Mapping near-surface structures in a geophysical test site using magnetic and electromagnetic induction gradients

Abstract

We applied Magnetic Gradiometry (MG) and Electromagnetic Induction (EMI) techniques to map and differentiate the geophysical response of 17 buried structures at <2.50 m deep in the Teoloyucan Geophysical Test Site (TGTS). To enhance the response of near-surface structures, we include the Electromagnetic Induction Gradient (EMIG) and complement MG and EMI acquisition and processing methodologies. This includes, acquiring and adding together orthogonal-transect surveys, computing the EMIG and sectioning area maps into close-up maps. The orthogonal-transect surveys clearly show distinct distributions of the physical properties resulting from the acquisition direction, the orientation of the transects and from the orientation and heading of the sensors. Compared with EMI, EMIG provides remarkable results for apparent conductivity although limited for in-phase data. For MG and EMIG, close-up maps yield better results than area maps. Also, adding the orthogonal-transect survey grids gives better results, followed by the E−W transect surveys and then by the S−N transect surveys. Most of the mapped structures present low-medium MG values and medium-high EMIG values. The proposed methodology is consistent for concrete and air-cavity structures, and for structures made with various materials and complex geometries. In some cases, the structures do not have enough contrast with their surrounding terrain and in others, the anomalies are shifted in <1.50 m. In this study, we demonstrate the convenience of computing the EMIG and the benefits of applying the used methodology to retrieve near-surface features that would be overlook with common-practice methodologies.