Posted By: Dave Johnson (1Cust161.tnt3.prescott.az.da.uu.net)
Date: Saturday, 29 December 2001, at 8:16 p.m.

There are three known causes of "mass effect", where you get ground noise from a spot (usually clay), but you break it up and the same amount of stuff doesn't give you near the grief.

CONDUCTIVITY
Clays hang onto both ions and moisture. This is especially true of the smectite group of silicate clays, less so of oxide clays which are common in the tropics. In order for electrical conductivity to happen, there has to be a good electrical path to form an eddy current loop. When you break up the dirt by digging in it, the electrical conductivity drops by an order of magnitude, and it no longer sounds off-- if that was the reason it was sounding off.

A spot can be electrically conductive for reasons that are not obvious-- unless you happened to see a cow piss on that very spot the day before yesterday.

NATURAL MAGNETIC "FABRICS"
Almost everyone is familiar with how black sand or iron filings line up with each other in the presence of a magnetic field. When particles are lined up end-to-end in filamentary structures rather than just being oriented randomly, the magnetic susceptibility of the bulk material increases. This makes the bulk material containing the magnetic particles more magnetic than it would otherwise be. Furthermore, when exposed to a magnetic field, the ends of the particles are forced through a larger hysteresis loop, increasing their magnetic loss angle.

When magnetic mineral grains are deposited in water or allowed to reorient themselves when soil becomes liquefied, they line up with the earth's magnetic field, and tend to attach to each other forming an anisotropic arrangement. This is well known in those branches of geology which are concerned with dating soils and lake sediments according to measurement of their magnetic properties.

If the magnetic particles in a soil have come into an anisotropic configuration, they are more likely to "sound off" on a metal detector. When they are disturbed by digging, they become like the surrounding (presumably previously disturbed) soil. Oxide clays, which are common in the tropics but which can be found in some temperate zone areas as well, often contain several percent or more gamma iron oxides, which gives metal detectors fits not only because of their magnetic susceptibility, but because of their relatively high magnetic loss angle. (High magnetic loss angle is what makes dirt "positive", in contrast to magnetite, a naturally occurring ferrite which has a low loss angle and is therefore "negative".)

RUSTED IRON METAL (ESP. NAILS & WIRE)
When iron metal rusts in place, you get a little deposit of iron oxides which are usually of the magnetic, high loss angle type. What's worse, if the object which rusted was a nail or piece of wire, the agglomeration of rust is linear, which is to say highly anisotropic. It's gonna sound off.