By Megan Metcalfe, Senior Marine Geophysicist at Wessex Archaeology
We know large areas of the Gulf of Mexico were once dry land and that these areas may have been home to Palaeoindians many thousands of years ago. The issue for archaeologists is how can we find these past landscapes when they are now covered by tens of meters of water? The answer is marine geophysics.
An example of different SBPs and their associated acoustic signals. Image courtesy of Wessex Archaeology. Download larger version (jpg, 257 KB).
When looking for submerged landscapes below the seafloor, the main geophysical tool we use is a sub-bottom profiler (SBP). Typical shallow water systems include boomer, pinger, and chirp. An SBP sends out an acoustic wave, some of which bounces off the seabed and the rest which penetrates the seafloor. When this acoustic wave hits a sediment horizon below the seabed, some of the sound bounces back. The time it takes for these acoustic waves to return are then used to calculate the depth at which these sediment horizons can be found. A horizon can be a change in the sediment type, for example between a layer of rock and a layer of sand. It can also show where there is a major change in angle and character of deposits, such as the cut of an ancient river channel, which has been filled by mud and sands.
In order to give us the best chance of discovering submerged landscapes in the Gulf of Mexico, we have decided to use two different SBPs. The first is a Chirp, which is a relatively high-frequency SBP that is known to reliably produce sub-seafloor data in the survey area. The second system is a parametric sonar.
To understand the significance of these two different systems, we need to take a quick side-step into the world of marine geophysics. Different SBPs will use different frequencies to map below the seabed. Lower frequency, higher power SBPs (such as boomers) can penetrate greater depths but are lower resolution, which means that we see less detail. Higher frequency SBPs use a lower power which means they can only penetrate shallow depths, but they provide a higher-resolution image, meaning they produce a more detailed picture of what’s below the seabed. Choice of system comes down to what question it is you are trying to answer and what sediment types you are likely to encounter.
Unlike other SBPS, a parametric sonar uses two different frequencies which are emitted at the same time. These two frequencies then interact with each other in the water column to produce a lower frequency signal with a very narrow bandwidth. This lower frequency signal gives the penetration, but without the interference from surrounding objects that you may have with other SBPs. To use an analogy, if you imagine a standard SBP were a lamp then a parametric sonar would be a flashlight; both will help you to see features, however a parametric sonar gives you a more focused view. This, along with a high sample rate, means that a parametric sonar can produce a much more detailed image, which in turn means we can map much thinner layers of sediment. Although parametric sonar data have been used to map submerged landscapes in the North Sea and United Kingdom (UK) waters, they are less commonly used in the United States for the investigation of past landscapes. That’s why my colleague at Wessex Archaeology, Louise Tizzard, and I have flown over from the UK to be part of this exciting project.
Wessex Archaeology is an archaeological company based in the UK. At Wessex Archaeology, we have carried out a variety of projects using parametric sonar. For example, at Area 240 in the North Sea where a combination of parametric sonar and a boomer interpretation were used to map the gravel-rich river channels in which 250,000 year old flint artifacts had been recovered .
Evans (left) and Metcalfe (right) monitor acquisition of the parametric sonar data onboard R/V Nikola. Image courtesy of Paleolandscapes and the ca. 8,000 BP Shoreline of the Gulf of Mexico Outer Continental Shelf. Download larger version (jpg, 3.4 MB).
For this project in the Gulf of Mexico, the Chirp and the parametric sonar will be used at the same time. Our hope is that these two systems will complement each other; the Chirp system will be used to get an overview of the old river systems, which may have been a desirable place for humans of the past to live. The parametric sonar will hopefully give us a more detailed image of any small, individual features or units which may be of archaeological interest, such as terrace features or possibly even shell middens. A combination of the two systems gives us an even greater chance of discovering more about this past landscape and will help us to decide which areas we want to target for further investigation.
