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

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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.)