Depth estimation of buried targets using integrated geophysical methods: comparative studies at Ahmadu bello university geophysics test site

Abstract

Accurate prediction of depth and position of underground structures is a critical step in structural foundation surveys such as civil engineering excavations to adequately maintain the existing underground utilities. The study presents the results of comparative studies conducted to evaluate the performance of electrical resistivity tomography (ERT) alongside the VLF-EM method regarding depth estimation and location of buried targets of known materials, properties and dimensions. A laboratory test was carried out on the buried targets to determine the electrical properties of the buried targets before burial. The pre-burial geophysical investigation indicates no major anomaly within the site that could influence the geophysical response of the buried objects significantly. The results of the post-burial geophysical investigation indicate high variations in electrical resistivity values varying from 47 Ωm – 1081 Ωm (before) and 0.113 Ωm – 19,879 Ωm (after) the buried targets, while the VLF-EM data indicates that the current density values within the site were significantly influenced due to the presence of buried materials, confirming major influence or distortion of geophysical signature of the site. In post-burial ERT investigation, the Wenner and dipole-dipole (DD) arrays registered 67 % and 80 % degrees of alignment with the actual depth of the buried targets, respectively. Both Wenner and DD arrays show strength in depth estimation. However, the DD array indicates higher strength in terms of depth estimation and it is potentially suitable for near-surface utilities investigation due to its high precision in depth estimation. In comparison, VLF-EM captured six (6) out of eight (8) buried targets with a 47 % degree of alignment with the actual depth of the buried targets, which is far lower than the ER method, and may not be considered the most preferable method for geophysical prospecting where depth estimation of targets is of prime interest. However, the depth of targets varies from one method to another and one array to another.