Geophysical Survey Systems, Inc.
SIR® 4000
Manual
MN72-433 Rev F
20
Offset
This is an internal system parameter that describes the time lag (in ns) between the SIR 4000 triggering
the pulse inside of the control unit until we consider it to have transmitted from the antenna itself. There is
no way to accurately measure the exact moment the pulse leaves the antenna, so we use the direct wave to
determine the appropriate point to set the offset.
•
The direct wave is the pulse that travels
inside of the antenna housing, directly from
the transmitter to the receiver, and it is
considered to be the first response in the
data. As long as the antenna dipoles are not
very far apart, as is the case with a separated
bistatic antenna pair, the direct wave occurs
sooner than any reflections from the ground.
If the direct wave is visible in the data, we
can be sure that we have 100% of our data.
The ground surface can also be used to
perform a depth calculation. The value is in
nanoseconds from trigger pulse inside the
SIR 4000. If you adjust this parameter use
caution not to lose the direct wave.
•
Adjusting the Offset is only available when Position Mode is set to Manual. When it is set to Auto,
the SIR 4000 auto calculates the Offset.
•
In the associated image (above), the antenna was moved up and down over the ground surface.
Notice how the surface reflection moves up to join the direct wave.
Surface (%)
This display option allows you to visually ‘cut out’ the flat part of the scan and begin at the direct wave,
thereby showing only the scan from the first reflected target. This should represent the ground surface.
•
The entire scan is still collected and saved, but not displayed during data collection. This allows
you to set the display to show an in-field time to depth calculation.
•
It is set as a percentage of the total vertical window. The SIR 4000 will examine the offset and the
antenna type selected from the Introduction Screen to automatically find the proper surface. It
will set near to the first positive peak of the direct wave.
Setup Menu - Process
Filters used during data collection or playback with the SIR 4000 will be saved as a separate DZX file.
When data are opened in the processing software RADAN it will open first without any applied filters
and then it will apply the DZX file and created a processed file.
Under Process, there are thirteen (13) settings that can each be accessed by rotating the Control Knob or
using the Directional Keypad. These settings are listed below as well as which modes they are available
in:
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