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Geophysics Methodology 101 – Part Two

Geophysics Methodology 101 – Part Two    

Welcome back! Part One of this section discussed how we picked the areas to survey. The following sections discuss how you actually do geophysics fieldwork. It will be divided into the following primers:

  1. Drawing squares and making grids
  2. What’s that doohickey? Or, getting acquainted with your gradiometer
  3. Walk, walk, fieldwork baby
  4. Murphy’s Law

Ready? Let’s go!

Drawing squares and making grids

Remember this photograph?

Swallowcliffebiggrid

In geophysics, you want your data to be recorded within very specific areas. In our case, it was 30 by 30 meter grid squares. Ideally, you want your grids to be ready before you go out in the field, with all of the grid points input in your GPS unit. Theoretically, the crewmember with the GPS unit will turn it on and the machine will tell them exactly where the points are. The crewmember can then put down bamboo canes to mark the points of the grids. The rest of the team can then begin surveying almost immediately.

You can create these maps in software like ArcGIS (Geographic Information System). Essentially, you take a map of the area you want to survey and make sure that it is properly georeferenced – meaning, that it contains information relating to longitude and latitude. This is important because you want your data to line up properly with your maps when you’re analyzing it later. Creating grids is easy: all you need to do is draw the appropriate grids over the georeferenced map and by appropriate I mean lining portions of the grid along something straight like a fence line, field boundary, or road. Once that’s finished, the data can be stored in your GPS unit and you’re ready to go!

Of course, maps and GPS units can be tricky – just ask anyone with a SatNav! We went out into the field and found that even though I programmed the points into the GPS unit, the map I’d used for georeferencing used the British National Grid, but the GPS unit itself used Universal Transverse Mercator (UTM) points. Oops.

Don’t panic! Not all is lost! This is where our good old friend Pythagoras comes in handy. Do you need to lay out 30 by 30 meter squares out by hand? Use A2 + B2 = C2 , three crewmembers, three very long tape measures, and lots of bamboo canes. Once you get the hang of it, you can lay out grid squares very quickly, and your friend with the GPS can come back along and record those new grid points to export back into ArcGIS with your data for later georeferencing and analysis.

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The takeaway message here is to be aware of your georeferencing systems. The other is to brush up on the Pythagorean theorem because even if your GPS is working perfectly, you’re probably going to end up laying out a few grid squares by hand anyway, because fields are never perfectly square and you’re going to survey sections that are irregularly shaped.

What’s that doohickey? Or, getting acquainted with your gradiometer

Congratulations, you’re ready to do geophysical survey! The only question is, what is geophysical survey? To put it simply: a non-destructive way of looking at what’s below the ground surface. There are many different tools that you can use to get these kind of results, and you’ve probably even heard of some of them: resistivity meters and magnetometers. There are different types of instruments within both of these categories, and each of them has a specialized function. For example, some are better at detecting solid stone structures while others are better at looking at shallow pits and ditches. On our project, we used a Bartington fluxgate gradiometer, which depends on magnetometry. Magnetometers, as the name suggests, measure differences in the natural magnetic field of a particular area. They measure variations caused by human activity, such as the digging of ditches.

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Here is a Bartington fluxgate gradiometer. These machines are ideal for geophysical survey because they do not need to probe the ground and take continuous readings (note that the machine is strapped to the surveyor). The long poles on either end are the sensors, and are programmed to take readings at certain intervals. The gradiometer takes those readings, puts them together, and voila! Raw data for later processing and perusal.

In the above photo, I’m doing something called balancing. Remember how I said that magnetometers record the differences in an area’s natural magnetic field? Well, the first thing you need to do once you’re out in the field is find out the baseline magnetic field for that particular area. Once that’s calibrated, all of the readings the machine takes will be measured against that baseline. And remember how we have three field, Fields A, B, and C? Each field has a different baseline, so the machine has to be balanced each time it’s used in a different field, as well as when you switch surveyors. This is because each person is a different: they’ll carry the machine differently, walk differently, and may have an entirely different magnetic signature than the previous person.

Essentially, these machines are delicate (and expensive) creatures and must be treated like the divas they are. Change fields? Balance the machine. Change surveyors? Balance the machine. Take a tea break? Balance the machine. Go and upload your data? Balance the machine. Do the macarena? Balance the machine.

Now, because magnetometers measure magnetic fields, that means surveyors and their teams must not have any magnetic metals on them while surveying, or it will screw with the data. That’s why geophysical surveyors are usually found in trackie bottoms with elastic waists, plain shirts, cheap trainers or wellies, etc. You’d be amazed how many clothing items have metal in them.

Now you know what your machine does. You’ve put on the appropriate clothing, and you’ve balanced your machine. Time to survey!

Walk, walk fieldwork baby

Remember those 30 by 30 meter grids you put in your GPS, and then laid out by hand? It’s time to start walking them.

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Within each 30 by 30 meter grid square, the surveyor has to start in one corner and walk up and down evenly spaced lines until they reach the other end of the grid. By the time they’ve reached the end of the grid, they’ve walked up and down thirty times, walking roughly 900 meters! Geophysical survey is very good exercise, no? And while they’re walking, the machine is taking constant readings. Most Bartingtons can store data for up to 12 grid squares before they need to be uploaded.

Surveying isn’t just a matter of walking. Surveyors have to walk at a constant pace, ensuring that the machine is taking the right readings at the appropriate intervals. Intervals and other considerations vary from machine to machine and crew to crew, but we programmed our Bartingtons to take multiple reading per meter, at two-meter intervals, in order to create a very detailed picture.

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Walking properly takes practice. The surveyor can set his or her pace in the Bartington, which will beep at a constant rate, just like a metronome. Some people like a faster pace, some like a slower pace. While the surveyor is walking, the other two members of the team are responsible for switching out the lines the surveyor is following, moving them down the grid every two meters. This is so that all the surveyor needs to worry about is walking up and down, up and down, up and down. The other members should also be wearing as little metal on them as possible – if they are, they need to take a few steps away from the surveyor as they approach.

Murphy’s Law

Sometimes, things go wrong. Georeference systems don’t match up, fieldwork is delayed because teams have to lay out grids, and Mother Nature decides to dump a thunderstorm on your head. We faced major delays our first day out in the field because of the mix-up with the coordinate systems. While half the team went out to manually lay out grids, the rest of us tried to figure out what was wrong with the GPS and the points (we didn’t find out about the British National Grid/UTM problem until later). At that moment, an enormous storm rolled in and the entire crew ran for the van. Swallowcliffe Down in on top of a high chalk ridge with little tree cover. The GPS unit sits atop a long iron pole, which would like carrying around a giant lightning rod.

Rain can cause major delays in in geophysical survey. Bartington machines are not waterproof, though they can survive in light rain when the main body of the machine is covered with plastic sandwich bags, and the sensors are covered with bin liners. Luckily, after the one thunderstorm, the weather cleared and even became unseasonably hot and sunny.

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Oh hey there random helicopter, flying low over Field B and scaring the bejeezus out of us. Other memorable distractions included hill-walkers, marathon runners (several of whom correctly identified our fieldwork as geophysics – thank you, Time Team), and the deposition of several very large stacks of hay right in the middle of Field C. Despite a few hiccups, we managed to get there in the end!

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Geophysics Methodology 101 – Part One

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:

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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:

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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!