This is a great educational read, tells you all about X/R signals and why mineralized soils (where usually most of the gold is) produces large X-signals.
METAL DETECTOR BASICS AND THEORY
Just google it to get the pdf link, it's on the minelab homepage
"As with most introductions, the above brief description is over-simplifi ed. The signal induced in the receive coil, by the magnetic fi eld of the eddy current, can be thought of as made up of two simultaneous components, not just an altered component:
• One component is the same shape as the transmit signal. This is called the reactive signal (“X”). Because it is the same
shape as the transmit field, the signal, by definition, responds immediately to what ever the transmit signal is doing.
• When this X component is subtracted from the eddy current induced signal in the receive coil, the shape of the remaining
signal depends only upon the history of the transmitted field, and not the instantaneous value. This signal is called the
resistive or loss component (“R”)."
"Unfortunately, soils are magnetisable and thus also detected by metal detectors and cause signals which interfere with metal target signals. The degree of the magnetic properties of soils varies considerably. This magnetic property of soil is usually called “mineralisation.” The mineralisation produces almost entirely X and only a small fraction of R signals. The R and the X signals of a deep metal target are typically much less than the soil X signal, so obviously it is better to use the R signal to locate metal targets, rather than X."
"USA goldfields are typically different from Australian goldfields:
• The USA soils are mostly mildly mineralised but in some areas may contain either nearly pure magnetite black sands or rocks, which
are problematic for metal detectors as they have very high X components (strongly attracted to magnets).
• Australian gold fields have highly mineralised soils, but very few black sands or rocks that contain nearly pure X magnetite. The
magnetic materials are in the forms of magnetite-rich small pebbles and rock coatings, clays and general “sandy” soils. These all
contain magnetic materials that produce high levels of X signals as well as R. The ratio of X and R is random, and the R component
arises from extremely small magnetic particles called superparamagnetic materials, which are discussed below."
"As stated earlier, mineralisation produces a large X component and a much smaller R component. Unfortunately X and R are unrelated and their relationship varies randomly from one location to the next. Salty soils produce R signals from the mineralisation, as well as from the salt conductivity. However, most goldfields do not have salty soils, and in these non-salty soils, the soil R/X ranges from about 0.5% to some extreme “hot rocks” with R/X about 5%. However, the nearly pure magnetite black sands and rocks exhibit very low values of R/X; <0.1%. In very highly mineralised soils, such as Australian goldfields, median soil R/X is between 0.5% and 1%, but this ratio varies from location to location. Some non-salty non-goldfield soils have R/X as high as about 10%."