Geophysical Fault Mapping Using the Magnetic Method at Hickory Sandstone Aquifer, Llano Uplift, Texas

Abstract

A magnetic study over a 95 m x 150 m area of the Hickory sandstone aquifer in central Texas was carried out as part of multitechnique geophysical investigation that included ground penetrating radar (GPR), electromagnetic (EM), seismic and seimoelectric. In geophysical exploration, the magnetic method can be utilized as an alternative to more expensive methods, such as seismic or it can be used to complement other methods. In this thesis, the magnetic method is applied to estimate the location of a previously mapped fault by Texas A&M geology students, and it is used to estimate the magnetic susceptibility contrast of the targeted fault. The main challenge of this study is imaging shallow faults using the geophysical magnetic method in a fractured aquifer with widely-scattered distribution of iron bearing rocks as in the case of the Hickory sandstone aquifer.

A Geometric—G858 Cesium vapor magnetometer was used to collect magnetic data. The data consisted of 19 north-south and 1 east-west lines acquired in October and November of 2012. Elementary data processing such as diurnal correction, regional correction, reduction to pole (RTP) filter, Euler deconvolution, forward modeling and inversion were employed to characterize the faulted zone. This faulted zone separates granite basement rocks from the Hickory sandstone. As a result, this study emphasizes that Euler deconvolution applied to RTP-filtered data increases the interpretability of geological and structural contacts. The results of the magnetic method have been compared to results of GPR, EM and seismoelectric methods. Understanding the magnetic mineralogy of rocks and their properties can improve the geological interpretation of magnetic surveys.