How accurate is a magnetometer for geological exploration? We are trying to determine volume of iron ore mine
That's a simple question, unfortunately the answer is rather complex. A magnetometer measures the intensity (or the gradient - depending on instrumentation) of the earth's magnetic field, nothing more and nothing less. The earth's magnetic field can be altered by magnetic minerals, of which the most common ones are magnetite, ilmenite, pyrrhotite, and some varieties of hematite. A properly executed magnetic survey should be 99.9% reproducible, so they are accurate. A base station needs to be used to correct for diurnal variations. Magnetite and hematite comprise the most common iron ore deposits, and therefore most iron deposits do have magnetic anomalies. The anomaly can be positive or negative, depending on a complex set of factors related to the polarity of the earth's magnetic field when the curie point of the magnetic minerals was set. A magnetic survey can almost certainly determine a substantial anomaly reflecting a large concentration of magnetite and usually hematite, but it will not tell you the size, grade, or the presence of impurities which can make an iron deposit worthless. While magnetic surveys are a very useful guide in iron exploration, there is only one way to determine the size of an iron deposit, and that is drilling.
