Advantages and limitations of 3D acquisition of magnetic field intensity measurements

Magnetic sources of small size and modest intensity produce a geomagnetic anomaly of low intensity and limited extent, which is not detectable beyond a few decimeters, one meter at most. It is therefore important to carry out geomagnetic surveys as close as possible to the source in order to detect them. This means that the sensor must be moved along a trajectory that follows the microtopography. This necessity complicates the problem, since in addition to the geomagnetic anomalies sought, there are also anomalies produced by topographical variations. The geometry of anomalies produced in space, above the surface, by low-intensity point sources differs from those produced by the humps and hollows of the surface topography of the area surveyed. Indeed, a point source produces a dipolar anomaly of circumscribed vertical extension, whereas topographical variations produce more diffuse anomalies, positive for humps and negative for hollows, with the dipolar component attenuated. To eliminate the confusion between a point source and a topographical effect, the solution is to explore the volume above the prospected surfaces. In this way, a 3D survey can distinguish anomalies due to microtopography from those due to modest magnetic point sources.

The density of the magnetic field intensity measurement cloud must be adapted to the size of the sources to be detected.  The smaller the sources, the tighter the measurement grid must be, and the closer the sensor needs to be to the surface. In practice, the size of the sensor determines the maximum spatial resolution that can be achieved. To achieve a high measurement density in an acceptable measurement time, measurements must be taken continuously at a high rate. The spatialization precision of the measurements remains an important factor for information quality. For decimeter-sized objects, the position of magnetic field intensity measurements is determined using a total station (S8, Trimble) at a maximum rate of 20 Hz by laser tracking a 360° reflector attached to the magnetic field intensity sensor. For metric objects, GNSS geopositioning with differential correction with local base performed in post-processing allows sufficient accuracy to be achieved. The 360° reflector, being non-magnetic, can be attached to the sensor, though GNSS antennas, being magnetic, necessitate the use of a miniature helical antenna offset by at least 0.5 m to mitigate its influence.

This type of 3D geomagnetic survey was originally used in prehistoric caves to locate hearths. A device equipped with a telescopic pole mounted on a tripod is used to scan the space, taking one measurement per 25 cm² of ground area. Measurements are taken at a rate of 10 Hz (G858, Geometrix). Tests with rates up to 100 Hz were carried out with a GSMP35U (GEMsystem), but it turned out that the measurement rate is not continuous, which poses problems for data fusion. In the field, these surveys have been carried out on Neolithic pebble hearths or on an antique shipwreck. For large surfaces, several hundred m², the device is mounted on a cart, or a cart with 4 superimposed sensors is used.