Tapi Irrigation Development Corporation Tender

Gul Medium Project Tal Chopda Dist Jalgaon

Electrical Resistivity Imaging - The direct current resistivity method has well-established data acquisition and interpretation techniques for standard survey configurations. The method uses pairs of electrodes to inject current into the ground and measure the resulting electrical potential distribution. Its application to dam seepage investigations is two-fold. The method may be used to monitor spatial and/or temporal variations in electrical resistivity in response to changing soil conditions caused by internal erosion and anomalous seepage. The method also may be used to characterize the electrical resistivity of the subsurface for the purposes of interpreting SP data. The method can also be used to detect changes in resistivity with time, which may be linked to the development of internal erosion in the core of the embankment. 2D Resistivity Imaging uses an array of electrodes (typically 60) connected by multicore cable to provide a linear depth profile, or pseudosection, of the variation in resistivity both along the survey line and with depth. Switching of the current and potential electrode pairs is done automatically using a relay box. The computer initially keeps the spacing between the electrodes fixed and moves the pairs along the line until the last electrode is reached. The spacing is then increased and the process repeated in order to provide an increased depth of investigation. In this way a profile of resistivity against depth ('pseudosection') is built up along the survey line. The modelled results are displayed as scaled resistivity-depth pseudosection as illustrated below. Blues represent areas of low resistivity whilst reds are relatively higher. The wedge shape of the plot illustrates the gradual reduction in the amount of data acquired as the current and potential electrode spacing are increased. The method can be applied in two ways. 3. Resistivity investigations as a onetime survey may detect spatially anomalous zones along the dam, and can be used to investigate suspected structural weaknesses. 4. Long-term resistivity monitoring to make use of the seepage-induced seasonal variation inside the embankment to detect anomalies not only in space, but more importantly in time, by studying deviations from the time-variation pattern. The second approach is more powerful as repetition of measurements provides additional evaluation possibilities for seepage analysis. However, the monitoring approach is also more demanding as installations are necessary as well as long-term instrumentation. In the present case, one line of ERI are proposed on NOF portion.

Streaming Potential- The streaming potential method consists of measuring the electrical potential by flowing water within a structure or subsurface. Self-potential (SP) is a passive technique that measures naturally occurring electrical potentials in the ground. This is the only one of these geophysical techniques that responds directly to fluid flow. Water flowing through the pore space of soil generates electrical current flow. This electro-kinetic phenomenon is called streaming potential and gives rise to SP signals that are of primary interest in dam seepage studies. SP is measured by determining the voltage across a pair of non-polarizing electrodes using a high-impedance voltmeter. This inexpensive and deceptively simple data acquisition procedure requires special care and attention in order to reliably interpret and correct for sources of electrical --noise that can mask the signal of interest. All noise sources – including time-varying telluric currents associated with solar and atmospheric activity, stray currents, and the corrosion of buried metal – must be recognized and measured. These noise sources can mask the relatively small signals associated with seepage anomalies. For this reason, telluric measurements and magnetic surveys should be carried out to assist in interpreting the SP data. Typically, SP anomalies on the order of tens of millivolts are associated with seepage anomalies of interest, although anomaly amplitudes largely depend on site-specific conditions. Interpretation of SP measurements to infer seepage patterns and concentrated seepage flows ranges from simple qualitative to more advanced quantitative numerical modeling approaches. Most common application of SP study is to identify the zones in the dam body through which seepage is taking place. The results are correlated with resistivity sections. In present case, one four profile of SP coincident with ERI profiles are proposed.