Geophysical Surveys of Providence Island with Gayflor Wesley

Meet Mr. Gayflor Wesley! Gayflor (Figure 1) is an Honors Graduate from the University of Liberia with a degree in Geological Sciences. He was born in Monrovia to parents from Lofa County, Liberia, he now calls Monrovia home. Since 2022, he has been an invaluable member of the BAHA team. We asked Gayflor some questions related to his work with the team. Read his answers below to learn more about Gayflor’s contribution to the project and his experience using geophysical technologies to learn more about Liberia’s past.

I have been working as an archaeologist on the Back-to-Africa Heritage and Archaeology (BAHA) Research Project since 2022. During this time, I had the opportunity to use advanced GPS equipment to produce a map for the project site of investigation, learn some excavation techniques to excavate artifacts from the soil, and conduct a geophysical survey to locate buried artifacts below the surface. As a recent graduate of the University of Liberia, where I studied Geological Sciences, I am particularly interested in improving Liberia's infrastructure and its environment and using technology like GIS to reduce the impact of natural disasters in Liberia. For this reason, when the BAHA team began deploying remote-sensing methods to better understand the archaeological significance of Providence Island to Liberia, I was inspired to contribute my skills and passion. I have worked with the team for three seasons of research, producing important archaeological knowledge about Liberia’s past. Figure 1: Gayflor Wesley excavating in Crozierville

What is electromagnetometry and what was the purpose of using it to survey Providence Island?

An electromagnetometric survey is a geophysical remote-sensing technique that produces high resolution data of subsurface contexts. The data can point to locations where archaeological materials may have been deposited long ago. Within the BAHA research project, we used the equipment to locate areas on the island that possess buried structures, middens, or artifacts below 2 meters.

Can you describe what we did and how we collected the data?

We collected our data in two different ways: manually (as I will refer to below) and with the CMD Mini Explorer Electromagnetic Conductivity Meter (CMDEM). As for the manual techniques, we set up archaeological units in locations on the Island that were likely used as dumpsites or gathering places by the African-American settlers and the indigenous people who resided on the Island long ago. Within the units, we conducted excavations by digging through the soil using trowels and dustpans to collect soil mixed with artifacts. We later transferred the soil to a screen for filtering. However, excavation is only one of the ways to look for artifacts below the surface. Our geophysical surveys using the Mini Explorer Electromagnetic Conductivity Meter offer a very useful alternative method.

Figure 2: BAHA team members Joe Durrant and Gayflor Wesley conduct surveys of Providence Island in December of 2022.

Firstly, it is non-invasive, meaning we don’t have to dig into the soil physically. All we needed to do was set up and calibrate the equipment to suit our context and research questions - and walk. One operator holds the equipment in their hands and moves in a north-to-south direction (Figure 2). While the operator walks, the equipment collects subsurface data every 0.2 seconds. This process produces an electromagnetic graph of subsurface conductivity, which was used to determine places on the island with artifacts or buried structures that can be targeted in later excavations.

As it relates to setting up and calibrating the equipment to suit our project, it is relatively simple and easy to execute. At first, we started with stationing the base for the Real Time Kinematic GPS and the Roving GPS and linked them to the CMDEM logger. With the use of Bluetooth connectivity, information flows easily from the GPS to the logger for storage. We then created the project file to track the workflow and defined our parameters. We used the deepest “looking” orientation of the CMDEM probe, down to approximately 1 meter, sampling at 5Hz (collecting one measurement in every 0.2 section). After all of the setup, we pressed OK on the CMDEM logger, and hit “go” on the CMDEM probe, and began walking in a south-to-north direction within transects of 0.5 meter spacing, filling in the little map on the logger (Figure 3). To generate a spatial map of the collected electromagnetic data, geophysicist Joe Durrant extracted the data from the logger and transferred it using a USB drive to a computer with ArcGIS installation. He then filtered out any poor quality GPS positions, and then gridded the data using Golden Software Surfer 19 and a nearest neighbor (IDW) gridding algorithm. Minimal filtering was applied - only a despike, which removes any extreme values typically 2 standard deviations from the mean of the data. Finally, a low-pass Gauss filter was applied to remove any noise in the data.

Figure 3: Drone photograph of geophysical surveys of Providence Island by BAHA team members in December of 2022.

What inspired the use of this survey method?

We felt the need to use remote-sensing survey methods to locate and excavate artifacts and structures that may have been deposited long ago due to natural phenomena such as erosion. From a visual observation of Providence Island, we noticed that the landform has changed over time due to erosion and human-inspired change. For the latter, we learned that certain portions of the island were refilled by the government in the 1970s to construct the Gabriel Tucker Bridge, which allows access to the Island and the Freeport side of Bushrod Island. Because of this, we determined that archaeological anomalies would be more easily found in shallow levels within those places on the Island that were not refilled or transported from one place to another and redeposited for a very long time. Using the excavation technique and the electromagnetometry survey, we effectively located the locations on the Island that had artifacts or buried structures that provide material information regarding the lives of the African-American settlers and the indigenous people who resided on the Island a long time ago. As a result, we were also able to uncover archaeological materials from the subsurface that had been hidden since deposition, making it easier to conduct proper preservation and interpretation. However, there is another geophysical survey that could be used for the project apart from what we applied to get our data, for instance, Ground Penetrating Radar (GPR). But this equipment is very complex to set up (requires a lot of wired connections), takes a longer time to collect data, and can’t cover as much space. Providence Island is approximately 11.22 acres, and we needed to complete the project within the fixed time of the field season. Thus, we determined methods that allowed for a straightforward set up process, fast data collection, and effortless data extraction that was not complex to access.

What purposes could these methods serve for Liberia in the future?

We should continue using these methods in Liberia for archaeological research because they are very easy to setup and relatively simple to apply in the field. To locate buried historical evidence and extract it in its original form for proper preservation and interpretation, we need to use gentle and non-invasive techniques in the country. Electromagnetic survey methods satisfy those needs.

For example, the CMDEM does not require a complex wire connection to set up compared to other geophysical equipment used for archaeological surveys, such as Ground Penetration Radar (GPR). Moreover, the data extraction process from the equipment for further processing is effortless and not complex. Additionally, the equipment is not heavy to control in the field and only requires a few people to improve the results of any archaeology survey significantly - at most, two persons are needed to obtain more accurate results. Geographically, Liberia is a tropical area and it experiences a lot of rainfall yearly. According to the Liberian Hydrological Services 2023 report, the average rainfall of Liberia ranges more than 4500 mm along the coast to about 2000 mm in the interior. Monrovia, the country’s capital and one of the wettest cities globally, has an average annual precipitation exceeding 5,000 mm. This is a clear indication that the Liberian environment is not too dense for archaeological assessment, and it is easier to detect ground conductivity graphs and locate archaeological anomalies below the surface.

What did you learn from this experience?

Upon joining the BAHA team, I had no practical knowledge of conducting geophysical surveys - only theories. This recent experience with the CMD Mini Explorer Electromagnetic Conductivity Meter was my first time handling such equipment and understanding its functions in the field. So, I paid vital attention to learning how to set up the equipment, how it relates to the project, and how to apply it practically. The CMDEM has multiple applications and is designed for survey depths ranging from 0.15 to as deep as 60 meters. When using the equipment in the field, the equipment application varies with respect to the project under investigation. Moreover, the equipment can be used for geological and geotechnical surveys, and it can also be used for different forms of environmental monitoring, such as soil profiling as a result of sediment deposition or human interaction with the environment over time. For our case, we needed to calibrate the equipment by rotating the CMDEM probe to 5Hz to have the highest image resolution of the subsurface and define our parameter to 2 meters to stay at a fixed depth. This setup produced a quality reading of the subsurface conductivity and detected buried structures and metal objects down to 2 meters.

Figure 4: Gayflor Wesley and Craig Stevens taking depths of an excavation unit on Providence Island in May of 2023.

What did the data tell us about Providence Island and its many pasts?

The data confirmed that a tremendous amount of trade occurred on the Island between the Indigenous people and the Europeans long before the coming of the African-American settlers in 1822. Additionally, the data provided apparent differences in the materials used on the Island by the settlers and the Indigenous People. These objects assist in explaining some aspects of everyday life on the Island. For instance, the refined ceramics and smoking pipes found on the Island were evidence of the trading that occurred long ago. There is also direct evidence of materials that were brought to Liberia by the African-Americans in 1822. Additionally, the locally-made ceramics and traditional beads found on the Island were used by the Indigenous people for traditional practices. These are some of the information the data tell us about Providence Island’s past.

Figure 5: Aerial view of Providence Island.

Thanks Gayflor! Stay tuned for more posts from members of the BAHA Liberia team.