NATURAL SOURCE MAGNETOTELLURICS
Section 13, Page 17 May 2002
13.5 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.
13.6 A NOTE ON PHASE
For scalar operations the phase difference between Ex and Hy should be between 200 and 1500
milliradians. In the presence of 2 and 3-D features, or when measuring fields in an anisotropic
environment, the phase values can approach π radians (3141.6 mr), but this is unusual.
For vector operations, keep the phase differences positive for both sets of measurements by
orienting Ex positive north, Hy positive east, and Ey positive east, Hx positive south.
For tensor measurements orient Ex positive north, Hy positive east and then Ey positive is
oriented east and Hx positive north. For this configuration the phase difference will be positive
for Ex Hy and negative for Hx Ey, which is the standard tensor setup.
If the phase differences are coming out negative when they should be positive (and vice versa),
this means that you have either the E-field or H-field orientations wrong. To correct this, just
switch one of the connections at the receiver, or rotate the H-field sensor by 180 degrees.
13.7 A NOTE ON SCALING
The following convention is used for all measured and calculated parameters:
• Voltage (magnitudes), displayed in volts.
• 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
• CC, displayed in units from 0.00 to 0.99
• E-field (magnitudes), displayed in volts.
• H-field(magnitudes), displayed in kilo-gammas
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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