Earth Field 'Diurnal' variations

[Willy Bayot]

For your info, this is the clear illustration of the influence of the man activities to the earth field.
In principle, in the middle of nowhere (sea, desert,large forest,...), the true diurnal variations are measured in a few nT (1 or 2) per minute. In those environments, the magnetic surveys are very well made with a single mag.

These two plots were taken in my garden by two distinct but synchronized mags whose single sensors were kept at a fixed location and separated by 3 meters for a period of 15 minutes. The readings were repeated at a rate of one per 1.5 seconds.

You can see how much the field varies in parallel on the two fixed stations between readings proving that they are not induced by the system themselves.
The main reasons for those large variations are the underground currents generated by powerful electrical activities with not too good groundings. The electrical railways are mainly responsible for those currents which are triggering their effects several kilometers away.
As an example, I know that the earth field observatory in Belgium installed in the middle of a large forest has blocked all projects of electrifying the railway network in their all region.

The variations are slightly lower in the countryside, far from any house or power line but, in my own experience, they are still there.

In conclusion, surveys made in the countryside, in rivers or in lakes should use a gradiometer or a differential PPM configurations to correctly detect and locate earth field anomalies of a few nT's (5 to 10).

Willy

[Willy Bayot]

Water pipe detection with PPM

HI,

The example I showed in here before demonstrated the influence of the small but still sizable diurnal field variations and those generated by human electromagnetic activities on the measurements while looking for tiny field gradients generated by buried foundations, wells, caves or fire pits. Those are the usual targets of archaeological survey work.

At the other end of the spectrum, this here is an example of the capacity of a PPM to detect ferrous object at long distances and still generating very large field gradients. These cases would make crazy many other types of PPM, even from the commercial network because they usually are limited in their capacity to detect large field gradients (nT/m).

A water pipe is buried under the road in front of my house. I have no idea at what depth but , I know very well where it exactly runs.
Look at this bar chart. It shows the earth field in nT measured by my PPM while I was slowly crossing the road. The peak of this plot is the exact location where the pipe runs. You can see that the system was able to correctly measure a total gradient of about 380nT in 8 seconds.
It is very easy to double-check the location by crossing the road several times.

This demonstrates the capacity of the system to detect and plot in real-time ferrous object buried underground or sunk in water.
The corresponding applications vary from public work companies looking for tanks, pipes and cables before starting digging with large excavators up to environmental organizations seeking deeply buried barrels with polluted or dangerous material.
Shipwreck hunters are also found of this type of instrument to locate sunken ships in complement to their usual SSS. The SSS is only able to 'see' things which are showing over the bottom of the sea but a PPM is able to 'see' (ferrous) things which are buried in the mud. It is true to the defence of the SSS that it is able to 'see' wood as well which the PPM does not.

Willy