Two-dimensional inversion of VLF-EM data

This report was prepared as part of research programme funded in part by the Commission of the European Communities, European Coal and Steel Contract (BC Contract No. 7220 - AF/136). A number of geophysical methods have been applied to a sequence of test areas in the vicinity of Saarbrucken, Germany. The studies relate to methods for the recognition of geological weakness zones and other surface discontinuities caused by underground mining in Carboniferous terrain. Progress to date (October, 1993) is summarised by Greenwood and Peart (1992). This report discusses a new method of data modelling applied to VLF data collected along three short (50 m) profiles in Saar Test Area 2.1. The data used in the study have previously been described by Greenwood and Peart (1992). The main aim of the report is to demonstrate an automatic method for the construction of a resistivity cross-section when suitable VLF-R data are supplied. The method used (two-dimensional, non-linear, regularised inversion) has not been used previously in an applied geophysical context. This report therefore discusses the scientific rationale and the known pitfalls of the procedure. The method is also applied to both synthetic models and the field data in order to assist in the interpretation of the results obtained.