Deployment in the mangrove forest
The first two days went very well, so we were ahead of schedule. On the third day, we started by going up a small river between the Sundarbans mangrove forest and the inhabited land, watching monkeys playing on the bank on the forest side. On the sedentary side, we have found our field open. At this site and the last before it, some of the rice fields have recently been planted with watermelons. This region grows the best watermelon in Bangladesh and it is their most valuable crop. Yet there were enough fields that were not yet sown for us to succeed.
We spotted our first monkey along the shore of the forest.
The second site of the day was more problematic. We couldn’t enter the Sundarbans until the next day, so we have to work on the last colonized lands on the left bank. When we drove into what we thought was a fallow field, it turned out to be a large construction site with backhoes moving around the imported sand. After what felt like an eternity in the scorching sun, we finally found a site in two dry rice ponds far enough away from heavy machinery for good measurements. By the time we finished it was too late for a third site. Still, I hiked almost 7 miles for both sites.
Our group on the field boat with equipment and guards heading to a site.
In the evening, the speedboat traveled to Mongla to drop off Anwar and a journalist traveling with us and pick up Celine Grall and Sanju Singha. She is my former postdoc and brings material for further scientific measurements. Sanju worked with me extensively on my GNSS (GPS) work. We will visit two sites later on this trip.
We settled into a routine of breakfast at 6, pick up and deployment, lunch at 12:30-1, deployment plus and dinner at 7:30. The food is a buffet like the lunch seen here.
To save time, we split up in the morning for site recovery. I returned to the first site with the speedboat, while the campaign boat took people to the second site. On the way back we picked up our two rangers and permission to work in the Sundarbans mangrove forest. With only two sites to recover, we were ready to start the deployment early and managed to install 4 sites.
Our speedboat had engine trouble and had to be towed by a country boat to a ranger station where our crew managed to fix it.
Deploying in the Sundarbans, the largest mangrove forest in the world, is different from working in populated areas. We simply pick locations the right distance downhill and enter the forest. We hike through the bush using deer trails where possible and try to find enough clearing to set up the center receiver location. For magnetometers, the challenge is to find a spot that is correctly oriented and free of tree roots, especially the aerial roots that allow mangosteens to live in salt water.
Nafis uses a compass to direct the people deploying the electrode, while he can still see them.
The electrodes have become the slowest part of deployments. Having to go straight N, S, E or W we couldn’t follow the trails but had to go straight through the trees and brush. We also can’t see 50m through the forest and had to find more creative ways to position the electrodes, like having intermediates with compasses. For the distance we have to use a tape measure because the laser rangefinder will not work. We also needed one of the armed guards to accompany the group to the electrodes because of the risk of tigers. At our last site of the first day, we came across some pugmarks – tiger tracks – probably from that morning. We saw pugmarks, some old, at about ¼ of our sites.
Part of the dense forest that we have to cross to access our sites. And bottom goran the forest is even worse.
Despite this, we manage to install 4 stations in the Sundarbans on the first day. Only having to retrieve 2 sites from the previous day gave us more time to install new stations. The next day it was the other way around. We had to take 4 stations. We were able to split into two groups by picking up two extra rangers, but they had to go back to their post after pick up. The afternoon passed very slowly because the goran the mangrove species at this site formed a very dense forest. We had to hack our way through with axes.
Installation of a vertical magnetometer among the thorny aerial roots of the mangrove.
Since tigers like this type of forest, we had to be extra careful. Our crew makes lots of loud noises to help keep the tigers away. As there are a lot of deer here, the tigers in this part of the Sundarbans are not very aggressive and the noise is enough to scare them away. At least we hope so. Almost all tiger attacks are further west than where we work. However, we have established a clear security protocol. Everyone stays in a group of at least 4 people with an armed guard. We make sure we have everything we need because one person cannot go back alone to pick up something that has been forgotten. We continually make a lot of noise.
Sanju’s hand next to one of the tiger marks we found.
Once a day we also deploy a gravimeter that Celine brought with one of my precision GPS. Although GPS accuracy with all trees is low compared to my geodetic work, it will be accurate enough for gravity measurements. Generally, Sanju and I install the GPS, while Céline does the gravimeter, all with an armed guard watching over us.
Céline takes a gravity measurement next to our GPS antenna mount and makeshift instrument box while a guard watches over us.
We had hoped to have 4 guards, instead of the two with us. However, we were able to get a third (and once a fourth) guard almost every day. As always in Bangladesh, we adjust our schedule according to the difficulty of the sites and the number of guards we have. On some days we can separate for a much faster retrieval of instruments. A group takes the speedboat to the most remote sites, while the field boat and the Kokilmoni cover the other sites.
One morning we discovered that one of our magnetometers had been dug up by a wild boar. Luckily, we logged enough data before that happened.
In the evening, after field work and showers, we watch the wildlife in the forest until it gets too dark. We saw deer, a crocodile, a wild boar, egrets, a stork and monkeys – but no tigers. For two nights we were able to stay at one of my favorite places in the Sundarbans near the place called Kokilmoni which our boat is named after. The small channel offers us a beautiful view of both banks.
Our group emerging from the forest after a successful instrument retrieval.
We posed for a group photo at one of the Sundarbans sites.
I also learn to differentiate the different species of mangroves. We try to avoid density goran forests, preferring the most open Gewa Where sundari Forest. Our maps do not differentiate the type of forest, so we sometimes sail along the coast in search of the most open forest. At one site we unsuccessfully took the country boat up a tidal creek to find more open forest. At our last site in the Sundarbans near the Bay of Bengal, we fanned out on a slight sandy upwelling of the mudflats. It shouldn’t flood until spring tide in a few days, but we’ve put the recorder and batter in plastic bags on top of the metal box just in case.
Arriving at low tide, we struggled through thigh deep mud to get to the forest of this site.
The Golpata palm lined tidal stream which we unsuccessfully ascended in search of more open forest.
On our last morning in the Sundarbans, we were delayed by thick fog. We had to wait for the fog to dissipate before we could collect the last 3 pieces of equipment. The speedboat crew who visited the last site found that the slight rise with the recorder and magnetometers remained dry, although some of the electrodes were under water, which is fine. In the end, we made 12 MT stations for four days on the Sundarbans. We then cruised through the Sundarbans towards Barisal, finally enjoying most of a day off during transit.
A group leaves by speedboat to collect the material from our last site of the Sundarbans.
Sunset over the Sundarbans mangrove forest.
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