COMPARISON OF ID AND 2D RESISTIVITY INSTRUMENTS TO IDENTIFY THE DC SHIFT FOR POSSIBLE CORRECTION

Abstract

ResistIvity technique is one of the commonly practiced geophysical methods in Sri Lanka to delineate subsurface geology and find water in the near surface. Over the years ID resistivity application become one of the major geophysical applications of GSMB to accomplish many subsurface explorations. With the limited applications and cumbersome maneuvers of the ID system, 2D versions of the same become more advantageous over the former system. The 2D system is generally known as Resistivity Imaging. However, the user has a limited control over the 2D method as the data inversion is almost automatic. In order to utilize two systems altematively there should be a common agreeable platform that enhances the confident level of two systems. With this objective is in focus, the following criteria has been adopted to compare and contrast the two systems in order to recognize their differences if any. A stretch of 50m, undisturbed and fiat ground with possible vertical resistivity variation has been selected as survey site. One-D system, ABEM 800 SAS, was deployed in the field during two working days and data were collected according to Schlumberger Array, Vertical Electrical Sounding (VES), with station interval of 4m. The collected data, apparent resistivity values, were arranged to a xyz file and a pseudo apparent resistivity section has been synthesized with data interpolation methods. The 2D system, AGI Advanced System, has been deployed along the same stretch overlapping the ID survey and 2D data were collected with 2m electrode spacing in the same array type. With the aid of 2D inversion software, a system that built-in to the AGI 2D System, raw data were converted into true resistivity values. One-D resistivity data, which acquired along the line at 15 VES locations, were converted into true resistivity values in RESIST ID inversion software. Both true resistivity values were plotted on the 2D inverted results for comparison. The results reveal that there are shifts when comparing both the resistivity and depth values of two systems. The shift can be identified; henee, possible corrective measure will be introduced with further analysis. However the 2D system is more accurate as we get the low RMS value than the ID system. In 2D system the depth control and penetration depths are limited compared to ID system.