Estimation of top depth to underground targets of Karous-Hjelt and Fraser filtering of VLF-EM measurements: The Thumb's rule approach

Abstract

Prediction of the exact location and depth of underground targets with the very low-frequency electromagnetic (VLF-EM) technique is one of the most important and difficult tasks in geophysical investigations. This study examined and compared the conventional 2D KIFFILT inversion pseudo-section and the use of Thumb's rule technique in the Fraser filter plot to estimate the top depth of underground targets. The VLF-EM measurement was performed over several empirical buried target models to identify anomalies or geophysical responses corresponding to subsurface targets. The Karous-Hjelt and Fraser filtering techniques were applied to estimate the depth of the identified anomalies using Thumb's rule and the conventional 2D KIFFILT inversion. The signal behaviours of the VLF-EM current density pseudo-sections and the application of Thumb's rule effectiveness in delineating empirical buried target models were examined. Thumb's rule shows 65 % accuracy with the actual depth of the empirical buried target models, while the conventional 2D KHFFILT inversion shows 30 % accuracy with the actual depth of burial. Thumb's rule is more effective and precise in predicting the accurate depth of underground targets. The influences of conductive and resistive materials on VLF-EM signals and the challenges of VLF-EM surveys were discussed. Thumb's rule is suggested as a substantial technique for estimating top depth to the underground target where depth estimation is of prime interest due to its large degree of accuracy. In addition, the total depth of the current density distribution was noted to be increased when the distance between measuring points increased. This means that VLF-EM signals with longer wavelengths indicate deeper depth penetration into the ground than signals with shorter wavelengths. The accuracy of Thumb's rule regarding top depth estimation of the anomalies has been successfully tested and validated, which can be used for VLF-EM investigation where accurate depth estimation is required. The VLF-EM technique can be considered reliable for depth estimation using Thumb's rule approach, which applies to a wide range of subsurface investigations.