The Application of Tilt Derivatives to EM Conductivity Data

In the processing of geophysical potential fields, a wide range of spatial derivatives are available to enhance the information contained in the basic data. Here the ability of the tilt and tilt derivatives to provide enhanced mapping of conductivity data is considered. Tilt and its associated functions are formed by taking combinations of vertical and horizontal derivatives of the data set. A theoretical forward modelling study is carried out to assess the performance of tilt derivatives in relation to the detection and definition of concealed conductivity structure. Case studies of the practical application of the procedures to survey data are performed. The case studies derive from large scale airborne EM data sets but the methods have a general applicability to a wide range of geophysical conductivity and resistivity data. The tilt functions embody Automatic Gain Control that normalise the detection and definition of both weak and strong conductivity gradients across an appropriate subsurface depth range. The use of high order spatial derivatives inevitably results in a degree of noise amplification that is survey and technique specific. Filtering methods for the reduction of undesired, usually high wavenumber, artefacts are available and are shown to be effective.