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Small frequency-domain electromagnetic induction sensors

How in The world does a small broadband EMI sensor with little or no source-receiver separation work?

Geophex, Raleigh, North Carolina, U.S.

Corresponding author: ijwon@geophex.com

Editor's note: The Meter Reader is a regular column in TLE, originated by the Gravity and Magnetics Committee and currently coordinated by John Peirce, that seeks to highlight new ideas in geophysical fields besides seismic—particularly gravity, magnetics, and electromagnetics. If you have a short contribution on these topics that is written in the relatively informal but informative style of TLE, please submit it to Dean Clark, editor of TLE in Tulsa, or to John Peirce, at GEDCO in Calgary. This article describes a new class of small EM sensors that have successfully overcome one of the inherent limitations that many of us were brought up to accept as gospel. The examples in this paper represent the solutions developed by one company. They are used as examples, and this usage does not imply they are the only solutions or that these products are endorsed by SEG. The author and I thank Ted Glenn for a very helpful review.

The first 20% of the full text of this article appears below.

Over the past several decades, we have witnessed the emergence of portable electromagnetic induction (EMI) sensors: to name a few, Geonics pioneered portable sensors such as EM-31, EM-34, and EM-61; Zonge Engineering successfully developed and marketed variations of controlled-source EMI sensors. These portable and popular EMI sensors have been used successfully for many geophysical investigations, particularly for shallow engineering and environmental surveys.

The most recent entry in this field is a new generation of one-man portable, digital, broadband EMI sensors developed by Geophex: the GEM-2 has a separation of 1.67 m between the transmitter (TX) and receiver (RX), while the GEM-3 has no practical separation because all coils are collocated in a concentric circular disk (Figure 1). Both sensors have a 30-Hz data rate at typical 3–10 simultaneous frequencies and, currently, a bandwidth of 30 Hz to 48 kHz, a span of about three decades. Each weighs about 4 kg (8 pounds) and comes in a single handheld package. The primary incentive for such small sensors is obvious: portability and speed.

View larger version (74K): [in this window] [in a new window]   Figure 1. Portable, digital, broadband EMI sensors. The instrument in the left photo has three linearly spaced coils with a maximum separation of 1.67 m. The concentric instrument in the right photo comes in three disk sizes up to 96 cm in diameter. The largest one is typically cart-mounted.