VLF wave impedance measurements have been successfully used to detect air-filled drainage galleries near the town of Alcala, Spain. The galleries are detectable by H-polarization electric field measurements due to the electric field anomalies associated with the galleries and overlying gravel deposits. The forced deviation of the primary current flowlines around the 2D void results in a higher-than-normal apparent resistivity and a relative phase low above the gallery.
The findings support earlier theoretical predictions that at very low frequencies (VLFs), galvanic current effects may dominate over vortex currents in moderately conductive terrains. Theoretical modelling confirmed that for a resistive target no detectable E-polarization response can be expected from either magnetic or electric field measurements since current line deviations and vortex effects are negligible under such circumstances.
The results demonstrate the importance of using at least two orthogonal VLF transmitters in order that anomalies arising from both galvanic and inductive effects may be identified, irrespective of orientation.
[1]
A. Sinha.
Interpretation of ground VLF-EM data in terms of inclined sheet-like conductor models
,
1990
.
[2]
J. Nissen.
A versatile electromagnetic modeling program for 2-D structures
,
1986
.
[3]
P. Kaikkonen,et al.
On the interpretation of vlf resistivity measurements
,
1985
.
[4]
Gerald W. Hohmann,et al.
Magnetotelluric responses of three-dimensional bodies in layered earths
,
1982
.
[5]
N. Paterson,et al.
Five years of surveying with the Very Low Frequency—Electro magnetics method
,
1971
.
[6]
W. J. Phillips,et al.
Study of the effectiveness of the VLF method for the location of narrow-mineralized fault zones
,
1975
.