magnetometry in the “Lac d’Antre”

The data collection was conducted on 27/11/2016 according to the methodology described. The number of recordings made is 5486 points.

Data statement
The data collection was conducted on 27/11/2016 between 11h and 12h30 according to the methodology described below. The equipment used is a Towfish Aquascan proton magnetometer and its MC5 microprocessor. A zodiac with an inflatable plastic bottom and a combustion engine made it possible to pull the fish hooked under a float (at 30 cm deep) with a cable length of 25 meters between the float and the boat, at an average speed of 4km. The number of recordings made is 5486 points.

The GPS position recorded is that of the float where the GPS sensor was positioned and connected to the boat by a 30 meter USB cable. So that the two cables do not separate, and they do not get caught in the vegetation. They have been inserted into lengths of polyester “chips” that allow the cables to float.

The distance between 2 passes is 18m. Function of the sensitivity of the Aquascan MC5 and given the average depth of 6 m, this allows, in theory, the identification of a magnetic anomaly of a minimum value of 40 kg of ferrous mass, with a variation from of 1 nano tesla. Note that these values ​​are only indicative since they also depend on the shape, the magnetization and the volume of the object.

Observation on raw data – validation of measures Calibration of the apparatus was carried out in the middle of the lake, far from any potential disturbance of the magnetic field, one will notice the position “zero” on the first measurements.

Diurnal variation of the magnetic field
In order to correct the diurnal measurements of the Earth’s magnetic field, the only information available in terms of recording these variations are those of the center of Chambon la Forêt, which gives the following information:
There will be a rather low variability over the recording period since the highest value of the Earth’s magnetic field is: 47 865.2 nT at 11.45 and the lowest is 47860.0 nT at 12.12, a difference of 5, 2 nT, while on the day she reached 23 nT
Each data has been corrected for the value of the magnetic field deviation. The same value is retained for each minute.

It is important to note that the recording of the magnetometer is a point data by its GPS position, but that the information rendered is in a wider environment. Thus the same magnetized mass is the subject of different recordings in the same trace, and function of the width between the traces recorded in the neighboring traces with necessarily different values.








The 3D view of the results allows an immediate identification of the positive anomalies but, except to multiply the axes, to materialize the negative anomalies.
A grayscale rendering attempt was attempted, but the result was difficult to interpret. Given the distance between each survey line, the software does a lot of interpolation.
With a color representation, it is easier to determine areas of anomalies

The recorded data very clearly show 2 lineations. They are visible on the map in color, but even more highlighted with the processing of data by B. Gavazzi. These 2 lineations seem to correspond:
– for the south-west one, either at the old lake bank or possibly at the old “Roman road” which ran around the lake.
– For the north is the shore of the lake which also materializes with the property boundaries between families BROCHOT and BESSON.
Regarding the possible lineation in the west and center, it would be necessary to redo measures to validate the data, but other measures indicate an anomaly in this sector.








54 point abnormalities were recorded. They have been grouped into 22 zones given their proximity to each other.

The data set is available in Appendix 13 with the readings of each trace participating in a zone, the 3D image of the zone and a theoretical interpretation of the magnetic masses.

Apart from a finding of a significant number of anomalies in the context of a survey whose results are validated, no interpretation can be formulated at present.



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