Page 84 - Geothermal Energy Systems Exploration, Development, and Utilization

60 2 Exploration Methods
Apparent resistivity response throughout period range
apparent resistivity
r 2 r 1 r 2 r 1 r 2 r 1 r 2 r 1
1 2 3 4
Period

T 0
TM TE
TE TM

Impedance polar diagrams of given period T 0

X

1 2 3 4
TM
TE

r > r 2
1
r 1 r 2
Figure 2.8 Impedance polar diagrams (at one frequency)
and apparent resistivities, at four sites on a simple 2D con-
tact model. TE and TM refer to transverse electric (E parallel
to strike) and transverse magnetic (H parallel to strike) field
polarizations, respectively. (From .. . ).

mode inversions. The determinant mode has been successfully tested in several
studies.
The goal of any MT interpretation is a representation of true resistivity with
depth. There are two ways such a representation is achieved: forward or inverse
modeling. MT multidimensional modeling techniques are well developed. Forward
modeling codes can resolve 1D, 2D, and 3D structures by creating a synthetic cross
section of the subsurface, computing its MT response, and then comparing it
with the actual MT data, using the time-consuming trial-and-error approach.
Inversion codes also exist and have been used routinely for computing 1D and 2D
responses. 3D inverse codes, even though they have been available for some time
(Mackie, Smith, and Madden, 1994; Newman and Alumbaugh, 2000; Zhdanov
et al., 2000; Siripunvaraporn et al., 2005), are mostly still in the development stage
(Siripunvaraporn and Egbert, 2009). Several field studies have shown promising