On our third day of fieldwork (February 24, 2023), we planned to install one site and pick up our first deployment. Our installment of the day was in a sugar cane field owned by a large company called CATSA. The farm was a major industrial operation and after a security check, we were guided to our site location, passing endless fields of sugarcane. The field where we would install our site had recently been burned to remove the debris left after the recent harvest, so it looked very post-apocalyptic and smelled like burnt molasses. The ground was covered in dark ash and the wind was howling with a vengeance, meaning we were all completely covered in soot. We quickly got to work!
An MT deployment consists of deploying 4 electrodes to sense the electric field and 2 magnetometers to sense the magnetic field these components connect to the central console which is the actual MT receiver. To start off our deployment, Darcy usually orients us and we begin walking out the electrode lines.
We deploy the electrodes like a compass, one in each direction, north, south, east, and west. The electric field is measured in both the north/south and east/west direction. In order to do this, we lay out a cable that connects the receiver to the electrode. This is usually around 50 meters on all 4 sides. This creates a 100 meter spread between the north-south electrode pair and the east-west pair.
These electrodes (shown in the diagram above in red) must be buried and kept moist to ensure an optimal reading of the electric field. To keep them damp we use clay-rich kitty litter that we put in reusable bags. and add water to each hole. The clay in it works well to retain the water and keep the electrodes damp for the duration of recording even in super dry and hot areas (like Nicoya!).
The magnetometers (shown in the diagram in blue as induction coils) must also be buried (the locations shown in diagram). These must be level with one pointing in the east-west orientation and the other north-south to record the magnetic field signal.
Once the electrodes and magnetometers are buried, its just a matter of hooking up the battery to the receiver and calibrating the receiver. The last step is to press start on the recording after we program the receiver to record data! And that’s it!
For this site, we used rocks to hold down the cables to decrease wind noise in the data and, despite the windy conditions, successfully deployed our fifth MT site of the field campaign.
As I detailed in my last update, we had been taking the first few days at a slower pace and working with all 5 of us together. But now that everyone was getting the hang of the workflow, we decided to split up after our first deployment at the sugar cane field so that one team could go check the battery level at the permanent site and the other team would go recover our first deployment site.
This seemed like a great idea until the recovery team (Darcy, Anna, and Gabriel) immediately got lost. We spent around 20 minutes driving in circles on some extremely bumpy dirt roads before locating our actual deployment site. We had taken the wrong dirt road and none of us even realized! Without the homeowner and his dog as our guides we were hopeless. Yes, we did have an exact GPS point of the location but after you drive down like 5 dirt roads, they all start to blend together. Google Maps is not the most reliable on rural dirt roads in Costa Rica and we learned that the hard way, when it lead us into a dead end of bushes and told us to keep driving. After finally locating the site—which was actually down a completely different network of dirt roads than the one we falsely remembered it being on—we quickly recovered the site. Recovery takes significantly less time than deployment because you only remove items from holes rather than dig the holes. Funny enough, we spent less time recovering the site than we spent driving around to find it!
This was our first recovery, meaning it was our first chance to see if all our hard work was worth anything. The moment of truth. The instrument was still running when we arrived. That’s a good first sign. After doing a quick QC of the data, they looked a bit noisier than we expected (perhaps due to shorter electrode lines constrained by a relatively small fenced area), but still completely useable. We have collected usable data! This is always a big relief at the start of any field campaign after all the planning and preparation (and hard work digging!).
February 24th progress update:
Yellow marker: currently recording. Green marker: recovered stations. The yellow house is our hotel in Santa Cruz.