AKULA 9000B USER MANUAL
/ SECTION 3
REV:EN05130513-03
Copyright 2006-2015 © Geoscanners AB
21
3.4.3
Filters Box
The antennas attached to your control unit are broad band
devices that transmit electromagnetic waves covering
substantial parts of the spectrum. This is one of the reasons
we can use ground penetrating radar to investigate the
underground and materials. This however also means that
these devices are subject to a wide spectrum of noises as well,
not to mention the noises the devices generate themselves. In
such a case it is hard to underestimate the importance of the
filters and the correct use of them.
The “Filter Box” has the set of the most used filters while
surveying the ground with the radar.
The LP field sets the low pass filter cut-off frequency we
need to use to ensure that our unit won’t be collecting “high
frequency” noise from intentional and unintentional radiators.
The HP field sets the high pass filter cut-off frequency so
vertical banding and “ringing” from the antenna are effectively
removed. Most of the time this is the only filter you need
and since Geoscanners control units have very small to non-
existent DC shift this can be successfully used instead of the
dewow filter.
The stack filter allows the selection of the amount of traces
selected for the stack. This will greatly remove instabilities in
the signal and acts also as a low pass filter removing “snow”
noise and amplitude deviations on the transmitted pulse which
aids a lot when doing layer tracing. The noise reduction when
using stack is equal to the square root of the amount of stacks
or if you prefer in decibels:
SNR = 10*log(N)
Choosing a very high amount of stacks will effectively reduce a significant amount of noise, but will also affect to
some extent the step response of your system. This means that caution must be exercised when using the stack
filter, specially if fast changes of the target geometry are expected.
The subtract background check box will as expected subtract a previously collected background averaged trace
from each and every collected trace in the incoming data. It is obvious then that the background noise must be
collected before you can use this filter. First specify the amount of traces you want to average to calculate the
background noise that will be subtracted from each and every incoming trace. When you have decided the amount
of traces you will be averaging press the button “BkGr” to actually collect this data. Please note that during this
time the radar will not accept any other commands so if you have selected a large amount of traces then you must
allow some time for the control unit to be able to collect those. When the background data has been collected
GAS™ will inform you about it and you are ready to subtract the background from the incoming data.
The filters contained within the filters box are a formidable tool for enhancing the quality of the collected data.
However, please exercise caution while applying different filters without considering the implications this has on the
data and how it can be affected by them. A good rule of thumb is to try to examine the incoming data and find out
the spectrum of it. According to the frequency values at -10dB levels the low and high pass filters can be set. I a
few occasions when the incoming data is contaminated with extraneous frequency components it is useful to apply
cut-off frequencies even above the -10dB levels in order to improve the quality of the data and the readability of it.
In many cases it is highly advisable to collect raw data so the possible mistakes done during collection in the field
can be mended after-wards. This is not applicable to utility detection applications where it is important to find the
target in the field and post processing in the office at a later time is not an option.
