Resistivity in Comilla
After returning from Sylhet, I left Dhaka the next morning for Comilla for 4 days to train a group of Dhaka University students and graduates on operating our resistivity imaging system. Many of the conclave people headed out to the Padma River, formed by the joining of the Ganges and Brahmaputra to do sampling for a remote sensing study of the rivers. I will be doing this later in the trip, but had other plans now. The transects of wells that we are drilling provide detailed vertical records of the sediments, but how do we connect the dots when the wells are 3-4 km apart? It turns out we can do it with electricity. Clay and mud has much lower electrical resistivity (or higher conductivity) than sands. The basic technique it to pound two pairs of electrodes (stainless steel rods) into the ground. We then use a car battery to
send a current between one pair and measure the voltage at another pair. The voltage depends on the rock type between the 4 electrodes. For our system, we have 84 electrodes that can be spaced up to 9 m apart and a long cable in 12 sections to connect them. A sophisticated resistivity device then sends current to one pair and measures the voltage at up to 8 other pairs at a time. The device is programed to do measurements with thousands of different combinations. The result is similar to doing and electrical catscan of the earth showing the distribution of sand and mud.
I came to Comilla with 6 trainees, Fayaz, Sojon, Jia, Rabi, Fuad and Paval. During the 4 days, they will work with me to learn to use the system
well enough to be able to carry out these surveys on their own. For the training site, we came to Comilla where to work around the Lamai anticline. It is the westernmost hill sticking up out of the floodplain. To help interpret the structure creating the anticlines, we need to know the dip, or slope, of the folded beds. This has been roughly done from topography, but the exposed topography is partly eroded. We want to image the boundary between the older Pleistocene sediments of the anticline and the younger Holocene sediments that cover them. Both sediments are similar, but the surface exposed during the last glacial period when sea level was 120 m lower has been altered to clay. This should show up as a dipping layer of low resistivity. We will do 4 lines, two on each side of the anticline to image the dipping beds.
Our first day was short because of the time it took to get here. We laid out a short line with 56 electrodes a short distance south of one of the wells that was drilled on the east side of the anticline. The car batteries we took along were not fully charged, so we hooked up Babu’s van to provide more power. That worked well and decided to use the car to run the equipmet the rest of the days. However, that meant the lines had to be where a car could go. Humayun did not join us because his wife has been ill. That meant we didn’t have a GPS to record positions or track where we were. We managed to use the GPS in my camera to get the positions of the line. We moved to our home for the next few days, the Bangladesh Academy for Rural Development or BARD.
The next day we shifted to the west side the next day. Google Earth became our tool for finding sites. With a USB modem we had slow, but continuous internet. We found a set of fields close to the one of our drill sites. We navigated to it by recognizing buildings, mostly gas stations from Google Earth. The fields were fallow, so we could cut across them. Everything was going smoothly. We finished early and I was able to spend the late afternoon teaching the students. That evening we processed the data for both lines. The first one showed the boundary we were looking for to be very shallow. A river had eroded part of the anticline. Thus we were on top of it and not on the flank. Good data, but it didn’t provide us with a slope. The other line had noise problems from a power line, but clearly showed the layer we wanted dipping ~3° to the west. The system was providing hard data.
We still needed a line on the east. We tried our first site, but Babu’s van could not drive to the line location. On to plan B, a country road on the west side. We drove our electrodes into the fields at the base of the road being careful not to disturb the growing vegetables. The data was marred by some power lines, but showed our layer for part of the line. It lined up well with the previous day’s results. For our final day, we need a good line on the east. I picked several candidates on Google Earth. The first was inaccessible, but I quickly found another road and we did it there. Because they were planting rice, the side of the elevated road was not useable. The top turned to not be that good a place. The data was much worse quality than any of the other sites. Still, it gave reasonable results. Humayun and Doug came out, so we interspersed working on the resistivity line with filming. That line competed our work here and the training and we all headed back to Dhaka. The students were trained and we learned how to select good sites.