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GCMarine EM Methods*
By
Steven Constable1
Search and Discovery Article #40175 (2005)
Posted October 31, 2005
*Adapted from the Geophysical Corner column, prepared by the author and entitled “Do You Need Marine EM Methods?,” in AAPG Explorer, October, 2005. Appreciation is expressed to Alistair Brown, editor of Geophysical Corner, and to Larry Nation, AAPG Communications Director, for their support of this online version.
1Scripps Institution of Oceanography, La Jolla, CA ([email protected])
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In the space of just a few years a new geophysical technique has appeared on the scene -- marine controlled source electromagnetic (CSEM) sounding, also known as Seabed Logging by Statoil and R3M by ExxonMobil. Such a rapid rise is bound to create some confusion, and so here I will try to explain just what CSEM methods are and what they can do for the exploration geologist. First, what is it?
Marine CSEM is one
of two electromagnetic techniques applied to offshore exploration (Figure
1). The first technique, the marine
The application of
Marine CSEM, however, behaves very differently than EM used on land, a feature that I will discuss below.
Actually, marine
CSEM is not that new; Charles Cox of Scripps Institution of Oceanography
proposed the
The most important
concept in any EM Skin depth = 500 meters x square root (resistivity/frequency).
At a period of one
second, the skin depth in seawater is about 270 meters; this means that over
each 270 meters the amplitude of EM energy decays another 37 percent. In 1000
Ohm.m basalt, at the same period the skin depth is nearly 16 kilometers; so
energy will propagate from the transmitter to the seafloor receivers mostly
through seafloor rocks, making the This behavior, because it looks a little bit like seismic refraction, has caused some confusion. Seismic waves decay geometrically as they spread, but retain a resolution that is proportional to wavelength no matter how far they travel. EM signals decay exponentially as conductive rocks absorb energy (and get heated by electromagnetic induction!) and have a resolution that is proportional to the depth of the target. This is not quite as bad as it sounds, since the skin depth provides an intrinsic depth measure; potential field methods (gravity, magnetics, DC resistivity) have no depth resolution other than that associated with spatial geometry. However, a target does need to be about as big as it is deep to be visible by EM methods.
“Discovery” and Use of Marine CSEM
So why, if
the
1. If the
water depth is shallow compared with skin depth, EM energy from the
transmitter reaches the atmosphere, where it becomes a true wave and
propagates geometrically. This “air wave” rapidly becomes the dominant
signal at the seafloor receivers and removes the sensitivity to seafloor
geology that we have in deeper water. Thus, until hydrocarbon
exploration moved to water around 1,000 meters deep, it was difficult to
take advantage of the marine CSEM
2. It has
long been known that the marine CSEM
One should
caution that evaporites, volcanics, and carbonates are all also
resistive; so the
Figure 2 shows how the The calculations represented in Figure 2 are quite complicated. To interpret real data without such modeling, it has become practice to divide the measured electric fields by the 1D background response (similar to using a reduced travel time in seismics), or even to simply normalize by the response of an instrument assumed to be positioned off target. Resistive features then stand out as anomalies in the data.
Since
resistors anywhere in the section can produce such anomalies, one needs
to be very cautious in using this simplified approach. Additional data
are always important, and so, for example, Figure 2 also shows only one frequency (1 Hz), but other frequencies -- having other skin depths -- will help resolve ambiguities in the interpretation. As in any geophysical interpretation, taken alone CSEM data will not yield a single unambiguous model. It can be seen from Figure 2 that at short ranges there is no sensitivity to the target. At larger ranges where the target is manifest the electric fields are very much smaller, and so the noise floor of the transmitter-receiver system determines how deep a target can be detected. The vertical axis is in units of electric field at the receivers (in volts per meter) divided by the transmitter dipole strength, given in turn by its antenna length (meters) times zero-to-peak transmission current, in amperes. Typical transmission currents are hundreds of amps; typical antenna lengths are hundreds of meters; and typical receiver sensitivity is hundreds of picovolts per meter. Another factor of 10 can be obtained by stacking, giving a total noise floor around -15 log units. Figure 3 shows the amplitude and phase of real CSEM data stacked into two-minute and 10-minute data frames. The phase varies over a smaller range than the amplitude but does not contain any independent information. Does marine CSEM work?
Undoubtedly yes, for big enough targets in relatively deep water.
However, even though the
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