3D Effect and countermeasure of 2D geoelectrical imaging of a subsurface linear structure

Abstract

This study was prompted by a field observation of a resistivity anomaly in a 2D Electrical Resistivity Tomography (ERT) scan during a fault investigation, suspected to be an artifact from an unnoticed 3D effect. Numerical simulations explored potential 3D effects when the survey line is not perpendicular to the strike of a linear feature, violating the ideal 2D assumption. Artifacts similar to the observed field anomaly arise when the survey line is oriented at an angle <60° from the strike. This 3D effect pattern, resulting from an oblique survey line, is quite different from the known mirroring effect of off-plane objects. Previously overlooked in practical applications, this phenomenon could lead to misinterpretations. A comprehensive investigation considering various influencing factors clarified the conditions and behaviors associated with this 3D effect. Further numerical studies of pseudo-3D surveys propose a practical guideline to avoid this effect with minimal effort. When field conditions raise concerns about surveying at an acute angle to the strike, adding one more survey line on each side of the main survey line, offset by two electrode spacings, is recommended. The 3D inversion of this narrow 3-line pseudo-3D survey can accurately resolve the in-line resistivity section.