GSSI SIR 20, User Manual

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Geophysical Survey Systems, Inc. SIR® 20
Manual
MN92-078 Rev F 32
10
This window will allow you to further customize your data collection parameters. The window is
divided into two main areas.
•
The top 2/3 of the window shows the data display as O-scope and Linescan.
•
The bottom 1/3 is reserved for parameter selection pop-up windows. These parameter selection
windows can be accessed by clicking on one of the blue and yellow collection parameter controls
at the bottom of the screen.
•
You can also check the file header by clicking on the File Header icon.
Position/Range
The first collection parameter button is for Position/Range. This controls the length of the time (in
nanoseconds) that the system will acquire data. The default vertical scale is two-way travel time. The
larger the value that you set for time range, the longer your SIR 20 will record data. This (generally)
translates to scanning deeper, however, total depth of penetration is a function of both the conductivity
(
σ
’
) and permittivity (
ε
’
) of the media, so the range and depth are not directly equivalent.
The Auto Position Servo and Position correction control enable the user to adjust the direct coupling
pulse (i.e. the transmitted pulse) of the antenna to ‘zero’ time, or to adjust the direct coupling pulse up or
down in the selected time range window. You will note that with the auto position servo ‘ON’ the
transmitted pulse is set several nanoseconds below the top of the O-scope display.
The Auto Position Servo automatically sets the transmitted pulse ‘down’ in the time range window
approximately 10% of the value entered for the range e.g. if the time range is set at 50 ns, the transmitted
pulse will be set 5 ns below ‘zero’. This is done to insure that the entire transmitted pulse is recorded. If
this function is
inadvertently turned off during system configuration, the transmitted pulse (zero time)
may not be present in the time range window. If this occurs, the data will be useless as you will have no
zero time reference form which to measure time/depth.
GSSI recommends that you leave the Auto Position servo ON until you are familiar with the effects of
moving the signal up or down in the time range window. Please note that the ‘zero’ time on the SIR 20 is
indicated by the value of 100. This value is arbitrary and has to do with the delay time of transmit and
receive signals within the electronics of the system. This value does NOT affect the stored value of the
range.
Gain
The second button controls the Time-Variable-Gain (TVG). Gain is signal amplification used to
compensate for the natural effects of signal attenuation. As the transmitted signal passes through a
material, it will weaken (attenuate) as the material absorbs some signal. Gain amplifies that signal after it
is received to compensate for signal losses and make weaker reflectors easier to see. No additional energy
is sent out of the antenna. The Gain function operates at a number of linearly distributed points along the
time range.
The Auto Gain Servo automatically sets the gain of the system based upon the received signal from the
antenna
at the point where the antenna is located when the servo is enabled . At large sites it may be
necessary to briefly scan the site to decide if the Auto Gain Servo gain values are too high or too low.
In either case, you should adjust the gains manually. You cannot set the exact time of a gain point;
however, you can manipulate the number of gain points so as to ‘position’ a gain point near at a particular
time range (or a particular reflector). You can then adjust the amount of gain applied.