new light on old sites

Geophysics Methodology 101 – Part One

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There were two major questions we needed to consider when it came to tackling geophysics for Swallowcliffe:

  1. How do you pick a place to do geophysics?
  2. How do you actually do geophysics?

We’ll walk through each of these questions and really break them down.

Question #1 – How do you pick a place to do geophysics?

As you can see from our previous post on the site’s location, the area we’re looking at is pretty large. English Heritage scheduled an area covering three separate fields:

Swallowcliffe_Down_1945_Field names

Dr. Clay’s 1920s excavation of Swallowcliffe Down took place in an area that overlaps Fields B and C, and parts of Field A were also mapped and possibly explored (the original excavation can be quite unclear sometimes!). When Elizabeth and I made our initial site visit in Spring 2014, we found visual evidence of the site in the lower right-hand corner of Field B:

OLYMPUS DIGITAL CAMERA

It’s difficult to see, but there’s a slight, semi-circular ridge a few meters beyond the fenceline. English Heritage also recorded a series of undated dyke systems at the opposite end of Field B. We also found a raised earthwork in the upper right-hand corner of Field A:

OLYMPUS DIGITAL CAMERA

For these reasons, we decided to prioritize the lower right-hand corner of Field B, all of Field C, and as much of Field A as possible (focusing on the upper right-hand corner). We wanted to survey the dyke systems on the left-hand side of Field B as well, but there were several issues with that:

Swallowcliffebiggrid

We’ll come back to this grid in Question #2, but for now, I would like to point out that each yellow square in the photo above corresponds to a 30 by 30 meter grid square. Therefore, Field B covers an area of about 93,600 square meters. Our team would only consist of six members working two machines in teams of three, and we would only have a week to complete the geophysical survey.

This is why we had to prioritize the specific areas that were mapped in Dr. Clay’s original report (the areas marked in red). We promised ourselves we would survey as much of the center and left side of Field B as possible, depending on how quickly we would get through the area of the site itself and how the data looked. While it would be great to find new things outside the site, we wanted to find the things Dr. Clay missed within the site – specifically, Iron Age roundhouses and four-post structures.

So, now you know how we picked the site. Coming up next: how to do geophysics!

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