Keep investigating: Watermelon Zephyr
The landscape is changing as we travel south on the M/V Kokilmoni along the Tetulia Strait in southern Bangladesh.The northern part of the channel contains many character (Shahe Island) is ideal for deploying our MT (Magnetic Terrestrial) instruments. We’re using them to image groundwater off the coast of Bengalades. In many places, groundwater is salt water, but beneath the salt water is fresh water from the last ice age. Since the conductivities of fresh and salt water are very different, we can use electromagnetic techniques to map them.
Kerry directs the MT unit on a tripod in a watermelon field.
As we continue, character Give way to farmland. We are always looking for fallow land with cows grazing on the grass. Now we find very few devices that are dry enough. However, we found several cow fallow fields surrounded by crops. We were able to put the central recorder and battery in a fallow surrounded by 3 magnetometers that had to be buried. Electric electrodes are placed 50 m away in the 4 cardinal directions, but they are small enough to be buried without affecting crops.
One of the ship’s crew members, Russell, dug a hole for the electrodes between the plants.
We were worried about Hurricane Assani, a rare March hurricane, but it made landfall well to our south. Nonetheless, instead of the 5km wide Strait of Tetulia, we stayed every night in the small protected passage. The weather looked worrying one morning, so Kokilmoni stayed put until the weather cleared around 11am.
Kerry checked the instrument readings before turning off the recorder.
We stopped looking for fallow land as we continued downstream. Everyone grows watermelons on all available land. Fortunately, the farmers agreed to let us deploy in their watermelon fields. The fields are cut by irrigation canals in which we can bury our 3-foot magnetometers. There is also a gap large enough between the plants to locate the recorder and electrodes. In addition, in the mature fields, we also got fresh watermelons. You can’t get fresher than eating a freshly picked watermelon in the field where it was grown. Farmers give us watermelons several times a day.
Adrian handed me a fresh piece of watermelon.
And just like that, everything went smoothly. We deployed 18 sites in 5 days. Then the weather changed and it was very windy. The wide fairway we’re travelling on leads into the Bay of Bengal and 20 knot winds are whipping up white waves. After being picked up in the morning, we entered a more protected side passage.
A fishing boat sails against the wind.
The country boat we tow rides the wind and waves. The installation site is too windy.
While waiting for the weather, I installed a sports GPS on a geodetic monument I visited before COVID 2020. The monuments were installed in 2002 by the Bangladesh Bureau of Investigation. Re-measurement of them in 2020 revealed how much they had receded in the 18 years in between. Since we were nearby, we went and installed it for another measurement.
Hasnat, Huy and Masud stand next to tripods with GPS antennas above the monument. The equipment box and solar panel are at the front.
We stopped for tea at a local shop and returned to the boat after picking up the GPS. The owners helped us in 2020.
The wind looks likely to persist for a few days, but as we approach the Bay of Bengal we need the MT station here to observe the transition to salt water. It’s already lunchtime and we haven’t installed any new stations yet. In Bangladesh, you have to be flexible and tough at all times. We have turned to the Andamanique to take shelter in the wide main channel. We decided to keep going along its route and follow it out to sea. It is much calmer in this narrow river in the middle of a large island. We managed to get to both stations before dusk.
Country boats parked on fallow fields beyond the dike.
Huy and Masud cross the ridge connecting the embankment and the land opposite it with the MT equipment box.
As we get closer to the Bay of Bengal, the water is getting saltier. We can see salt deposits on dry soil. Lush watermelon fields have given way to fallow paddy fields. They can grow rice here only when monsoon rains and flooded rivers bring fresh water. During winter and spring, the land is fallow. There is no lucrative watermelon crop here.
Céline uses her parasol for gravimetric measurements.
Kerry takes a selfie with some local girls. They were standing on the dry wreckage of a fallow paddy field.
The next day we continued along the Andharmanik River and set up the last 3 stops of the transect. On the last stop, we took a country boat along a small river channel full of fishing boats. The access is just a few kilometers from Kuakata Beach. We have reached the end. We even sailed past some mangroves near the coast, but thought open fields would be better than going through the forest again.
The land in front of the embankment was eroded. Often, new land is being planted on the other side of the island, reflecting the constant change of the island. Still, it’s a good step forward for carrying the device.
We have completed 23 MT stations along the transect for a total of 44 stations. There will be more work to do near Khulna at the end of the cruise, but I’ll be off the boat until then. We will now switch to a different type of electromagnetic measurement, which will be towed behind the boat.
Fishing boats are tied to the embankment secured with sandbags.
