Tea Garden Rescue
We are installing magnetotelluric (MT) stations around northeastern Bangladesh to image deep below the surface. Everyone goes out for a few days at a time. They use solar radiation to probe beneath the Earth’s surface. We need long wire electrodes in north-south and east-west directions to record the electric field, plus three magnetometers pointing north, east, and perpendicular. These instruments are very sensitive to pickup effects from several kilometers below the surface, and they need to be kept away from any power lines. In densely populated Bangladesh, this is difficult to do. Power lines and roads criss-cross. We want to image the depths of large thrust faults, the depths where large earthquakes can occur, and the thickness of sediments. Maybe we can image the bottom of the crust, but that’s unlikely to be disturbed. We are giving up the use of harvested rice fields because we are not far enough from the power lines.
A woman picks tea leaves at the Surma Tea Plantation. Plants remain waist-high for easy picking.
The first site we tried in Sylhet in northeastern Bangladesh was where we had previously installed the seismic station. It is located on a tea plantation (plantation) on the side of an anticline mountain. Sylhet is where sediments from the Ganges-Brahmaputra delta fold into hills through the long-term movement of the giant thrust fault below. Tea trees grow in well-drained soil, so they are mostly planted on the flanks, and sometimes on the top of the anticline as well. Tea also fills the valleys only near the Indian border where the land slopes upwards. Sylhet, with its active hills and valleys, is where most of Bangladesh’s tea is grown. Geology and tectonics are also why we are here. For me, this is my eighth visit to Sylhet.
Women pick tea in the fields next to our deployment. They don’t repackage in another 7-10 days, which is much longer than our deployment time.
The tea garden manager was very welcoming and we walked around in the heat with the assistant manager until we found the right spot. With acres of tea trees, it’s easier to find a spot away from power lines. We started deploying our instruments, but we couldn’t complete the deployment until Samer’s team picked up from the last two sites west of the Meghna River. With all the broken magnetometer cables, we don’t have enough cables without cables from other sites. It was a tough day due to the need to dig trenches and bury all the cables, although the tea garden crew did a lot of digging. By the time the others arrived, we were exhausted and let them finish the scene.
Biplab completed the excavation of the perpendicular magnetometer, while Shofique, Oliver and the people of the tea garden watched.
Our new paradigm is being deployed in tea gardens. We also buried all the lines in shallow trenches. Enough to keep foxes from nibbling on them. We shortened the distance between electrodes from 100 meters to around 50 meters. Just burying about 130 m of cable is enough – 230 m is too much. Our pattern is now an L instead of an X, with two long and two short electrode cables. While longer times are better for measuring electric fields, it is also more sensitive to localized electrical noise.
A worker was transferred from taking care of the young tea trees we deployed to dig trenches for us.
The next day, I thought the weather was bad, so I stayed in the hotel, and the two teams visited multiple locations, including another tea garden with an earthquake station. The permission for the afternoon tea garden was passed, and they began to deploy. When a few people returned to the hotel to get more equipment, I joined them to end the deployment, although it took a little while to find them among the acres of tea trees. As we were leaving, we saw a fox, our nemesis, on the side of the road.
A view of our second tea plantation. The trees provide shade for the tea trees.
The tea pickers “two leaves and one bud” when picking.
It was pouring rain the next morning, so I used this time to prepare my lecture for the next part of the trip. We have four American students, four other Bangladeshi students and more professors coming and we will be running a small field school while continuing the MT investigation. They will gain hands-on (and dirty hands) experience primarily in cool mornings and afternoons during lectures and field trips. In the afternoon, I continued to prepare, and the two teams went to scout. For dinner we had some jackfruit we bought during our scouting. Fresh jackfruit is great.
Masud and a tea plantation worker put the magnetometer cable into the trench before covering.
The next day was drier and we took the equipment from the first tea plantation and installed it in the third tea plantation further east. Trenching worked. All cables are intact. I am increasingly optimistic that we will get enough data to make an impact. This time the deployment was on an open meadow in the tea garden, with cows grazing nearby, and the steep slope of the tea hill. As before, the workers in the tea garden did most of the trenching work. Using kodal, they cut into grass and soil and flipped a section in one or two hits. Even two 40- to 50-meter cables won’t take long. In the evening we had a fresh salad made by a driver with more jackfruit.
Biplab and Arman took the MT equipment box to the beautiful third tea plantation site.
It rained again the next day, and we took the time to fix the broken wire that was bitten by mice and foxes. In the afternoon, Samir and Masood went to scout. I continue to focus on my presentation preparation. That night, it was fresh pineapples and oranges, and delicious lemon juice from Bell. That’s one reason to come to Bangladesh this time of year – all the tropical fruits are ripening and I want to be able to taste them most nights.
A delicious plate of jackfruit, the national fruit of Bangladesh. There can be 100-500 cements, each containing a large seed.
The next morning, we all went to the fourth tea garden on the anticline east of our hotel. This was by far the most beautiful one as we drove through the valley surrounded by steep tea slopes. We had to walk a kilometer from our car to the site because the car could get stuck if there was a sudden downpour. However, it is easy to pull the cart and enjoy the scenery at a slower pace. We took our time and when we were done the driver greeted us with a big bowl of fresh pineapple.
A one-minute movie through a tea garden (while retrieving equipment from a van). Click to play (I apologize for the dirty footage).
Now, Babu and I take off tomorrow morning to Dhaka to pick up students and professors for the field school. Everyone else stayed on to continue reconnaissance, retrieval, and deployment of MT. We stopped at Ujan Vati where we stayed a while ago for a quick lunch at 3:30. We arrived in Dhaka in the evening. In the morning we met or picked up Bangladeshi students and professors, then picked up Americans from the airport and drove back to Sri Mongar in two cars, this time stopping at the same place for lunch after 3pm. Tomorrow we go from pure MT to a mix of MT and field school with 11 students, 5 professors and 5 drivers.
Lunch on the way back to Sylhet without new students.
