Design of digital geo-electrical equipment and its application on noise level reduction


Electrical resistivity survey is used to explore subsurface layers in hydrology, geology, mining, geotechnical and environmental investigations. The resistivity measurements are normally made by injecting current into the ground through two current electrodes, and measuring the resulting voltage difference at two potential electrodes. Electrical resistivity is a function of injection current, measured with difference potential and geometrical factor of electrodes. From the current (I) and voltage (V) values, an apparent resistivity (?a) value is calculated as ?a=KV/I, where K is the geometrical factor. Increasing the distance between two current electrodes caused an increase in the depth of penetration. To determine the true subsurface resistivity, layers thicknesses, an inversion of the measured apparent resistivity values must be carried out by means of a computer program. The ground resistivity is related to various geological parameters such as the mineral, fluid content, porosity, saline and degree of water saturation in the rock.
One of the new developments in recent years is the use of 2-D electrical Imaging/ tomography surveys to map areas with moderately complex geology (Loke & Barker 1996). Chambers et al (1993) use a multi-electrode system and dipole-dipole array for prospecting oil and gas. The results were satisfactory. El-Qady and Ushijima (2001) were used neural networks and inversion of DC resistivity data to interpret deep sounding electric. Busby (2000) has used azimuthally apparent-resistivity measurements for determining fracture strike orientations. Christiansen and Auken (2004) have made a dynamic system for detecting lateral variability.
Recording waveform of resistivity data can be done through moderate seismic digitizers. Distinguishing basement type fault by means of this system is very important and significant in seismology, especially in areas of large cities.
Electrical noises have an important role in the accuracy of geo-electric data. These noises can originate from geology conditions, self- potential, induction polarization, power electrical cables, and underground railways, pumping engines and electromagnetic fields. In the first step, most noises have been identified and the effects of important noises have been investigated which can be reduced using different field array and processing data. One of the most important ways to decrease the gain of noise is the recording of electrical data in digital way. To do this electrical data have been measured in digital waveforms in different parts of Iran. Then we applied different filtering tools to increase gain of signal to noise ratio. Results of our research showed that using electrical waveforms leads to an increase in the precision of measurements from 1mv (in analogue measurements) to 0.01 mv.