Vertical electric source in transient marine CSEM: Effect of 3Dinhomogeneities on the late time responseстатья
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Дата последнего поиска статьи во внешних источниках: 11 января 2018 г.
Аннотация:The time-domain marine controlled-source electromagnetic
method, based on injection of electric currents into the sea
via a vertical cable and measurements of the transient vertical
electric field, is characterized by high sensitivity to resistive
reservoirs and robustness with respect to distorting effects of
lateral heterogeneities. This is due to the fact that a vertical
electric dipole induces in a stratified medium only a transverse
magnetic (TM) field. In addition, the vertical electric field is not
directly contributed by the transverse electric (TE) component
of the field scattered by heterogeneities. Nevertheless, a closedform
solution shows that the first order effect of a lateral heterogeneity
displays itself as a parallel shift of the late time response
curves. The effect is also observed in 3D responses evaluated by
numerical solutions of the integral equation of the modified iterative
dissipative method. Moreover, in “favorable” conditions,
scattering on heterogeneities may change the law of the field
decay. The parallel shift of the late time curves is caused by
vertical polarization of the scatterer, while its horizontal polarization
leads to an abnormally fast decay of the vertical electric
field. The latter effect, observed against the background of the
general decay of the free electromagnetic field, can be associated
with “energy channeling from the TM to TE field.” Neither
of the effects necessarily deteriorates the method sensitivity.
Unlike the vertical electric field, the horizontal electromagnetic
field is contributed by the scattered TE field. As a result, the
abnormally fast decay of the vertical electric field is accompanied
by an abnormally slow decay of the horizontal components.
The transient horizontal electric field may become almost insensitive
to the reservoir resistivity. In addition to unrealistically
harsh requirements to the transmitter tilt, this may render
accurate measurements of the horizontal electromagnetic field
of a vertical electric bipole not feasible.