Dear UBC-GIF
I am currently working on a project that involves the acquisition of
electromagnetic data (through electromagnetic induction). AC current will be
passed through the primary coil and the response received via the secondary
coil will be digitized and analyzed. The data will then be manipulated to
produce resistivity information.

Could you please send information on the following:
a.     how to determine the depth of penetration of the signals;
b.     how to eliminate direct coupling between the transmitter and receiver coils;
c.     the expected magnitude of the AC signal;
d.     how to obtain the in-phase and quadrature signals and how to incorporate and
process these data.

I also have a couple of questions on how to use the data acquired. I plan to
use the data to present a 3D image of the resistivity profile of the earth. I
would like to know: a. how to extract 3D information from the data. b. how to
quantify the signals received and hence obtain the conductivity (resistivity)
information from the data. c. where I can get more information about linear
inversion and software algorithms to process the data.

I plan to write a software programs that will control the logging process and
present the resistivity profile of the sampled region.
a. I would be grateful to receive some general 3D programming techniques and algorithms

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Hello,

You have tackled a very challenging project for several reasons. The best I can
do to help is point to some references that are easy to obtain. But before I
list some sources, I should point out that all your questions are good ones,
and that there are whole companies and university research groups working on
different aspects of these questions. It might be a good idea to focus on
certain aspects of either the instrumentation or application of results.

a) Penetration depth is usually described in terms of skin depth. This involves
frequency, ground conductivity and magnetic permeability.

b) In common instruments, the direct coupling signal is usually "known"
electronically (and involves maintaining fixed coil spacing and orientation in
the field). Alternatively Using two coils oriented perpendicularly can
eliminate direct coupling, but this is tricky unless coils are maintained with
perfect orientation in the field. Direct coupling can also be avoided by
working in the time domain using pulsed signals. Time domain EM is commonly
available, but physics, math, and instrumentation are very complicated.

c) Extracting inphase and quadrature phase is an electronics problem which
requires the receiver to be carefully synchronized to the transmitter.

Extracting 3D information from the data is a problem that no one has solved
satisfactorily yet (2002) in the 50-60 years of EM geophysics. Researchers are
getting close to developing solutions, but the details involve subjects that
keep experienced professors and PhD graduates busy for many years. Many
instruments today still generate only "apparent" resistivities, and
interpetation of geology is attempted from these very simple results.

Generating conductivity from raw EM data can be done using several differnt
types of approximation. The references from Geonics (below) should help.

References on linear inversion can be found listed at the UBC-GIF web site.
There are not many on EM yet but that is because application of inversion
theory to EM problems is only now (2002) becoming practical.

You must talk to computer specialists to learn about programming techniques.

Now, some general references:
Many very good technical notes are available from Geonics at
http://www.geonics.com/. They are free but to get some of them you may need
to send in a request.

Older text books often provide outlines of how instrumentation works. Modern
text books discuss inversion to some extent, but electronics is rarely
mentioned.

Some modern texts are listed at
http://www.science.ubc.ca/~eoswr/geop/appgeop/appgeop-links.html
This is a large page with many useful links. Use your
browser's "find" tool to search for things on this page (For example, use the
browser's Edit menu, Find option, and type "text" in the box).

The SEG (http://seg.org) has many excellent books, student membership is not
expensive, and students get price reductions for  publications. As usual, most
information is about using geophysical data, not how to generate the data.

I  wish you luck in your studies.
Regards, etc ....