A geophysical field in the Eastern Amazon

Between June 9 and 24, 2025, I took part in an unforgettable journey in the interior of Maranhão. Our destination was the Formoso, Coqueiro, and Cabeludo lakes in the Baixada Maranhense—a region marked by flooded landscapes, where water and land mix in delicate balance. We went there to investigate traces of ancient submerged stilt villages, in an effort to reveal part of the history of the indigenous peoples who inhabited these shores centuries ago. More than data and images, this experience provided us with human exchanges, collective learning, and moments that go far beyond what can be contained in a technical report.

The work was part of the second phase of the project “O povo das águas” (The People of the Waters), coordinated by Prof. Dr. Jorge Porsani (IAG/USP), and brought together a multidisciplinary team that included Dr. Luiz Antonio Pereira de Souza (Laps), archaeologist Dr. Alexandre Guida Navarro (UFMA), Dr. Leonardo Gonçalves de Lima (UFMA), doctoral student Thalita Borba (IAG/USP), technician Marcelo Stangari (IAG/USP), and myself, representing IPT.

Expedition team: standing – Prof. Leonardo Gonçalves de Lima (UFMA), Thalita Borba (doctoral student at IAG-USP), Prof. Jorge Porsani (IAG-USP), Diego and friend (residents of Lake Coqueiro), Marcelo Stangari (IAG-USP); seated - Prof. Alexandre Navarro (UFMA), myself, and Luiz Antonio Pereira de Souza (Laps!).

Our schedule was intense. We arrived in São Luís on June 9, and the next day we were already on our way to Penalva, our base for the first nine days. From there, we sailed daily to Lake Formoso, where we applied a combination of geophysical methods: GPR, Side Scan Sonar, echo sounding — in addition to photographic records with a drone. Reaching the study areas wasn't always straightforward. Much of the challenge began even before we boarded the canoes: land access to the lake shores required navigating long stretches of unpaved roads, makeshift bridges, and flooded areas. The logistics of transporting the geophysical equipment—bulky and heavy—amid these conditions required not only planning, but also a great deal of determination (and two sturdy pickup trucks).

Part of the road to Lago do Formoso: crossing a wooden bridge on a dirt road, typical of access routes in the interior of Maranhão.

Our first study site was Lago do Formoso, located in the Baixada Maranhense, an extensive floodplain in northeastern Maranhão that forms part of the Amazon biome and is internationally recognized as a Ramsar Site due to its ecological importance. The lake is shallow, with depths usually ranging from 1 to 4 meters, and is embedded in a dynamic alluvial plain shaped by the tropical hydrological cycle. During the rainy season (typically from January to June), the lake expands significantly, while in the dry season (July to December), its surface recedes, exposing large areas of land.

Previous research and local oral histories suggest that human occupation around the lake dates back at least 1,000 years, with communities building their homes on wooden stilts driven directly into the lakebed. Over time, the natural flooding of the region led to the submersion and burial of these structures beneath layers of organic sediments — a low-oxygen environment that favors the preservation of archaeological materials, such as ceramic fragments, wooden posts, and possibly funerary urns.

Geophysical equipment: lots of cables and little space.

We spent five consecutive days on board, facing dense vegetation, shallow waters, and the natural challenges of a flooded environment. Alongside Laps, I was responsible for operating the side-scan sonar (SSS) and the echo sounder, two of the main instruments used to map the lakebed and detect submerged features. Working in such conditions demanded constant coordination, especially given the limited space and the delicacy of the equipment. Our canoe became a compact but efficient data acquisition unit, navigating through narrow, plant-choked channels and open water alike.

Laps and I aboard our small acquisition canoe, equipped with the geophysical instruments used in underwater prospecting on Lake Formoso.

And it is impossible not to mention Eduardo, our local pilot during our days in Formoso. A native of the village of Coqueiros, Eduardo piloted our handmade boat with the same skill with which he whistled his tuneful tunes — always singing, always smiling, making the morning crossings almost poetic. With his steady hand, he guided us through floating macrophytes and narrow channels, revealing the territory he knows like few others.

Eduardo piloting our canoe (possibly for the first time with an assistant instructor).

More than just helping us navigate, Eduardo showed genuine curiosity and enthusiasm for our work. In every pause, he asked about the instruments, their purpose, and the images we were collecting from beneath the water. One moment that stayed with us was when he carefully held the side-scan sonar in his hands — eyes wide with interest, as if realizing for the first time that the lakes he knew so well could reveal even more when seen from another perspective. Moments like this highlight the importance of involving local communities in scientific research. When residents understand the tools and objectives of a project, the knowledge exchange becomes richer, more respectful, and more meaningful. Eduardo wasn’t just our pilot — he became part of the process, contributing with his wisdom and learning with us along the way.

Eduardo, local pilot from the village of Coqueiros, holding the side-scan sonar used during our surveys. His interest in the equipment and the research brought an invaluable human connection to the scientific process. Photo: Laps.

Due to the shallow depth of the lakes and the need for high-resolution data, the vessels used had to be small, which limited space and stability. Conducting geophysical surveys aboard adapted canoes is far from a simple task—it requires organization, patience, and, above all, balance. Every movement had to be calculated so as not to compromise the equipment or the safety of the team.

Geophysical surveys aboard adapted canoes is far from a simple task!

In addition to the limited space, navigating through dense aquatic vegetation made everything even more challenging. Floating macrophytes became entangled in the propellers and sensors, requiring cleaning and inspection of the equipment. Operating geophysical instruments—especially the echo sounder and sonar—in this environment required extreme caution, as any carelessness could damage sensitive components or compromise data quality.

Echo sounder partially covered by aquatic vegetation during the survey at Formoso Lake. The density of floating plants was one of the biggest obstacles to navigation and data acquisition. Photo: Laps.

After Penalva, we headed to Santa Helena, hoping to continue our work at the Cabeludo archaeological site. But, as with any field campaign, unforeseen events are also part of the journey. On June 17, we had planned to conduct geophysical surveys at Lago do Coqueiro. We arrived early, set up the equipment on the boats, and set off for an initial survey of the site. That's when we encountered an unexpected challenge: virtually the entire surface of the lake was covered with floating aquatic vegetation, which made safe navigation impossible, especially with the side-scan sonar and echo sounder.

Widespread and dense presence of surface aquatic vegetation!

Each lake we visited held its own mysteries. Each section of the bathymetric line was another attempt to understand the submerged past. But, deep down, what struck me most were the moments outside the graphs and coordinates—the conversations with the locals, the stories told at dusk, the collective effort amid the humid heat and the constant rocking of the boats.

We returned home on the 24th, tired, with clothes smelling of the river and hard drives full of data. But we also returned with the certainty that there is still much history hidden beneath the calm waters of the Baixada Maranhense — and that it is worth every effort to bring it to light.

Technical team of the project that participated in field activities and the (essential!) support of the residents of Santa Helena.