Comparative study between filtering and inversion of VLF-EM profile data

Abstract

This paper discusses the practical use of filtering and inversion in VLF-EM data processing and interpretation. The advantages and disadvantages of both mentioned techniques were outlined to avoid the misleading interpretation of such data in some case studies. Much concern is taken to show the interval distance effect upon the correct depth identification of the anomalous body by either inversion or filtering. The methodology of the study is going through proposing an initial model, generating the synthetic VLF-EM data of the model by means of forward modeling, filtering (Karous-Hjelt filter) and inversion (Inv2DVLF software) of the synthetic data and comparing between the results of both methods and the initial model. The study reached to (1) Karous-Hjelt filter provides misleading depths for his limited depth of resolution and cannot provide estimates of deep targets if the profile is too small, whereas inversion provides exact results, particularly in case of shallow anomalous target; (2) crossover between in-phase and out-of-phase data could resulted from a small shallow conductive target or a large deep one; (3) selection of a reasonable environmental resistivity has an important impact on the inversion process; and (4) the numerical reflection resulted during VLF-EM data inversion could lead to an erroneous interpretation.

Very Low Frequency Electromagnetic Induction Surveys in Hydrogeological Investigations; Case Study from Poland

“…A secondary magnetic field is generated which is out of phase with the primary magnetic field. The intensity of the secondary magnetic field depends on the conductivity of the ground (Khalil and Santos 2010). Resultant magnetic field, which is produced by the interference between the primary and the secondary magnetic fields, is elliptically polarized.…”

Exploration of Lead-Zinc (Pb-Zn) Mineralization Using Very Low Frequency Electromanetic (VLF-EM) in Ishiagu, Ebonyi State

“…Finally, the in-phase readings on stacked profiles (Figure 7) and the current density maps (Figures 8 and 9) were used for the interpretation of the geologic structures in terms of conductive mineralized veins and this provided more detailed information on the extension, thickness and depth distribution of the Pb-Zn mineralized veins in Ishiagu field [15][16][17][18][19]. Generally, good 2-D structures with less overburden were detected by induction, hence, the Pb-Zn mineralized veins lying in the area of survey with less overburden (in this case closer to the Abakaliki anticlinal axis) were utterly inducted; while those concealed by very thick overburden were not easily inducted, but accomplished through current gathering and are thus, of moderate responses [20][21][22].…”

The VLF-EM imaging of potential collapse on the LUSI embankment

“…They used smoothness and initial model constrained based on least-squares approach (Sasaki, 2001). Thus, 2-D resistivity resulted by the software depends on resistivity initial model (Khalil and Monteiro Santos, 2011). The Inv2DVLF was developed based on a forward solution using FEM.…”

“…To investigate the subsurface resistivity distribution, we applied the regularized inversion technique (Inv2DVLF software) to acquire the 2-D resistivity model. An environmental resistivity set of 50 Ωm, 70 Ωm, 80 Ωm, 100 Ωm and 150 Ωm has been selected before we choose a 2-D resistivity model with the lowest root mean squares error (RMSE), as suggested by Khalil and Monteiro Santos (2011). Fig.…”

“…The figures demonstrate that the RMSE of NA-MEMD is lower than that of EEMD. It means that the filtered NA-MEMD is easier to invert than the filtered EEMD and the inversion of NA-MEMD, as in Khalil and Monteiro Santos (2011).…”