Large scale 3D inversion of HEM data using a moving footprint

Helicopter electromagnetic surveying is a standard reconnaissance method in the mining industry. Typical surveys may cover tens to hundreds of square kilometers with hundreds of thousands of multifrequency soundings. Interpreting this volume of data is problematic. Yet each sounding location is sensitive to an area of less than 1 km2, not the entire survey area. We suggest an inversion scheme, based on the integral equation method, in which the entire survey is inverted simultaneously with each transmitter-receiver pair being sensitive only to a relatively small area around each sounding location. We show that this method is as accurate as standard integral equation inversion methods, yet it inverts the data much faster. The technique is able to invert entire HEM surveys with over 500,000 inversion cells and tens of thousands of transmitter positions in less than one day on a single PC.