Magnetic Gradiometer Surveying in Areas of High Ambient Noise

Magnetic Gradiometer Surveying in Areas of High Ambient Noise
Magnetic Gradiometer surveying in areas of high ambient noise.
Surveying with a magnetometer in urban environments can be difficult due to high
ambient noise. In the following example, we show that it is possible to make high
resolution magnetic surveys in areas where there is high cultural noise from fixed
sources such as buildings, but also from sources that vary rapidly in time.
The area surveyed was approximately 35 metres by 20 metres and was a flat area of
grass and with a single large tree in the middle of it. A steel man hole cover was noted
4 metres off from the bottom right corner of the grid. A large hotel and conference
centre bounded the site to the West, and roads, both surface and overpass bounded
the site to the North and East
Lines were surveyed at a 2 metre interval with a G-858 Cs vapour vertical gradiometer
running from South to North, with the bottom sensor 50 cm above the ground and the
top sensor at a height of 1 metre above the ground and a system sample rate of 5
readings per second with the instrument set in continuous mode. In this mode the
recording instrument has a sensitivity, or base noise level, of 0.075 nT/m
Whilst conducting the survey, variations on the gradiometer display of the
magnetometer were seen with a total amplitude in the 100 to 200 nT/m range.
Data was exported from the magnetometer to a PC using MagMap 2000 where the data
was corrected for positional errors only, and prepared for final display using WinSurf.
Figure 1 shows the recorded total field for the top sensor, huge anomalies of the range
of 4000 nT can be seen here.
Figure 1
However, when the data was plotted in vertical gradient from
using WinSurf several vertical gradient anomalies can be seen
which have a maximum amplitude of 150 nT. It was supposed
that the vertical gradient anomalies area due to previous
excavations and constructions on the site, which used to form part
of a graveyard. On closer examination, these vertical gradient
anomalies as shown in figure 2, were seen to be spacially
independent of the extremely large total
field anomalies as shown in Figure 1 and
recorded by both the top and bottom
sensors, How could such large total field
anomalies, producing huge horizontal
gradients, not have a vertical gradient
component as well ?
We noted when overlaying the data measurement positions on
the data maps that these large total field anomalies did not in
many areas spread laterally across more than one line. Thus it
was concluded that the strong total field anomalies were varying
with time and were probably caused an external cultural noise.
Figure 2
To test this theory, we conducted two experiments, first we ran a base station using two
sensors in a vertical gradiometer mode, the results of which are shown in figure 3. This
plot shows how the total field varies with time, with a total peak to peak amplitude of
some 4000 nT, and with a period in
some extreme cases of just a few
seconds. Note, as we conducted the
survey using a gradiometer, the use of a
base station would not normally be
considered as necessary to correct the
data for diurnal or other time varying
magnetic “noise”. The results of the short
base station trial as expected, show tiny
variations in the vertical gradient with
respect to time, but as expected huge
variations in the total field with respect to
time.
Figure 3 Base Station data.
Next as the site was small and readily
accessible we conducted the whole survey again. As we predicted the results of this
second survey showed great variation with the first survey in
respect of the total field measurements, as is shown in figure 4,
but the gradiometer data as shown in figure 5, shows excellent
correlation with the first survey. Note that due to survey points
disappearing in between the first and second surveys, there are
some slight differences in the area surveyed on the two surveys.
We re-examined the area around the site for possible sources of
this time varying noise and observed an electric tram way beyond
the road to the North and about 60-70 metres away from the site
On enquiring, this was found to be powered by DC, and our
conclusion was that the time varying magnetic anomalies were
due to differences in load on the power lines caused by trams
stopping, starting, accelerating etc.
Figure 4
Examination of the total field signals from
the survey data show time based variations of the magnitude of
4000 nT, in the time taken to survey one line, however gradient
anomalies as small as 20-30 nT/m can be seen through this noise.
In conclusion, this survey is an excellent example of how, in a high
noise urban environment with very high sources of cultural noise,
both static and dynamic, a true simultaneous gradiometer, Ie one
that reads both sensors at exactly the same time, can yield
excellent results in resolving small magnetic gradient anomalies
and filter out totally the cultural noise.
Figure 5
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