Remote Sensing

Remote Sensing in Archaeology

Some of the most significant innovations to archaeology have been in the realm of remote sensing. Remote sensing refers to the ability to collect information about places or objects at or under the surface of the ground, usually without having to disturb the ground

Authors: David Cranford, Assistant State Archaeologist, Raleigh Office
Chris Southerly, Deputy State Archaeologist-Underwater, UAB

 

Though archaeologists tend to focus on people and their technologies from the distant past, the cutting-edge tools we use today are anything but ancient. Advances in everything from computing power to satellite imagery and global positioning systems (GPS), to drones and LiDAR (Light Detection and Ranging) have revolutionized how we do archaeology. These new technologies help archaeologists in identifying and investigating archaeological sites both on land and underwater.

Some of the most significant innovations to archaeology have been in the realm of remote sensing. Remote sensing refers to the ability to collect information about places or objects at or under the surface of the ground, usually without having to disturb the ground. While many remote sensing technologies are mounted on aircraft or rely on satellites in space, others can also be used directly on the ground surface or in the water. Some of these techniques measure subtle changes in the earth’s magnetic field or map differences in the soil’s electrical properties to determine where people once lived, cooked, or left their trash.

One type of remote sensing that is transforming the practice of archaeology is Ground Penetrating Radar or GPR. GPR works by transmitting tiny pulses of high-frequency radio waves into the ground and then recording precisely, in nanoseconds, the time it takes for those signals to reflect off objects or contrasting layers in the soil and return to the antenna. The longer it takes the signal to bounce back, the deeper the item or feature is. As a GPR system is pushed across a site, all the different reflections are recorded and displayed as vertical slices of the ground. When GPR data are collected in a grid pattern, these lines, or transects, can be stitched together to create a 3-dimensional map of the site which can be sliced horizontally at different depths to visualize what is below the ground.

GPR Equipment
OSA archaeologist using GPR equipment to locate underground features.

Archaeologists often use GPR to map cemeteries where some of the graves may not be well marked or easily visible from the surface. In other cases, GPR can help figure out where buildings once stood or how deep a site is buried below the ground.

Because water and land are different in texture, density, and chemical makeup, water requires different types of remote sensing technology. The most common instruments used by underwater archaeologists are marine magnetometers and side-scan sonars. Both instruments are usually towed behind a boat by a cable connected to a computer onboard. Data are combined with GPS locations to create an accurate “image” of the survey area.

Magnetometers accurately measure the earth’s background magnetic field in units called nanoteslas (nT). Submerged or buried ferrous or ferromagnetic materials (iron and other magnetic metals) distort or warp the earth's magnetic field. The magnetometer can detect these distortions, and the closer an object is to the sensor, the more it distorts the magnetic field and the easier it is to tell that something is present. These distortions tell us something is there but not what is causing the anomaly, or abnormal reading. It might be a lost crab pot, or it could be part of a shipwreck. Using side-scan sonar can help determine what an anomaly is if it is not completely buried.

Side Scan Equipment
Side Scan SONAR equipment

SONAR (SOund Navigation And Ranging) sends out a micropulse, or ping, of high-frequency sound waves and measures the time delay of the echo coming back to the sensor, detecting the location of objects underwater. The method is similar to the GPR but uses sound waves rather than radio waves. Modern digital side-scan sonars send multiple pings per second out in a fan shape to both sides of the sensor. The returning echoes are processed and recorded by special software that generates a 3-dimensional image of the ocean floor and what is resting on it. Depending on the frequency of sound used, this technology can be accurate to within 1½ inches!

OSA archaeologist using side scan sonar
OSA archaeologist using side scan SONAR to document a shipwreck.

Images:
-OSA archaeologist using GPR equipment. Image by NC Offce of State Archaeology
-Side Scan SONAR equipment. Image by NC Office of State Archaeology
-OSA archaeologist using side scan. Image by NC Office of State Archaeology