An Unexplored Realm in the Heartland of the Southern Gulf ... - Famsi

Magnetic Data Collection, Conditions, and Field Methods
A magnetometer survey is a passive, non-invasive, geophysical remote sensing
technique employed to measure the physical and chemical properties of near-surface
deposits (Clay 2001; Kvamme 2001, 2003). The most effective range of this equipment is
the uppermost 1 to 3 m of the surface, which is the maximum depth in situ cultural
material has been noted or recovered at El Marquesillo. Kvamme (2003:441) states that,
“it is almost as if nature designed archaeological sites to be made visible by the magnetic
variations they exhibit.”
The geophysical conditions along the Southern Gulf Lowlands are conducive for
the implementation of magnetic geophysical surveys, and previous surveys have provided
outstanding results at other regional Olmec sites (Breiner and Coe 1972; Morrison,
Benavente et al. 1970; Morrison, Clewlow et al. 1970; Welch 2001). At these sites,
magnetic prospecting of stone sculpture and architecture was positive due to the high
contrast between the volcanic rock and the sedimentary fill. The favorable soil
composition, postulated intrusive magnetic elements, exposed open terrain, plus the
technique’s rapidity and effectiveness all suggested that a magnetometer survey should
be an integral part of the project’s design.
Successful magnetometer prospection is due primarily to the contrasts between
the magnetic properties of natural or intrusive material and the surrounding soils. The
sedimentary soils that compose the region around El Marquesillo are neutral in their
magnetic content. The Pliocene basalts used in the production of Formative period
monumental sculpture, constructions, and features retain a high level of remanant
magnetization. Williams and Heizer (1965) describe the basalt used in the monuments of
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