2.75-D ERT - Zigzag Electrode Acquisition Strategy To Improve 2-D Profiles
A. Robbins and A. Plattner
Event name: 23rd European Meeting of Environmental and Engineering Geophysics
Session: Best of SAGEEP
Publication date: 03 September 2017
Info: Extended abstract, PDF ( 621.78Kb )
Price: € 20
Although 3-D electrical resistivity tomography (ERT) has been available for more than two decades, its widespread use has been limited by higher data acquisition and processing costs compared to standard 2-D ERT. Alternatively, the viable method of processing 2-D profiles with a 2.5-D approach has overwhelming popularity due to ease of data acquisition and processing. However, 2-D profiles do not account for resistivity variations perpendicular to the profile. This limits the retrieval of valuable information and may lead to biased resistivity profiles for subsurface objects that intersect obliquely with the survey line. In principle we could use 3-D processing to calculate a resistivity solution from the 2-D array. Unfortunately, this leads to inversion results that are symmetric with respect to the profile because the sensitivity pattern for each measurement shares this type of symmetry. We propose an acquisition strategy that has the simplicity of a 2-D profile in terms of work in the field and equipment requirements, but overcomes the symmetry issues of classical 2-D profiles. Rather than along a line, we arrange our electrodes in a zigzag pattern of alternating +/- offset along the y-axis. This approach, which we dub “2.75-D ERT” can be implemented by simply shifting the electrodes away from the center profile in an alternating pattern and does therefore not require any additional equipment or setup in the field. The resulting data needs to be processed with 3-D electrical resistivity code. With modern computers and software this does not pose an obstacle anymore even when only moderate computing power is available thanks to free high-performance programs such as BERT or E4D. In a field experiment, we compare the results of a 2-D array to a zigzag array both transecting a known target at an angle. Unlike the solutions for the 2-D array, our zigzag array captured the known target’s asymmetry with respect to the profile.