GDP-32
II
INSTRUCTION MANUAL
May 2002 Section 11, Page 16
11.5 A NOTE ON VARIABLE A-SPACING
When using variable A-Spacings, the key to getting the correct A-Spacing entered and used, is to
remember two things: 1) the last field changed controls the A-Spacing value that is used, and 2)
the A-Spacing that is used is ALWAYS the value in the Variable A-Spacing list.
For example, if the operator sets the A-Spacing value in the header to 50 M, then all of the
numbers in the Variable A-Spacing list are set to 50 M. If the operator then gets into the
Variable A-Spacing list and changes all of the values to, say, 25 M, then the A-Spacing value left
in the header will still be 50 M but the actual values used will be 25 M - as shown in the Variable
A-Spacing list.
The computer remembers where the latest changes were made and upon startup of the program.
The A-Spacing in the header will still be 50 M, and the values in the Variable A-Spacing cache
will still be 25 M. However, if the operator sets the A-Spacing in the header to, say, 100 M on
startup, then the Variable A-Spacing values will all be set to 100 M.
11.6 A NOTE ON PHASE
The CSAMT program is an absolute phase program - that is, the program measures the absolute
phase shift between the transmitted signal and the received signal. For normal operations at low
frequencies, the real-time phase shift should be around -100 to -200 milliradians for the E-
fields, and around +π/2 (1571 mr) for the H-fields. If the E-field phase is closer to π radians
(3142 mr) then you have some wires reversed. You can get rid of the π radians of phase shift by
either reversing the transmitter wires at the transmitter, or reversing the leads going into the
receiver. For multi-channel receivers, it is easiest to make the change at the transmitter, and then
maintain the same polarity through out the survey. If the phase for the H-field channels is
negative π/2, you have a 180 degree phase shift. The easiest way to solve this problem is to
rotate the H-field sensor by 180 degrees.
11.7 A NOTE ON SCALING
The following conventions are used for all measured and calculated parameters:
• E-field Voltage, (magnitude) displayed in volts.
• H-field Voltage, (magnitude) displayed in kilogammas when decalibrated with antenna cal.
• Current, displayed in amperes.
• Phase, displayed in milliradians.
• Apparent resistivity, displayed in ohm-meters.
• Dipole spacings, displayed in meters.
• Coil calibrate magnitudes, entered and displayed in millivolts per gamma.
• SP, displayed in millivolts
• SEM, displayed in milliradians
If scaling is necessary on these values, the following labels are appended to the end of the
number string:
M - Mega units
K - Kilo units
m - milli units
u - micro units
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