Geologically-constrained 1D TEM inversion

Abstract

During 1D inversion of TEM data it is desirable to constrain the inversion with all available information. Accordingly a program has been developed to perform geologically-constrained 1D TEM inversion. Three different types of inversion are permitted: geometry inversion, homogeneous property inversion, and heterogeneous unit inversion. During geometry inversion, the layer boundaries are allowed to move, except at fixed points, e.g. drilled contacts. During homogeneous property (?stratigraphic?) inversion, the conductivity of geological units is optimised, with each unit assumed uniform in conductivity across the entire survey area. During heterogeneous unit inversion, the conductivity is allowed to vary within geological units. These inversion options allow the interpreter flexibility to explore geological scenarios. The inversion program operates in a fully 3D geological context, but a 1D model is extracted at each location for adjustment. The method is illustrated via application to airborne TEM data recorded over shallow coastal waters. Inversion is constrained by sonar and marine seismic surveys, and by conductivity measurements of seawater and of sediment core. In this environment it is reasonable to assume that the sediment is fairly uniform in conductivity, and therefore an inversion sequence was designed to define the model with minimum intra-sediment conductivity variation. Seawater depth and conductivity were treated as known. After construction of a 3D starting model, inversion proceeded in three stages: first, the optimal uniform sediment conductivity was defined; then the geometry of the sediment-bedrock contact was adjusted; finally, internal conductivity variations were permitted in the sediment. At the end of this inversion sequence the surviving internal conductivity variations within the marine sediment are considered indicative of actual lateral variations in the sediment.