Barisal and the Eastern Strait

We completed a line of magnetotelluric (MT) stations along the Psur River to map the distribution of fresh and brackish groundwater. This electromagnetic geophysical method can distinguish them because saline groundwater is more conductive. Preliminary observations on the data are consistent with more fresh water beneath the shallow brackish groundwater here. Then we head to Barisal and further east, the Strait of Tetulia, where the water is fresher. It took a whole day to traverse the winding passages of the Sundarbans mangroves.

Kerry Huy and Shohan test the TEM device during transport. We will tow the system behind the boat later in the cruise.

We came out of the Sundarbans in Shalancora very close to where I had a GPS station that stopped working. My plan with Sanju was to go there via our speedboat and rejoin Kokilmoni. However, it was foggy again, so we headed to the ghat (pier) instead and took a motorbike taxi for 4km. Masood and the captain also came. Since it was a Friday morning and taxis were scarce, I shared one with Sanju while the others continued to wait.

Some of the motorcycle taxis we use to get to our GPS site.

When we arrived, Bachu, who was looking after our equipment, gave me a big hug. He unlocked the school that had GPS, and we started swapping out the old GPS unit for another. After doing some checks and fiddling with the modem that was initially unable to connect to the network, we left. We had tea at a stall before the extra motorbikes arrived in our caravan back.

Buffalo grazing in open fields is ideal for deploying MT gear. Kokilmoni waits in the background.

We spent the day continuing on to Barisal, where Hasnat and his students joined us, while Sanzhu and Chauhan left us. In Barisal, Kokilmoni refueled and re-supplied. The next morning we started our journey to our first station at 5:45am without fog.

Country boat returning from another successful MT deployment.

While our map shows very little bare ground, we found that the well-known sand island is covered with a lot of grass. While we saw some crops like watermelon, squash and peas, there were cows grazing lazily in many places at the time. Seeing cows grazing now is a good sign. This means that the land is not planted with vegetable crops, but remains fallow. It also means that some green areas on our satellite map are favorable for deployment.

The black garbage bag in the middle of the open field houses the MT recorder and its battery, while the magnetometer and electrodes are buried in the surrounding field.

We are developing a new protocol for finding sites. We picked an area that looked promising about 5 km downstream from the last site. Then, as we approached it, Kerry and I went to Funabashi to find promising sites. Sometimes when we visit them, they are so wet that we need to move to a different site. We have the luxury of choice because we have become so efficient at installing sites.

People cross a bridge made of areca tree trunks. We sailed around in small country boats rather than carrying equipment over there.

On the flat opening, we can go out and bury the electrodes in the north, east, south, and west directions at a distance of 50 m from the recorder. It’s also easier to dig a hole to bury 3 magnetometers in charred sandy soil. The ship’s crew does most of the heavy lifting, handling and digging. Kodals, which are essentially large hoes, are much better than shovels for digging trenches with horizontal magnetometers. The post hole excavator we bring is perfect for vertical magnetometers. We can now install a station in less than half an hour. Sometimes scheduling an overnight deployment with a shift crew takes longer than the installation time.

I set up an active GPS every day and now have a suitable tripod to get the precise altitude for the gravimetric measurements Céline is doing.

To pick up the instruments, we split into two teams. The speedboat picks up two more distant spots and Kokilmoni picks up two closer ones. Now, for the first time, we’re able to deploy 4 instruments a day for two days in a row – and have enough remaining daylight for a swim call.

Take the parts of the MT system back to the boat after waking up in the morning. We paid a guard every night.

However, just when we felt like we were on our way, Cyclone Asani began to form in the Indian Ocean. Located to the south of us, it formed near the Andaman Islands and is expected to make landfall in the Ayeyarwady delta in Myanmar. The wind has only increased a little so far. Still, Kokilmoni is keeping it safe and anchored every night in a protected channel to keep us safe. Then they set sail at 5.30am, and we got back to our starting position for the day.

Enjoy a swim at the end of the day early. For safety, lifebuoys are attached to the boat.

It’s getting harder and harder to find good locations as we work south. For our purposes, the characters here are too low and too wet. On more stable land, we sometimes have to travel a long way to find a good location. For the last stop on March 21, we ended up stopping at a ghat (dock) to explain to the townspeople what we were doing, then walked over to their newly planted fields, using some of the fallow and the bump between the fields Deploy from borders. Since the process took longer, we only completed 3 stops, the last at sunset. Many children and many villagers followed us, fascinated by what we did. Still, it was nice to walk through the village, something we rarely encounter on our boat trips.