What’s up, Xplorers?!
The word “geophysics” comes from “geo,” meaning “earth,” and “physics,” meaning “natural science.” So, geophysics is basically just the natural science of the earth. It’s all about observing natural properties like electricity, magnetism, and to actually see what’s underground before we even pick up a shovel.
If you’re anything like us, you’re thinking, “That still sounds intimidating as heck!”
We get it! You’re definitely not the only one who had a rough physics class in high school. (Thanks Mr. D***.) The good news is: Geophysics does not have to be scary! Keep in mind — we’re not trying to understand all the little nuance-y, science-y things today, okay? This post is all about understanding how we can use geophysical techniques to see beneath the soil without digging.
Thankfully, we have as our guest captain today the brilliant and talented Mr. Steve Filoromo! Steve is TerraX’s resident geophysical expert and lead historical archaeologist, and we’re glad he’s here to help us out. Are y’all ready to get started? Good, because together, we’re going to gain a good understanding of what geophysics really is.
Get Ready to Feel a Spark
The first geophysics method that TerraX uses is called “electrical resistivity.” In English, electrical resistivity is about using an electrical current to look through the soil.
It’s actually really cool! Basically, when he’s running an electrical resistance survey, Steve will place these fancy probes in the ground. Then, he’ll run an electrical current through the ground and get a read out of the voltage at different depths within the soil, and the different levels of voltage can tell us quite a lot about what’s underneath the surface!
It may sound complicated, but it’s really not. Think of it this way: The more tightly compacted the soil is, the faster it can conduct electricity and the higher the voltage will be. On the other hand, the more loose the soil is, the slower the electrical current will pass through and the lower the voltage. By knowing where the soil is more tight and where it isn’t, we can learn all sorts of things!
To illustrate, Steve gave this example,
“When you were in college, did you ever see footpaths on campus where the grass had been trampled? In those areas, because of all the years of feet trampling over the ground, the soil gets more compacted. If we ran electrical resistivity tests there, we would see higher resistance in those walking areas. In exactly the same way, we can use electrical resistivity to see where walkways might have been, and that can tell us about pathways between or through pre-contact Native American Villages! Alternatively, it is possible to see other importance features like house floors, architectural ditches, and middens–which are far more common in our area”
Pretty cool, right?! By using electrical resistivity, we’ve been able to map out villages with incredible accuracy. We can understand exactly how and where people walked on the earth long ago, and that teaches us so much about their various lifeways.
And houses and walkways aren’t the only things we can find with electrical resistivity. These tests can help us detect pits that may have been used for cooking and other features common to archaeological sites. It’s an incredibly effective method, and can be completed with only minimal disturbance to the earth.
Archaeologists Are Pretty Magnetic, Don’t You Think?
Now that we’ve gotten in our goofy joke for the week, let’s talk about magnetometry! This geophysics technique uses — you guessed it — magnetism!
Have you ever been watching TV really late at night and come across one of those infomercials for metal detectors? The ads always have footage of people who look like they could be your weird great uncle walking around on beaches, metal detectors in hands, looking for jewelry, watches, coins, keys — basically anything metallic that has magnetic properties. While the products shown in those long, annoying infomercials are usually not super powerful, the truth is, they’re not too far off from what archaeologists actually use.
By using special magnetic devices, called “magnetometers,” archaeologists like Steve can identify all kinds of magnetic features and metallic objects. Often, we use magnetometry to find middens,, architecture, and building materials, to name a few, since they’re commonly made of metal or have magnetically enriched properties. We can find objects like nails and old tools this way, and they can tell us a lot about the types of structures that were once built on a dig site as well as how and when they were constructed.
Don’t trip, though, magnetometry is useful for much more than finding old rusty nails in the ground. We can also use magnetometry to discover grave sites that may or may not have above-ground markers.
Unfortunately, one of the realities of archaeology is that sometimes we do find human graves and cemeteries that haven’t been properly marked. It’s essential to treat such sites with the respect they deserve, so we want to avoid disturbing graves as much as possible. It would be horrible to think that we could go about conducting a regular shovel test and accidentally discover a burial site! So, by using magnetometry, we can sometimes discover the burial locations, ensure they’re properly protected, and avoid digging in those areas.
Ground-Penetrating Radar — Possibly Our Favorite (#SorryNotSorry)
When it comes to something as important as grave protection, however, we don’t want to leave any technique out. Possibly even better than magnetometry is GPR, or ground-penetrating radar.
GPR is one of our absolute favorite techniques because it is so minimally invasive yet it can see so deep into the ground. Here are the basics: We have a special piece of machinery that can send electromagnetic signals through the soil and measure reflections between that help map the depth and location of the interface between soil layers and archaeological features back. Objects — including burials — will appear on our read outs as changed in the frequency of the reflected signals. Depending on the soil and its conditions (how wet, how much clay there is, and more), we can detect even a few meters beneath the soil — that’s pretty deep! And it’s incredibly helpful when we’re searching for unmarked graves. GPR can help us locate burials so we can avoid them in our shovel testing.
GPR can also be useful, however, in special cases when we need to actually move graves. Sometimes we come across unmarked burials that, for whatever reason, were not interned in a marked cemetery. In such cases, out of respect for the deceased, we send in our cemetery experts and, with the permission and supervision of the living descendants, will carefully remove the graves and rebury them in appropriate locations with markers denoting the individuals who have been buried. By searching beneath the soil, we can pinpoint exactly where we need to dig in order to carefully remove the burials and completely avoid causing damage to any human remains. This technique is critical for the appropriate handling of graves and grave materials, and at TerraX, we take that responsibility seriously.
GPR is great for cemetery location, but its applications in archaeology can be virtually endless. We can use GPR to find just about anything you can imagine might be found underground! We can use GPR to find architectural features like basements and other construction features, which can inform us about the buildings and lifeways of people who used to live on our dig sites. We can use it to learn more about the stratigraphy — or layers — of the soil, which can tell us about the environmental history of the area and the changes that have occurred on the land over time.
Of course, GPR isn’t perfect. According to Steve, “There can always be ‘noise’” — or strange reflected signals that appear in our readings but that don’t always signify the presence of anything archaeologically significant. However, it’s still an incredibly important and helpful tool, and we’re grateful for the ability to use it!
It’s true, y’all. Geophysics, as a field, is an incredible innovation. It combines all our knowledge of both geology and physics, and by putting it to work for archaeology, we can discover some incredible new things that would have been inaccessible to us before.
With electrical resistivity, we can map out ancient villages and literally walk down the same pathways our ancestors used long ago. Through magnetometry, we can unearth materials from long-destroyed buildings and understand how humans lived long ago. And with the power of ground-penetrating radar, we can accurately discover the locations of countless archaeological and historical sites including, most importantly, unmarked cemeteries, which allows us to take care of graves with the respect and care they deserve.
At TerraX, we could not be more excited about the opportunities we have because of these techniques, and we ABSOLUTELY could not be more grateful for the expertise of our geophysics wizard, Steve.
Thanks so much for joining us today, Xplorers! We hope you learned a little bit more about some techniques you may not have heard of. If you have any questions, you know where to find us! But just in case, we’ll drop links to our Facebook, Instagram, and LinkedIn accounts here. Feel free to reach out — we’d love to hear from you!
Until next time, keep learning, Xplorers.
— The TerraX Team