Gadgets used in Garrido property investigation: "ground-penetrating radar," magnetometers

Authorities are using an assortment of technologies to analyze the contents of property belonging to Phillip Garrido, the accused rapist/kidnapper whose alleged abduction and abuse of Jaycee Dugard is the subject of previous Boing Boing posts. Bone fragments have been found on the patch of land in Antioch where he, his wife, and his victims lived. Along with cadaver dogs, authorities are using "ground-penetrating radar" and forensic archeology tools including magnetometers, in hopes of finding (or ruling out the possibility of) remains of other girls who disappeared around Dugard's age. Here's the website of Bill Silva, an archaeologist assisting in the case. He reported finding an "anomaly in the soil that will require further investigation." Does anyone know more about the specific devices used for this sort of operation? I am interested to know more about the technology involved. Contrary to CSI, none of this is particularly glamorous or fast-paced work.

(PHOTO: Lance Iverson / SF Chronicle. Investigators pore through the back yard of the house next to Phillip Craig and Nancy Garrido.)


  1. Archaeologists often employ both ground penetrating radar and electromagnetic conductivity measurements to find anomalies in the soil (if you ever get the chance to see the BBC show “Time Team”, they use both quite a lot). The results vary greatly depending on the ground that is being measured, but short of digging everyplace its about the only way to narrow down a below ground search. This link is to a site for a company that does both, and they have some nice graphics on what the results look like:
    (I know nothing about the company itself, by way of disclaimer)

    Its very cool stuff if you’re into that kind of thing, but Indiana Jones it aint.

  2. Oh! Hey! They used GPR on Bones this week (TV show). I don’t recall hearing the full name of it on the show. I’ve never heard of the technology before, but it’s cool to read about a real application of it.

  3. I have worked on projects with both and you have to work with both. Each can be foiled in varying ways by electromagnetic radiation (buried power lines), water in the soil, large rocks.. you have to use a combination of technologies to get an accurate picture and the person running the machine has to really know what they are doing because there’s a lot of post field interpretation. It’s not like seeing an x ray of the soil. I’ve seen gpr miss human remains but that particular project missed a lot of other stuff so we may have just had a bad day. An anomaly in the soil is just that, you don’t know until you dig.

  4. What JBAD said is essentially correct.Particularly:
    An anomaly in the soil is just that, you don’t know until you dig.

    I’ve worked on sites in a variety of geographical environments directing backhoe stripping after remote sensing has been done.It is little better than “dowsing”in my experience.Subsurface water,old treestumps/taproots, buried pipe,gravel bars etc are indistinguishable from cultural features(pits,houses,graves).In a region and site type I’m familiar with I could do a better job “remote sensing” based on surface observations of the subtle topography of rises and swales.Give me a split spoon and a plumbing probe for even more precise “remote sensing”.Stripping a relatively thin upper layer of soil with a smooth backhoe bucket and I can tell you if their were any holes dug in the past 3k years or so and their size and shape.In my experience,GPR and magnetometers are nearly useless.I’m sure they perform better in some environments but I suspect the greatest success is achieved when the operator relies on other cues than just the remote sensing data.

  5. The live girl they found must be pretty messed up to not be able to simply tell the police if he had other victims.

  6. Most of the anomalies I have found with GPR were empty fuel oil tanks which I wanted to dig up, and sewer lines, which I did not. But every time I had one out I wondered if I’d find a grave.

    For the most part, GPR are good to about 6-10 feet in depth. They measure, along a line, at what depth any reflective surfaces start and stop. Make several passes in a grid, connect the dots, et voila.

    My first one had a scrolling paper output and a clicker you’d mark out every ten feet with, then get to work with the graph paper. Modern GPR rigs have much greater resolution, computerized distance wheels, and some offer integrated local GPS input to let a computer connect the dots for you in a GIS environment.

    But even the modern GPR won’t give you anything close to the ultrasound-like image that CSI:Miami implies.

  7. English Heritage has produced a set of guidelines that discusses which geophysical techniques (if any) are appropriate for different types of archaeological investigations and ground conditions. They consider GPR is best used when you know roughly what you’re looking for and that thing is large or consists of stone.

    Buster Ancient Farm, a fascinating experimental archaeology site in Hampshire, England, has a test area for geophysics equipment. They’ve reproduced typical features (walls, robbed out walls, post holes etc), you can bring your device and give them the results and interpretation and they’ll tell you how accurate you are.

    Oh, and Time Team is made for and broadcast by Channel 4 in the UK, although it may well be shown on BBC America who use a fair amount of Channel 4 material (Skins, Gordon Ramsay, How Clean Is Your House).

  8. GPR, or ground penetrating radar, only detects, as other people are saying, any differences in the soil. Something that is non-homogenous. An underground stream, pipeline, etc., will give a long, generally linear trace, a grave will be over a smaller area of course, and frequently will have homogenous ground around it.

    However, as noted previously, many things can affect it, much like a metal detector finds old tin cans as well as buried treasure. You either have to dig or do soil samples, as the person who mentioned the split-spoon was talking about (that’s a disturbed soil sample, pushing a device into the ground about 6″ to see what you get back. It’s hammered in, which is why it’s disturbed, so to speak. A Shelby tube is pushed in slowly to get an undisturbed sample, but for chemical analysis, either work.

  9. We use GPR on construction sites all the time to locate underground utilities. This is a pretty novel application for it.

  10. #9 – The news reports I heard said the two possible murder victims were from 1988 and 1989, which predate Jaycee Dugard’s 1991 kidnapping.

  11. I’ve used a magnetometer (a Fluxgate magnetometer, actually) quite a lot on archeological digs. It’s this big rectangular box that you walk with, holding it a couple inches above the ground, back and forth and back and forth, mapping out an area.

    The magnetometer measures the strength and direction of magnetic field lines. Virtually anything you do to the ground will affect the field lines. Undisturbed rocks tend to line up their magnetic particle’s with the Earth’s magnetic field. If you then dig a ditch and fill it back in, all the earth you moved will have jumbled-up magnetic particles, and the contrast will show up well with a magnetometer.

    Also burning affects the field lines, so an old fire pit or something will show up.

  12. The Robert Pickton case in Canada required the use of many different types of equipment/gadgets to sift through the soli of the killer’s pig farm. The RCMP were overwhelmed by the amount of soil that had to be analyzed, however their diligent work secure a conviction on 6 murders and charges on 20 more.

  13. Not much to add to the comments already added (I’ve used them all–GPR, magnetometers, etc) except that I would be surprised if the backyard was much anything besides noise on the machines. It was pretty junked out IIRC and erecting all the tents and things surely disturbed the soil. Stripping off the top soil with a backhoe is really the only way to be sure, and a skilled operator…well, can peel bananas:

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