Conical source transient electromagnetic response characteristics with overburden based on Occam constrained inversion

The overburden thickness and conductivity can affect transient electromagnetic response in shallow subsurface investigation. In order to further verify the overburden detection effect with a conical source device, combining all-time apparent resistivity conversion of transient electromagnetic field, the electromagnetic response characteristics of the underlying strata influenced by the overburden was studied by introducing an Occam inversion with stratum information constraints. The results indicate that electromagnetic response intensity of the target layer is related to the resistivity ratio of the overburden to the target layer. With the overburden resistivity gradually increasing, the resistivity fitting degree between inversed data and real value increases and then decreases. The critical point appears at the moment of the overburden resistivity equaling to the underlying high-resistivity layer. The electromagnetic response of the underlying stratum attenuates with the overburden thickness increases, which leads to the abnormal peak of the electromagnetic response coming earlier, and the inversed resistivity peak depth increasing. Besides, by using constrained stratigraphic inversion, the inversion fitting degree and the layer interface resolution can be improved. The research can provide a theoretical reference to the design of measurement parameters and data processing and interpretation.