Geophysics of a War of 1812 Fort

Ground-penetrating radar (along with other kinds of remote sensing) is an emerging field for archaeologists, and it's very exciting. It's also kind of complicated! Happily, Peter Quantock, a graduate student at the University of Denver who is completing his Master's Degree in archaeology, is here to explain it all to us.

Geophysics at Fort Hollingsworth
by Peter Quantock

In recent years, geophysical survey has become an increasingly popular archaeological tool in Maryland.  Archaeologists at sites such as Port Tobacco, Nottingham, and Benedict conducted magnetometry and ground-penetrating radar surveys as part of their investigations. In Jim Gibb’s 5 part series about Fort Hollingsworth on this blog, he talked briefly about the geophysical techniques that were used to help locate the fort.

Today, I’m going to give you a little more information on the ground-penetrating radar (GPR) survey conducted at the site through images of the data. GPR is a technique that collects and records information about the subsurface.  It involves the transmission of high frequency radar pulses from a surface antenna into the ground.  This data is converted into vertical profiles and plan view maps to aid in interpretation.

GPR Gear: GSSI SIR-3000 with 400MHz dual antenna and survey wheel used to conduct the survey at Fort Hollingsworth.

In March 2012, with the help of ASM members and other volunteers, I conducted a GPR survey over top of the magnetometer survey I did the year before.  The goal was to find the trench used at Fort Hollingsworth.  And we were successful! Read all about it in the "Finding Fort Hollingsworth" series on this blog.  

To interpret GPR data we first go to the vertical profile, which allows us to look at the depth of features in the ground.  Below is an example of a vertical profile collected at the site.  This profile runs north south within the larger grid.  It looks like a bunch of sharp, squiggly lines, doesn’t it?  

Yes, it does!

What I see is much different…I see depths and distances of features underneath the ground.  The areas where the black and white reflections are stronger in contrast to the area around it (circled in red in the image below) tell me that there is something of interest there.  These are called high amplitude reflections and they indicate buried features.  THIS is what the fill from the trench looks like in a radar profile!  

These vertical profiles are collected along each transect that is surveyed within the grid.  Our grid was 50m x 50m and we had transects at an interval of every .5 meter.  That gives us 100 vertical profiles to interpret.  We can then put all of these profiles together to create a plan view map.  These are called slice maps.  They are “slices” in time, each representing 10ns of depth (or about 10cm in depth).  The slice map below represents the data from about 30-40cm below the surface…right where the trench fill was located! It's not exactly an underground photograph, but the linear shape definitely suggests the outline of the earthwork we identified here.

The GPR survey conducted at Fort Hollingsworth was a success and great fun!