March 17th was an ambitious day for everyone: we planned to recover three and deploy three stations. This was planned to be the last full strenuous day for us, after which we would be doing mostly recoveries, aside from one deployment, until the 19th. We knew we could power through, especially with the promise of fieldwork ending a day early so we could head to San Jose for a Successful Field Campaign party.
The team of Oliver, Alexa, and Anna would be recovering two and deploying one, while Samer, Kennet, and Gabriel would be recovering one and deploying two. Any team with Gabriel on it was automatically the stronger of the two teams which is why they would be doing the two deployments rather than us. Our first recovery was smooth, but we could not say the same about the second. When I went to check the data at the site, there was no data to check! The only thing on the SD card was a file that I was not able to open and the file from the previous test run we had completed. One possibility is that the battery died before it had any chance to collect data. It seems that the batteries we bought were not very reliable. We did some light problem-solving and replaced the battery before heading on our way to scout for our deployment of the day.
We scouted in a very mountainous region where there was a large gap in our data set. On our first try, we met the nicest family. The husband, Jose, was the son of the landowner for the parcel of land we were interested in. He rode with us to guide us to their home and after Alexa did an amazing job of translating the project objective, we got permission and were on our way. We wanted to install it that day, but while scouting out the area, it began to rain. We would have to walk down a steep hill to get to the flat area we selected for deployment and walking downhill with heavy equipment while the ground is wet and muddy is a quick and easy way to hurt yourself. We opted to wait out the rain. Jose and hisfamily graciously offered to host us for coffee and pastries while we waited. Within the next 40 minutes, the rain stopped, and we were able to knock out a deployment. Once everything was completed, I checked the resistivities and I noticed that the AC voltages were wildly swinging up and down. This was super unusual as normally the AC will settle out after a few seconds. Normally a high AC voltage means you are near a powerline, a water pump, or some other large source of noise, but in this case, we couldn’t see anything that would cause this kind of AC spike. A high AC reading normally means you will find yourself with some very noisy data, and with fieldwork ending soon, we would very likely not have time to reinstall if this was the case.
I called Samer to ask if there was anything I could do to fix this issue. Samer asked me if I was hearing any thunder nearby, which at first, I thought was a weird question but quickly realized he was asking because lightning striking close to the receiver will cause it to record sporadic jumps in AC voltage. He said this would eventually level out and the quality would improve once the storm passed. The processed data appeared to be clean from our small test run so we headed back to the hotel for dinner. On our way, we stopped to say goodbye to our kind hosts, and we were greeted with a huge bag of oranges to take with us on the road.
Local lightning can become a source of noise that saturates our MT signal (and even potentially damage equipment!), but on a global scale, lightning is a source of electromagnetic signal for the MT method which provides higher frequencies (>1 Hz) than other magnetospheric sources. The lightning acts like a powerful antenna, pumping out electromagnetic radiation in all directions similar to a large radio tower except at much, much lower frequencies than your local FM radio station which operates in the MHz (megahertz) range. The EM waves produced by a single lightning strike can revolve around the globe around 7-8 times, because the atmosphere is very resistive, and thus the signal weakens slowly (we call this attenuation). The EM waves bounce around between the relatively conductive Earth and conductive ionosphere (approximately 100 km above the surface), constrained within the resistive atmosphere. This “bouncing around” results in particular EM frequencies known as the Schumann resonances between 7 and 60 Hz. Because there is always lightning activity occurring somewhere on the globe, this EM source is constant and can be used as a natural source for imaging with MT in addition to other, lower frequency sources.
That evening we were joined by a very special guest for dinner! Marino Protti, the director of OVSICORI (the acronym stands for Observatorio Vulcanológico y Sismológico de Costa Rica in Spanish which translates to the Volcanology and Seismology Observatory of Costa Rica). Marino is a collaborator on this project and he was recently recognized at the 2022 AGU meeting as a role model in international collaboration. Working with Dr. Protti, it’s easy to see how he was recognized for this. We were so thankful he was able to join us as we reached the final stretch of our field campaign!