high resolution audio frequency magnetotellurics
Introduction:
A new exploration system has been developed
that significantly improves magnetotelluric
exploration for hydrocarbons. The system uses
natural electromagnetic energy as its source.
Many experts say that this source is
"many orders of magnitude greater than the
strengths of fields that can be generated with
man-made sources on the surface of the earth."
The technique varies from classical thinking by
analyzing the higher frequency harmonics of the
recognized lower frequency carrier waves which
propagate within the earth. The result of this
departure from the recognized operating frequencies
is a significant improvement in the resolving power
of the instrumentation, allowing vertical subsurface
sampling as small as .8’ at 5,000’ depths with depth
accuracies of +/- 25’ in most areas.
High quality geophysical instrumentation records a wide-band signal in the field for both the magnetic and electric components of the electromagnetic field. These time domain signals are properly filtered, sampled, and stored to digital tape. These digital signals which contain information for all depths are demodulated by analog or digital computer to final form for analysis. The recorded signal contains frequency-phase vs. amplitude information relating to the incoming field at the surface, the decaying earth carrier field, and the modulation resulting from the earth’s resistivity reflection coefficients. The former two fields are extracted using least square methods. Only the earth’s resistivity profile remains as a function of frequencies. Simplification of the well known “skin depth” equation is used to convert the profiles to depth. The result of this complicated process is a series of electric and magnetic reflection coefficients. These can then be combined to form the apparent resistivity series defined by Z = E/H as a function of depth.
These systems yield valuable information on the porosity zones in the subsurface. Since electromagnetic energy seeks the least interrupted travel path, the amount of connate or bound subsurface waters dramatically effect the apparent resistivity. The more the porosity, the more bound water, and the lower the apparent resistivity curve, yielding an excellent set of data for point-to-point correlation as well as diagnostic information about available subsurface porosity zones. The reflection amplitudes of magnetic and electric signals react differently to hydrocarbon and water. These differences are readily apparent when the two reflection coefficient profiles are cross-plotted and this display becomes an aid in the estimation of the hydrocarbon potential of surveyed intervals in the subsurface. In addition, the presence of hydrocarbons in the subsurface impedes the transmission of electromagnetic energy which appears as a unique phase distortion in the modulation of the carrier waves, visually apparent in the digital modulation trace of the DIGILOG, and detectable by analog analysis with the Z-SCAN.
All of the above processes are combined in visual displays similar to standard electric log format. The digital computer outputs an apparent resistivity log, cross-plotted with both the magnetic and electric sequences. In addition, a fourth curve showing the modulation pattern of the data is also included. Within the Z-SCAN system, a graph is made of apparent resistivity changes vs. depth with reservoir fluid estimates of oil, gas, and water. Whether the DIGILOG or Z-SCAN systems are used for point-to-point correlation or detail fluid analysis, they are an excellent addition to your remote sensing arsenal.
Interpretation:
The key to DIGILOG OR Z-SCAN interpretation
is the ability to anchor correlation using
distinctive packages in concordance with
consistent markers which give a characteristic
response. In general, analyses of thicker
sections will enhance correlatability,
excepting inclusion of highly stratigraphically
variable sections or sections with very thin
beds which introduce sampling bias or aliasing.