President / William R. Petrick, Ph.D.
Bill received a Ph.D. From the University of Utah where he received the Outstanding Ph.D. Research Award. His emphasis has always been electrical methods of exploration. Specifically multidimensional forward and inverse solutions.
The Industrial Imaging Company (IIC) was incorporated in 1993 as a geophysical consulting and software development organization. We have been in business continuously ever since. Our clients include the worlds largest exploration / mining companies. In 2007 we developed our own 3D magnetotelluric inversion program. This was one of the first 3D inversion algorithms commercially available. Initially our clients would provide us with their MT data which had been collected by third party contractors. This situation was not always ideal. At about the this time, under a grant from the U.S. Department of Agriculture, we developed our own 3D audio frequency magnetotelluric (AMT) field hardware. The goal of this project was to develop new hardware capable of collecting 3D AMT data that could compete, on a cost basis, with 2D data acquisition. This capability provided us with full control over the data quality from survey design to final delivery of the 3D subsurface image. We did our first survey, with full 3D inversion, in 2010. Ever since that time 3D AMT data acquisition and inversion has been predominantly all we do. On occasion we will provide clients with 3D inversions of their legacy MT or AMT data.
AMT is a powerful exploration tool. Both AMT and MT use naturally occurring electromagnetic energy as source fields. The difference between AMT and MT is the frequency range. Simply stated; the MT frequency range is below about 1 Hz, the AMT range is above one Hz. Practically speaking the useful AMT range for mineral exploration is between 1 Hz and 1 kHz. Our field system collects data nominally from a few Hz to a few hundred Hz. Our field work is focused in the western U.S. In this environment we typically achieve a depth of exploration of about a km.
3D imaging is key to correct images of our clients exploration projects. While some geophysical contractors claim the ability to do 3D imaging it is the rare occasion when they actually do 3D. Virtually all geophysical contractors who cater to the mining industry do profile data acquisition and 2D inversion. Their preferred technique is controlled source AMT (CSAMT). You won’t find oil companies doing 2D CSAMT. When they do MT they do full 3D MT. That means 3D surveys and full 3D data inversion (aka 3D subsurface imaging). Full 3D has been expensive. That’s why oil companies do it and mining companies have been inclined to settle on profile CSAMT and 2D inversion. We’ve been able to cut the cost of full 3D AMT to the point that it is frequently less expensive than 2D CSAMT.
What’s wrong with 2D MT inversion and why contractors should stop doing it.
2D MT inversion is a bad idea. It’s a bad idea for many reasons, the most egregious of these are:
Poor position and conductivity estimates of the subsurface
Profile orientation: The subsurface image depends on the direction of the survey profile
These are all serious problems with 2D inversion. All of these problems go away with 3D data acquisition and inversion. The static shift problem still exists in 3D but it is significantly mitigated compared to 2D profile inversion.