HEM Data Processing - A Practical Overview

The theory of frequency-domain helicopter electromagnetic (HEM) surveys is relatively straightforward, but in practice there are many issues to deal with, both in hardware development and in the software to handle the data processing and analysis (DPA).

This paper gives an overview of standard HEM data processing and analysis techniques. This is not new technology, and is intended only to fill a perceived gap in the literature by summarising existing basic DPA procedures for typical HEM datasets.

Assuming that the inphase (IP) and quadrature (Q) data for each frequency has been calibrated to parts per million (ppm) during data acquisition, the main procedures are as follows.

1. DATA PROCESSING
   • Apply standard geophysical pre-processing techniques such as parallax (lag) corrections.
   • Remove non-geological noise from the raw data using appropriate filters.
   • Apply drift corrections (zero level corrections). Remove system and instrumentation drift from all EM channels.
   • Archive data and analysis results along with complete documentation of all archived parameters and necessary metadata.
2. DATA ANALYSIS
   • Calculate apparent resistivities and depths.
   • Pick anomalies for conductive target body location and analysis. Requires an EM interpreter, but automatic picking provides a first pass.
   • Analyse anomaly targets. Calculate apparent conductances and depths to conductors.
   • Presentation of results; various combinations of:
     • Standard geophysical maps - survey lines etc.;
     • Apparent resistivity and depth maps;
     • Profile maps of IP, Q, resistivity, anomalies;
     • Classified anomaly symbol maps with annotations e.g. Identification letters, apparent depths, etc.);
     • Detailed multichannel profile plots for each survey line.
     • Conductivity depth sections.
     • Anomaly report.