I am wearing $28,000 of sensitive electrical equipment and sinking deeper into the rice paddy with every step. By this point, its pretty apparent that I am going to need some help.
I am carrying a device called a gradiometer. It measures anomalies in the earths magnetic field. Our team is using it to look for Mongol shipwrecks buried somewhere (we hope) beneath meters of mud and rice. The gradiometer’s cesium sensors pick up fluctuations caused by buried iron – perhaps corroding armor or fasteners in the hull of an ancient ship – and heated material, such as brick, burned rock or ceramics
The device is really expensive, meaning: don’t get it wet. To make things worse, there are a number of people watching, including a pack of laughing Vietnamese children. Vietnamese children always seem to be laughing or smiling, even when they are working hard. At this point, they are running circles around me, almost if they could run on the top of the water itself.
The rice field is inundated with runoff from the irrigation ditches. Some places you can maintain your footing. This was not one of them. For the third or fourth time today, a slurry of dirty water was threatening to come over the top of my rubber boots. Normally, I am not bothered by a little mud – but this is rural Vietnam, and the water is teeming with parasites that will knock you flat if you get them in your system.
I sigh. “Veronica – I am going to need some help.” My partner has been standing back so that she doesn’t interfere with the sensor readings. She returns the sigh with the air of one much put upon, and makes her way over. I shouldn’t be stuck… again. In my defense it is hard to judge the consistency of the muck before you put your foot down. Plus, the equipment adds some 45 lbs.
Veronica reaches down, grabs my rubber boot, and pulls my foot out of the mud with a squelch. As she pulls, she sinks in a little further, so its my turn to pull her foot out. Then my foot, then her foot and my foot again… in this way we crab our way back to firmer sludge.
We have been surveying this field for hours. The process is arduous and repetitive, but I relish being in the field, doing archaeology. A magnetic sensing survey is like mowing the lawn. We walk back and forth across the paddies, each transect a few meters apart. As we move, a GPS receiver on my heavy pack maps my position. To make sure we walk in a (somewhat) straight line, we follow cues from a head-up-device called a swather. It’s mounted on an arm that extends over my left shoulder, and reminds me of the targeting apparatus Luke Skywalker used to take pot shots at Death Star.
Its been a long morning. I am exhausted and my shoulders hurt; moreover, getting stuck is evidence that I am making poor choices about my survey path. Its time for me to switch out and let Veronica take over. This is an unnecessarily complicated process.
The gradiometer is cumbersome. I am carrying a two meter boom with a pair of cesium sensors mounted on one end and a heavy counterweight on the other. A tangle of wires connects the sensors to a computer hanging around my neck. I am wearing a non-magnetic frame pack with a Trimble GPS antennae, transceiver and swather, along with a pair of ridiculously heavy batteries. Without the sensor pole, the pack weighs some 35 lbs – and I am sure it was designed by an engineer who really disliked archaeologists. The crew calls it the “The Rack of Pain.”
Transferring the entire rig without setting it down (in the mud) is a tricky dance, especially for only two people. I let the counterweighted end of the boom rest on a dike and hand the computer to Veronica for safe keeping. She steps behind me and lifts the pack while I unclip myself and step out. I balance the boom, take the computer back, grab hold of the frame and rest it on my knee while she flips around front side and into the harness. She clips back in, takes back the computer, slips the boom into position and powers back on.
While the sensors are on, they sing. The pitch goes up when the gradiometer senses a strong magnetic field, and down when the field changes. When you find a really strong magnetic signature, the device sounds like a robot in estrus. It terrifies the heck out of most of the Vietnamese children. I convinced some braver individuals to wave their sickles at the device (yes – the kids run around with sharpened sickles). The tone changes when the blade is close, and drops when they take it away.
While we are in the field, we can “feel” out an anomaly by passing the sensor rod back and forth over them. The pitch rises and falls, and we can get a sense of the anomaly’s shape and strength. We laugh about it – but when all is said and done, mag survey is high tech dowsing. We are led by spirits in the earth.
Special thanks to the entire Bach Dang crew: Randy Sasaki, Jun Kimura, Dr. Mark Staniforth, Dr. Le Lien Thi, Britt Burton, JB Pelletier, John Pollack, Dave Ross and especially Minh, without whom we would never have been able to find our way around!