Tweet
Hi all,
Some people asked for a rule of thumb to evaluate the field gradients generated by iron/steel targets of given mass at given distances and detected by a magnetometer.
You must first know that most of the targets act as a magnetic dipole as seen by a mag but, if the target is much longer than thick but if one end is far from the point of measure, it is felt like a monopole. It is also felt like a monopole if the shape of the target is compact or round.
The field gradients are directly proportional to the mass of the target and inversely proportional to the third power of the distance for dipoles and to the second power of the distance for monopoles.
This is a ZIP'ed EXCEL file which grossly evaluates the expected field gradients in Gamma (nT) for monopoles and dipoles of various masses and at various distances.
As a general physical rule, instruments which depend on transmitting some sort of signal and then detecting a response will have a sensitivity which varies as one over the fourth power of the distance. This is the case of all the metal detectors and all the EMI sensors whatever their make and design.
A magnetometer doesn't transmit a signal so its sensitivity will vary proportionally with how the effect of an anomaly changes along the field lines; this is, very roughly, something like one over the third power of the distance for dipole targets and one over the second power of the distance for monopole targets . Of course, the downside of the magnetometer is that it only detects materials which have some magnetic permeability.
This is the reason why a mag is always more sensitive than any EMI system provided the target is made of magnetic material.
Willy
Some people asked for a rule of thumb to evaluate the field gradients generated by iron/steel targets of given mass at given distances and detected by a magnetometer.
You must first know that most of the targets act as a magnetic dipole as seen by a mag but, if the target is much longer than thick but if one end is far from the point of measure, it is felt like a monopole. It is also felt like a monopole if the shape of the target is compact or round.
The field gradients are directly proportional to the mass of the target and inversely proportional to the third power of the distance for dipoles and to the second power of the distance for monopoles.
This is a ZIP'ed EXCEL file which grossly evaluates the expected field gradients in Gamma (nT) for monopoles and dipoles of various masses and at various distances.
As a general physical rule, instruments which depend on transmitting some sort of signal and then detecting a response will have a sensitivity which varies as one over the fourth power of the distance. This is the case of all the metal detectors and all the EMI sensors whatever their make and design.
A magnetometer doesn't transmit a signal so its sensitivity will vary proportionally with how the effect of an anomaly changes along the field lines; this is, very roughly, something like one over the third power of the distance for dipole targets and one over the second power of the distance for monopole targets . Of course, the downside of the magnetometer is that it only detects materials which have some magnetic permeability.
This is the reason why a mag is always more sensitive than any EMI system provided the target is made of magnetic material.
Willy