Difference between gradiometer and magnetometer?

[zenit52]

Hi to all friends,

what is Difference between gradiometer and magnetometer ?
which is better for use ?

with these devices we can detect pits or holes ?

Thanks...

[zenit52]

can someone answer to me... ?

[BigBang]

We can say

Induction Balanced Magnetometer = Gradiometer

[spiridon]

can someone answer to me... ?

One gradiometer = 2 magnetometers

[signman]

Gradiometer vs magnetometer

A magnetometer is a single sensor magnetometer which measures the magnetic field strength that the sensor sees. If you are going to walk around with a mag looking for significant changes in field strength to indicate whether there is a ferrous object in the vicinity of the sensor, then this may be suitable for your needs.

A gradiometer is essentially 2 magnetometers and sensors connected together. The 2 sensors are at somewhat of a distance from each other. For example, maybe you have a vertical pole with 1 sensor 2 feet off the ground and the 2nd sensor 6 or 8 feet off the ground. One reading is subtracted from the other. If they both see the same magnetic field strength (no ferrous objects in the vicinity) then the subtracted reading will be near zero. If the operator begins to move near a ferrous object in the ground, the lower sensor will begin to see a higher or lower reading than the higher sensor. So as the 2 readings are subtracted, the result will be a number more positive or negative from zero. As the sensor staff continues to move closer to the object, the number (magnetic field difference between sensors) will continue to get further away from zero.

The advantages to a gradiometer is that it is insensitive to diurnal changes in the earth's magnetic field strength. This means, as you use a magnetometer, the strength of the field is changing and sometimes quite significantly in a relatively small period of time. If you are using a single sensor, and you are doing a large survey grid, this can become a problem as the ambient readings vary. With a gradiometer, the 2 sensors are reading the same ambient field readings which means no matter what the background reading is (as it changes) the 2 sensors will still subtract to near zero if no anomaly is present in the ground. If you were to do a large survey which required multiple days to complete, or it begins to rain and you must temporarily halt a survey until a later time, or the diurnal shifts are quite large in a relatively short time, this will be a problem using a magnetometer but not if you are using a gradiometer. If you stand in the same spot and take a measurement with a magnetometer and it reads 50,000nT and 2 hours later it might read 49, 875nT. If you stand in the same spot and take a reading today it might read "5" and tomorrow even though the magnetic field has shifted 125nT from the previous day, the gradiometer will still read "5". I hope I made this clear.
Thanks,
Signman

[DELPH]

Gradiometer vs magnetometer

As indicated by Signman, the first objective of a gradiometer is to improve the sensitivity of the field measurements.

- When using a single magnetometer, to distinguish local field anomalies from the environmental noise like diurnal variations, data must be filtered or reference station data be substracted. This processing may also alter the local field information.

- When using a gradiometer, we look at the local fields from multiple positions, simultaneously. The integration of those, and of the spatial progression of the magnetometer allow the computation of field gradients in multiple axis. Whatever the diurnal variation as we measure and compare measurements at a same time from known positions.

Different gradiometers exist, usually 2 and 3 axis sensors that measure:
- A transverse/lateral gradient from the side probes
- A vertical gradient from the a additional probe located above/below the transverse axis
- A longitudinal gradient that is either measured by an additional probe ahead in the sensor. The longitudinal gradient may also be estimated by integrating the sensor positioning and computing an along-track gradient.

To have good gradiometer data, the system must be stable and all probes be keeping their position on the respective axis. Motions must be compensated on all axis to re-estimate the proper gradient axis, particularly roll & pitch effects. The Z axis still has to be compensated (altimeter, pressure sensor for marine applications) to keep a same reference level.

Philippe
http://delph.ixsea.com

[zenit52]

Thanks dear signman and DELPH,

Thus a gradiometer is better for exact results... ok?

my other question is: with gradiometer we can detect pits or holes ?

[Willy Bayot]

As indicated by Signman, the first objective of a gradiometer is to improve the sensitivity of the field measurements.

- When using a single magnetometer, to distinguish local field anomalies from the environmental noise like diurnal variations, data must be filtered or reference station data be substracted. This processing may also alter the local field information.

- When using a gradiometer, we look at the local fields from multiple positions, simultaneously. The integration of those, and of the spatial progression of the magnetometer allow the computation of field gradients in multiple axis. Whatever the diurnal variation as we measure and compare measurements at a same time from known positions.

[B]Different gradiometers exist, usually 2 and 3 axis sensors that measure:
- A transverse/lateral gradient from the side probes
- A vertical gradient from the a additional probe located above/below the transverse axis
- A longitudinal gradient that is either measured by an additional probe ahead in the sensor. The longitudinal gradient may also be estimated by integrating the sensor positioning and computing an along-track gradient.
]
To have good gradiometer data, the system must be stable and all probes be keeping their position on the respective axis. Motions must be compensated on all axis to re-estimate the proper gradient axis, particularly roll & pitch effects. The Z axis still has to be compensated (altimeter, pressure sensor for marine applications) to keep a same reference level.
[/B
Philippe
http://delph.ixsea.com

The part of this explanation marked in red only applies to fluxgate mags or any vector-type mag.
It does not apply to a PPM which measures the total magnetic field. A PPM is not sensitive to roll & pitch.

A gradiometer is only necessary to detect very small field gradients, i.e. of a range of values equivalent to the natural micro-variation of magnetic field. This is usually a few nT's per few seconds if captured in the countryside far from powerful electromagnetic human activities and devices.
The real diurnal variations are larger (10's of nT) but much slower during the day, their period is expressed in minutes and do not disturb a survey made from one reading per second.
An underground magnetic disturbance generated by a magnetic target generates a very sudden peak or drop of field value. This is very easily shown in a survey plot made by a single mag in spite of the slow changes of total magnetic field.

Now, to respond to the second part of the question from zenit52.
Detecting underground holes and pits with a mag in archaeological surveys essentially depends on the general magnetic susceptibility of the natural ground. In ground with very low susceptibility, these surveys are very difficult as the contrast between the NO-GROUND (hole) and the GROUND will generate very weak field gradients. In ground with high susceptibilty (volcanous rock or dust), this will be very easy.
Detecting buried wall foundations depends on the nature of the rocks or material used to build thsoe foundations. If their magnetic susceptibility gives a large contrasts with the natural ground, they will also be easily detectable.
However, in difficult cases, one can use a true gradiometer (two carried sensors) or a differential mag (two separate mags, one with a fixed sensor and one carried). The two configurations are able to remove the effects of the micro-variations.

Willy

[mikebg]

Hi to all friends,
what is Difference between gradiometer and magnetometer ?
which is better for use ?
with these devices we can detect pits or holes ?
Thanks...

I found this:
http://www.ehow.com/list_7394286_dif...adiometer.html