• The central beam should be treated with a mix of linseed oil and white spirits every 6-12 months, (or
more often dependin on circumstances) – this will help to repel water and prevent waterlo in of
the central beam which can be a source of measurement errors.
• Always use the white mud caps and rommet to keep mud away from the probe mountin position
under the beam or win .
• Take special care to fit all the insulators as shown in the dia rams – if the metal probes conctact with
the wooden or metal win s potential leaka e current paths can be created which in turn can cause
measurement errors, especially in wet weather.
• Touch up any scratchin of paint on the win s to avoid any possible current trackin to or from
exposed metal.
• Ensure the bolts securin the handle to the beam (located underneath the beam) are still encapsulated
with plastic filler and rubberised sealant.
• When usin win s ensure the nylon bush and washer kit that insulates the struts from the handle has
not broken.
• If you experience stripin problems with Parallel Twin data you may wish to follow the practice of
some users who wrap heavy duty plastic sheetin around the beam system and seal it up with tape.
This is reported to reatly reduce or eliminate stripin problems, especially in older frames.
Striping Defects wit t e Parallel Twin Configuration
Even with ood care and maintainance, stripin can still occur with the Parallel Twin mode, usually in wet
conditions. Normally, all will be well when the weather is fine but after a short period of workin in rainy
conditions or in lon damp rass stripin will start to occur. Typically this is a low level phenomenon, about 2
ohms in ma nitude. Two example surveys illustrate this problem and show two different methods of dealin
with the error should it arise.
Method 1
An example of stripin can be seen in the survey opposite, fi ure 1. The first ima e is the raw data, and the
second shows the data after hi h pass filterin (X=10, Y=10, Wt=U) so the stripin is clearer. The survey is an
amal am of surveys made with Sin le Twin and 2 Parallel Twin (3 probe), with the stripin occurrin only
with the 2 Parallel Twin confi uration.
The data can be corrected usin standard Geoplot process routines applied to the complete composite. First the
data must be despiked and saved. Then apply a low pass filter across the saved data with X=10, Y=0,
Wt=Gaussian. Repeat the low pass filter 3 or 4 times and save this new data set. Low pass filtered data for the
raw data above (not the hi h pass filtered data set) is shown in fi ure 2. This new data set has effectively had
most archaeolo ical features smoothed out and retains just the back round eolo y and stripin defect due to
leaka e.
Next subtract this new data set from the ori inal despiked data set to effectively remove the stripin defect. You
can see from the results in fi ure 3 that all trace of the stripin has been removed. Note also that the data has
effectively been hi h pass filtered, thou h there are differences from the re ular hi h pass filtered data in that
localised features are more visible. Note that any archaeolo ical features that are parallel with the traverse
direction will be removed by this processin technique, so, dependin on their importance, you may need to use
the Cut and Combine function to temporarily store and preserve such features in a temporary composite and
then repaste them back in their ori inal positions after the above processin .
