Electrical resistivity is another survey method we will employ this season in Week 3!
When you were in primary school science (and if you were really clever, secondary school physics), you probably encountered several lessons on electricity. You may not have gotten as much firsthand experience out of the formal classroom lessons, say, as someone who licked a battery or stuck something in an outlet, but then again we all have our learning styles. (DO NOT DO EITHER OF THOSE THINGS. WE’RE SERIOUS.)
Electricity is clearly a Big Deal in modern society, as we rely on it for everything from light bulbs in our flats to the smartphones in our hands. Electricity is also part of the human body, conducted by our cells (through a little bit of chemistry involving ions), telling us about stimuli, reacting to stimuli, and making our hearts pump. But in every circumstance, it all comes down to subatomic particles: protons in the nucleus of an atom are described as positive (+), while electrons that encircle the nucleus are described as negative (-). Electrons can get moving between adjacent atoms of conductors like copper, and zap! You’ve got an electrical current. We’ll get back to this in a mo, just keep that image of flowing electrons in your heads.
The equipment we are going to be using has two metal electrodes 50cm apart that you stab (gently) into the earth at predetermined, regular intervals along a measured axis. Most of our team has done this insert-take readings-repeat process along a surveyor’s tape-anchored grid system, recording a reading or two every metre across fields, animal paddocks, and the occasional golf course so far in our careers, but not yet at Bamburgh! Once you’ve done one whole length of the survey area, you do the same distance again, this time just sliiiiiightly (a metre) to the left or right. Eventually you will cross the length and width of the survey area multiple times without tracing over your previous footsteps. This builds up a picture of the subsurface one grid square, and then strip, at a time. (There’s another pair of probes outside of the survey area which we will talk about further when we actually see the full setup of the equipment that the Soton team is bringing; unhelpfully, a “twin probe array” like the one we are expecting actually has 4 prongs in the full system: the mobile pair and the stationary pair.)
Wait, wait, wait…but what are the machine and the various electrodes actually doing? The rig is sending an electrical current through the soil between the prongs. Conductivity means the current flows relatively easily; resistance is therefore the opposite of conductivity. How conductive the earth or buried features may be is heavily reliant on moisture content and the arrangement of pores within the medium the current is meant to pass through. If the current passes through with little resistance, features like pits and ditches that have been filled back in with material may lie under the surface. Areas of high resistance may represent buried foundations, trackways, or rubble.
This system is adaptable to a degree: you can configure it to work at varying depths by changing that initial spacing of the mobile probe pair, but this also changes the resolution of the data we are gathering. Even though we are conducting currents, the readings are not going to be skewed by metal artefacts below the surface. This is however a slower method of survey than GPR because you must stop and prod regularly rather than walk comfortably rolling an array along each row. Also, it’s not ideal in winter surveys because of the amount of prodding into frozen soils.
It is surprisingly difficult to get hands on the output of resistivity surveys from other sites to share as examples, so for now take a look at our last resistivity survey here. Figure 4 shows you the output from the processed data. It takes a lot of training to be able to read and interpret these surveys, so please don’t be put off by that initial black and white image. Figure 5 shows you the interpretation by Dr. Groves and their team. The small high resistance areas (light blue) could be stone-lined graves, while the small low resistance areas (light green) could also be graves, but only filled with soil. The other larger blue anomalies seems to be dune-related, while the large linear green anomaly could be a soil-filled ditch.
Nota Bene (yes, we did get a little Latin up in here, simply meaning “note well”): We often point to 18th-century American mega-flirt Ben Franklin as the first mainstream electrical experimenter, but knowledge of electrical phenomena such as shocks from electric eels (weirdly enough, not truly eels but closer to catfish) and static generated using amber can be found in sources as far back as the Egyptians in the Bronze Age for the former, and Archaic Greece for the latter. Franklin was studying lightning when he confirmed it was a type of electrical discharge, but he was building his experiments on the work of scientists from the century prior; one such scientist was William Gilbert who wrote extensively on magnetism, but disbelieved its connection to what we call electricity. Gilbert was not only physician and philosopher to the stars (Elizabeth I), but also the first scientist to posit that we ourselves are living on a giant magnet. More on that when we get to magnetometry! We now know that these two forces are united through electromagnetism: an electrical current generates a magnetic field, creating a basic force of nature that affects things as small as individual atoms to as large as our whole planet.
This blog is brought to you as part of our 2023 grant-funded activities through the generosity of the Castle Studies Trust.
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