A fluxgate gradiometer is a formidable piece of equipment that takes the form of a sophisticated passive sensor for use in ground scanning. It measures, quantify and records local variations in magnetic flux density, measured in Teslas. Due to the subtle nature of most sub-surface archaeological remains, magnetometry readings are usually output in NanoTeslas (nT) and fluxgate gradiometers are capable of resolutions ranging from 100 to 0.1nT. The readings are relative since the data gained is not an absolute value of the ground's magnetic flux density, but rather the degree to which it differs from a pre-determined zero point. To find a suitable zero datum is usually the first thing to be done on arriving on a site to be scanned. This instrument provides a very essential part of the total system, it is used to locate the whereabouts of a pig should it become lost in a line. The gradiometer may take the form of a portable hand held unit, a fish for offshore location of pipelines or magnetic pigs, or be used in conjunction with an R.O.V. or with an airborne vehicle. It is designed to sense magnetic fields, which exist around all magnetic objects. These steady magnetic fields penetrate soil and water, and so indicate the presence of magnetic objects, which may be buried, or hidden from view.