Not even the rain can stop us

March 8^th brought with it plans to move base camp yet again. This time we would be moving a three-hour drive south from Sámara to Hotel Villa Grace in Cóbano, located near the southern tip of the Nicoya Peninsula. We planned to recover one site along the way, but upon arrival, we found that it had only collected six hours of data before the battery died unexpectedly. We decided to replace the battery and leave it for another day. Then, we headed to deploy the site that we had scouted the day before. This deployment was quick and easy thanks to Gabriel’s unmatched digging skills. Oliver laid out the electrode lines and Alexa buried the electrodes, while I prepared the receiver for recording.

This included hooking up the battery and GPS to the receiver and running a receiver calibration that takes around 15 minutes. While this was happening, I made sure to note the serial numbers of all the equipment we were using, including the electrodes, magnetometers, receiver number, and SD card. We do this to identify any malfunctioning equipment and isolate it if something were to go wrong during the recordings.

I also recorded the distances to each electrode. This step is crucial in allowing us to measure the electric field with the electrodes. We get a recording of the electric field in the north/south (E1) and the east/west (E2) directions. Just as a refresher, for the E1 direction, north is the positive direction, and south is the negative direction. For E2, east is positive, and west is negative.

The MT receivers record the potential difference between the positive electrode and the ground and the negative electrode and the ground (for the ground, we either used a metal rod or another electrode) for both E1 and E2. The potential difference is simply a voltage reading, like when you measure the voltage on a car battery. However, we are interested in measuring the electric field, which has units of volts per meter. Therefore, to convert this potential difference reading into an electric field measurement, it must be divided by the length of that electrode wire, which is why it is essential to record the electrode separation distances in the field.

Once the calibration was finished, and we had all the electrode distances, I removed the SD card and programmed it to record MT data. I made sure to input the electrode line distances, change the recording settings so that we didn’t use up all the SD card memory,  input the type of magnetometers we were using, and then saved these settings back onto the SD card. After this, all that was left was to copy the calibration file from the SD card before removing it from the computer and placing it back into the receiver. Then, I turned on the receiver and waited for it to get a GPS lock (essentially, wait for it to get a lock on GPS satellites so that the receiver can keep a very accurate record of the time). While I waited, I took the time to fill out the installation sheet. This is a spreadsheet we use at every site to record serial numbers, distances, and every detail we need about the site. Once we got the GPS lock, I started the recording. The receiver ran a few tests to make sure all the equipment was working correctly. Then, it spits out some values for electrode contact resistance and DC voltage, which also had to be recorded in the field notebook and the spreadsheet. If the resistance values were less than 10,000 ohms (better still, less than 1,000 ohms) and the DC voltages less than 0.1 V, we were good to pack up and leave the equipment to do its thing.

After our deployment, it started to rain. Rain is unusual this time of year in this part of the world. Unfortunately, our kitty litter in the bed of the pickup truck did not fare too well.

Progress after March 8th:

The issue of the rain persisted into March 9^th where it “soiled” our plan to deploy early in the morning. The team consisting of Samer, Luis, David, and Gabriel went to recover several sites and swap batteries at the semi-permanent site in Hojancha that required way more driving than would be preferred (nearly four hours round trip!). They also would be dropping David off in his hometown (which was conveniently close to one of their recoveries). College would be starting soon and David’s short time with us was sadly coming to an end.

Alexa, Anna, and Oliver had planned a deployment in a nearby mango farm, but this had to wait until after the rain cleared. While waiting we went to a nearby restaurant for lunch.

Deploying in the rain itself is not fun, and it gets even less fun when you consider that around half of the roads outside of towns on the Nicoya Peninsula are dirt roads that quickly turn into mud when water is added to the mix.

These can quickly become mud pits for your car to get stuck in if you are not careful, thus we decided to err on the side of caution. We managed to deploy later that afternoon when Samer’s group returned. The mango field was gorgeous, and the digging was quite easy. This farm is intended for exportation so we were not allowed to take any of the mangos for ourselves, which was a little sad but also this was the least we could do given that the landowners graciously gave us permission to dig holes on their property.

Progress update after March 9^th: