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CONVENTIONAL INTERPRETATION TECHNIQUES
The ultimate goal of doing any geophysical survey is to deliver a map or model indicating the subsurface distribution of physical properties, i.e. conductivity in the case of TEM and to interpret this data in terms of geology. When geological information are added to the geophysical model it is possible to link lithology or structures to distinct geophysical units of different conductivities and this serves as a very useful tool in constructing a final geological model.
This study is concerned mainly with the conversion of TEM data to a reliable subsurface conductivity distribution. Traditionally there are two separate classes of exploration targets for TEM surveys and based on these classes different survey geometries and interpretational procedures are followed. In modern day geophysics these two classes of surveys increasingly overlap but it is still important to understand the different approaches that were followed in the past as most of the interpretation techniques being used today were developed for very specific target conditions.
The first type of target is considered to be a confined conductor(s) in a resistive host rock. The theoretical assumptions made in this case are basically that the host rock has no influence on the electromagnetic propagation of fields and conductors are considered to be suspended in free space for all mathematical purposes. TEM surveys to detect this type of target are designed to emphasize lateral variations in conductivity and are called profiling surveys. The geophysical model derived from interpreting a survey like this would contain the location, conductivity and geometrical parameters of one or more finite conductors (two- or three-dimensional). These conductors would be simplified geometrical shapes such as spheres, ellipsoids, plates or prisms approximating true geological units. Typical geological targets are massive sulphides or linear structures like weathered faults or shear zones.
1 INTRODUCTION
1.1 General overview.
1.2 Objective
2 OVERVIEW OF ELECTROMAGNETIC THEORY FOR GROUND, INLOOP
TDEM DATA.
2.1 Introduction.
2.3 Analytical TDEM responses for four common models
2.3.1 TDEM central-loop response over a conductive half space
2.3.2 Thin, conductive sheet (S-layer).
2.3.3 Finite conductor in a resistive host rock (half space).
2.3.4 Finite conductor in a conductive host rock (half space)
2.4 Conclusion
3 CONVENTIONAL INTERPRETATION TECHNIQUES.
3.1 Introduction.
3.2 Profiles versus soundings .
3.3 Forward Modelling and Inversion
3.4 Limitations on automation of inversion technique
3.5 Decay curve analysis.
3.6 Transforms (Depth imaging)
3.6.1 Conductivity Depth Images (CDI’s).
3.6.2 Stationary current images (SCI) (SCI – trademark of Geoterrex-Dighem Pty Limited).
3.6.3 S-layer differential transform
3.7 Combining Strategies
4 AUTOMATED INTEGRATED ANALYSIS OF TDEM DATA.
4.1 An integrated analysis algorithm..
4.2 Visualization of the field data for rapid anomaly detection.
4.3 Decay curve analysis.
4.4 Numerical calculation of the S-layer differential transform
4.5 The S-layer correction factor.
5 APPLICATION TO FIELD DATA.
5.1 Introduction
5.2 Ground survey
5.3 Airborne survey.
5.4 Conclusions and recommendations
