Introduction

Summary

Conventional controlled source electromagnetic (CSEM) surveying was designed for deep water. In shallow water (<300 m) the magnetotelluric noise and the air wave present problems for the method. The source signal is continuous with energy concentrated in a few discrete frequencies. A transient broad-bandwidth source signal, with the same energy spread out over the bandwidth, allows significant gains in signal to noise ratio. Deconvolution gives more than an order of magnitude increase in signalto- noise ratio; correlated noise, including magnetotelluric noise, can be reduced by a further order of magnitude; and the air wave becomes separable from the earth response – a further gain of about 15 dB. For the same source energy the transient source signal is superior by about 40 dB (a factor of 100).

Conventional controlled-source electromagnetic (CSEM) surveying uses a horizontal current bipole source, towed about 50 m above the sea floor from a vessel moving at about 1.5 knots, and receiver nodes placed on the sea floor. The source emits a continuous signal, usually a square wave, which has energy concentrated at discrete frequencies (Srnka, 1986). Between these frequencies the energy in the source signal is effectively zero. The method was designed for deep water (Eidismo et al., 2002). As Eidismo et al. (2002) explain, the air wave dominates the overall response at source-receiver separations greater than about five times the water depth. The air wave propagates from the source through the water to the sea surface, travels through the air at the speed of light, and propagates from the sea surface to the sea floor receiver. Propagation through the conducting sea water at the source and receiver attenuates the signal exponentially. The deeper the water, the greater the attenuation of the air wave, and the easier it is to detect the signal from below the sea bed. The first successful demonstration of this method was conducted in water about 1200 m deep offshore Angola (Ellingsrud et al., 2002). In shallow water, say less than 300 m deep, not only is the air wave a problem because the attenuation is much less, but magnetotelluric noise becomes another important source of noise, as there is less water to attenuate the noise originating in the ionosphere. In this paper we argue that the use of a broad-bandwidth transient signal instead of the conventional continuous signal yields significant gains in signal-to-noise ratio. These benefits far outweigh the initial advantage in signal amplitude the conventional method has at its discrete frequencies. This is even more important in towedstreamer implementation. The current bipole source in transient EM is essentially the same (and also controlled, so it is also CSEM), but the signal has a beginning and an end and has a generally smooth frequency spectrum over a limited bandwidth. It is particularly easy to choose the frequency bandwidth of the signal using a pseudo-random binary sequence, or PRBS, because it is easy to switch the polarity of electric current very fast, (Duncan et al., 1980).

This content is only available via PDF.
You can access this article if you purchase or spend a download.