the raw data. For frequency-domain soundings several apparent-resistivity definitions exist. One definition us es an asymptote for the field of a magnetic dipole in a homogeneous half-sp ace and is useful only for low induction numbers. Another definition uses o nly the amplitude information of the total magnetic field, although this re sults in a non-unique apparent resistivity. To overcome this non-uniqueness , a complex derivation using two different source-receiver configurations a nd several magnetic field values for different frequencies or different off sets is derived in another definition. Using the latter theory, in practice , this means that a wide range of measurements have to be carried out, whil e commercial systems are not able to measure this wide range. In this paper , an apparent-resistivity concept is applied beyond the low-induction zone, for which the use of different source-receiver configurations is not neede d. This apparent-resistivity concept was formerly used to interpret the ele ctromagnetic transients that are associated with the turn-off of the transm itter current. The concept uses both amplitude and phase information and ca n be applied for a wide range of frequencies and offsets, resulting in a un ique apparent resistivity for each individual (offset, frequency) combinati on. It is based on the projection of the electromagnetic field data on to t he curve of the field of a magnetic dipole on a homogeneous half-space and implemented using a non-linear optimization scheme. This results in a fast and efficient estimation of apparent resistivity versus frequency or offset for electromagnetic sounding, and also gives a new perspective on electrom agnetic profiling. Numerical results and two case studies are presented. In each case study the results are found to be comparable with those from oth er existing exploration systems, such as EM31 and EM34. They are obtained w ith a slight increase of effort in the field but contain more information, especially about the vertical resistivity distribution of the subsurface.