Geophysical Survey Systems, Inc.
SIR® 4000
Manual
MN72-433 Rev F
23
FIR Low Pass and IIR High Pass
These options, and the following seven (7), allow you to set data collection filters to either remove
interference or smooth noise. A Low or High Pass filter is an antenna-specific frequency filter and should
be modified each time a different antenna is connected. These filters are used to downplay external
interference, and clean up the signal.
FIR filters have a Finite-duration Impulse Response and, when encountering a feature in the data, are
guaranteed to output a finite filtered version of that feature. These filters will produce symmetrical results
so reflections will not be shifted in time or position. By setting these at different ends of the antenna’s
bandwidth, you are determining the range of frequencies that the antenna is filtering out.
Pressing FIR Low/High Pass with either the Control Knob or Enter button will allow you to scroll
through different frequencies.
•
FIR Filters should not be used with 4096 samples/scan or higher.
•
The maximum Low and High Pass filter values are 5000 MHz.
•
Select Off to disable either filter.
FIR Low Pass:
The selected frequency in MHz is the upper limit of the filter. Any frequency lower than
selected Low Pass frequency will be allowed to pass through the filter and be recorded by the system (i.e.
system will record any frequency
lower
than XX MHz).
FIR High Pass:
The selected frequency in MHz is the lower limit of the filter. Any frequency higher than
selected High Pass frequency will be allowed to pass through the filter and be recorded by the system (i.e.
system will record any frequency
higher
than XX MHz).
FIR Stacking
In addition to FIR frequency filters, another option is Stacking. Stacking is a high-frequency noise
reduction filter which operates in the horizontal direction. Each new scan has a 1/n influence in the data,
so this filter has a tendency to smooth high frequency targets and accentuate low frequency horizontal
features, like layers. As the number of stacked scans (n) increases, the influence of each new scan drops.
The FIR Stacking filter input is in number of scans. Pressing FIR Stacking with either the Control Knob
or Enter button will allow you to select a number of scans.
•
High frequency noise generally has a ‘snowy’ appearance.
•
The larger the number you put in here, the smoother the data will be. It is possible to over smooth
and ‘smudge’ out real data.
•
A larger number also means that the system is performing a great deal of extra calculations and
data collection speed may be reduced.
•
The Stacking filter maximum value is 1023 scans.
FIR BG Removal
Background Removal is a filter which removes horizontal bands of noise similar to those seen in the
image below. Sometimes these bands are caused by ‘real’ horizontal reflectors, but they can also be
caused by low frequency noise such as antenna ringing. These layers can obscure other ‘real’ horizontal
or point source reflectors.
Summary of Contents for SIR 4000
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