5.5 VLF-EM Depth Of Penetration Case Study

3.5.5 VLF–EM Depth of Penetration

The question now how deep electromagnetic waves penetrate into the ground and the resolution as a function of depth is of prime importance in EM surveying. In an isotropic resistive medium, EM waves would travel virtually indefinitely. However, in the real world, where surface conductivities are significant, the depth of penetration is often very limited. Theoretically, the maximum penetration depth at which an EM plane wave has been attenuated to e-1 (or 37%) relative to its initial amplitude in a fairly conductive homogeneous half-space and results in magnetic and electric field components that are detectable by an VLF–EM equipment at a particular operating frequency, is known as skin-depth 'δ'

δ=√(2ρ/(ωμ_0
d≈1.5δ≈755√(ρ/f) (3.27)

3.5.6 VLF-EM Survey Procedure and Quality Control

The VLF–EM data are normally collected along profiles. When planning a VLF–EM survey several considerations must be taken into account. (1) Is the target’s strike direction. In VLF-EM work the profile direction is almost irrelevant, the critical parameter being the relationship between the known (or presumed) geological strike of a subsurface conductor and the azimuth (or bearing) of the radio transmitter. A subsurface conductor which strikes towards the radio-transmitter is well-coupled, as the magnetic field is at right angle to it and current can flow freely. Otherwise, the current flow would be restricted, reducing the strength of the secondary field. Profiles must be located perpendicular to strike so those anomalous zones can be compared to background levels. Every effort should be made to avoid putting profiles in areas that contain a number of cultural features that may mask anomalies associated with the intended target. (2), consideration must be given to which transmitting stations are available for use during
These transmitters provide strongly observed radio-signals and show a wide azimuthal distribution for instance 0o, 15o, 30o, 60o and 190o. Their expected geographic locations from which they were initially emitted, based on the measuring azimuths were also noted. Therefore, reliable measurements could then be conducted along 28 surveying profiles oriented in S–N, SW–NE and W–E directions. It was not possible to have a perfectly regular grid of data points due to irregular topography, and the landfill installations themselves. Profile lengths ranged between 120.0 to 540.0 m. Global positioning systems (GPS) was used for exact spatial positioning of collected data. Readings were taken at station interval of about 5.0 to 10.0 m in both the E–W and S–N directions, oriented roughly perpendicular to the crossed main geological strike of El-Sallum basin which is characterized by numerous faults in E -W, NE-SW and NW-SE direction. Survey profiles were denoted as VLF–1 to VLF–28 (Figure 3.19). The present measurements were carried out using the WADITM VLF–EM system (ABEM Instruments AB, Sweden) (Figure 3.20). The accuracy of measurements depends not only of the ground earth model, but also of field strength, remoteness and departure from the true azimuth of the radio-transmitters, and