Detection of fracture zones in mountains using semi-airborne magnetometric resistivity method

Abstract

Detecting hazardous geological objects in mountains is of significant importance for the construction and maintenance of infrastructures, like tunnels and bridges. Conventionally, hazardous geological objects can be delineated by surface-based geophysical methods. But those methods can have difficulties because of severe topography and other obstacles of accessibility. In this study, a new approach, semi-airborne magnetometric resistivity (SA-MMR), is proposed to detect water-rich fracture zones in mountains without any field work on the mountain. SA-MMR, a variant of the classical MMR, establishes steady-state electrical and magnetic fields by a pair of grounded electrode, and measures the total magnetic intensity (TMI) data in the air. The combination of steady-state fields and TMI data has a significant advantage in data quality because the data are less interfered with by high-frequency cultural noises and are not sensitive to the rotation of airborne magnetometers. In order to study the feasibility of SA-MMR, we design a mountain model containing a low-resistivity fracture zone. The positive and negative current electrodes are placed on the opposite sides of the mountain. TMI anomaly 5 m above the topography is simulated on a data grid over the entire mountain area. Our modeling results show that if the current electrode pair is aligned with the strike of the fracture zone, a 10 nT anomaly highlighting the location of the fracture zone can be observed. However, an electrode pair perpendicular to the strike direction of the fracture zone only generates an anomaly of a few nT’s. Therefore, it is critical to deploy sources at multiple azimuthal angles in field surveys. Our synthetic feasibility study serves as a reference for the design of SA-MMR instruments and field operations.