USE OF DECAY PATTERNS FOR THE CLASSIFICATION OF ANOMALIES IN TIME‐DOMAIN AEM MEASUREMENTS

Geophysics ◽  
1976 ◽  
Vol 41 (5) ◽  
pp. 1031-1041 ◽  
Author(s):  
G. J. Palacky
Keyword(s):  
Time Domain ◽  
Field Measurements ◽  
System Response ◽  
Electromagnetic Measurements ◽  
Time Domain Electromagnetic ◽  
Decay Pattern ◽  
Body Parameters ◽  
Flight Computer

Interpretation of time‐domain electromagnetic measurements normally comprises visual anomaly selection and determination of body parameters, such as conductance, depth, and dip. A study is made to examine the possibility of in‐flight computer interpretation on the basis of decay patterns. Analysis of system response over conducting loops, vertical and dipping sheets, horizontal strips, and a half‐space indicates that identification of models and some of their parameters by decay patterns is feasible. By the simultaneous use of vertical and horizontal coil receivers, a reliable recognition of models may be achieved. While the secondary magnetic field over a conducting loop decays exponentially, other models show distinctive nonexponential patterns. Decay patterns are affected by conductance and conductor size, but less by depth and dip variations. Field measurements indicate that decay pattern may be used to distinguish between geologic bodies of various types.

The 2D coastal effect on marine time domain electromagnetic measurements using broadside dBz/dt of an electrical transmitter dipole

Geophysics ◽  
2011 ◽  
Vol 76 (2) ◽  
pp. F101-F109 ◽  
Author(s):  
Mark Goldman ◽  
Eldad Levi ◽  
Buelent Tezkan ◽  
Pritam Yogeshwar
Keyword(s):  
Time Domain ◽  
Electric Dipole ◽  
Field Measurements ◽  
Resolving Power ◽  
Receiver Coil ◽  
Vertical Electric Dipole ◽  
Horizontal Electric Dipole ◽  
Time Domain Electromagnetic ◽  
Actual Field ◽  
Receiver Arrays

Galvanic transmitter-receiver arrays commonly are used in marine controlled-source electromagnetic (CSEM) exploration of electrically resistive targets such as hydrocarbons, gas hydrates, etc. These arrays utilize vertical electric currents and, as a result, are expected to provide better resolving capability for exploring subhorizontal resistive structures than arrays including horizontal coils. If, however, a subseafloor resistive target is located within a transition zone at distances of up to a few kilometers from the shoreline, the 2D sea-coast resistivity contrast significantly affects the resolving capability of the measurements. An extensive multidimensional modeling supported by numerous offshore measurements showed that the inductive array consisting of a horizontal electric dipole transmitter and a broadside vertical magnetic dipole (horizontal coil) receiver exhibits much better resolving power in time domain compared to all other arrays but those with a vertical electric dipole. This effect takes place only if a short offset receiver coil is located between the transmitter dipole and the coast. If the coil is located at the seaside of the transmitter dipole, the signal lacks the resolving capability almost entirely. At large offsets, the resolving capability of the measurements is relatively low at both sides of the transmitter dipole. Although actual field measurements were conducted only to explore a shallow target (fresh subseafloor groundwater body), calculations show that the same phenomenon exists in case of deep targets (e.g., hydrocarbons).

On: “Induced‐polarization effects in time‐domain electromagnetic measurements” by M. F. Flis, G. A. Newman, and G. W. Hohmann (GEOPHYSICS, 54, 514–523, April 1989).

Geophysics ◽  
1989 ◽  
Vol 54 (12) ◽  
pp. 1655-1656 ◽  
Author(s):  
Richard Smith
Keyword(s):  
Time Domain ◽  
Three Dimensional ◽  
Polarization Current ◽  
Electromagnetic Measurements ◽  
Charging Current ◽  
Time Domain Electromagnetic ◽  
Time Domain Response ◽  
The Time Domain ◽  
Theoretical Results ◽  
Insight Into

Flis et al. provide useful insight into the time‐domain response of three‐dimensional polarizable bodies; however, their inference that negative transients are caused by a polarization current which reverses direction disagrees with the previously published theoretical results of Smith et al. (1988) and Smith and West (1988), who found that the polarization current is always negative (provided that the chargeability m and charging current are positive).

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