TIME DOMAIN INDUCED POLARIZATION (TDIP)
Section 9, Page 17 May 2002
9.5 ALGORITHMS
The equation used for calculating the time domain (see below) is the equation used in Swift
(1973). By inverting the negative half-cycle, chargeabilities are averaged over each cycle until
Escape
is pressed or until the specified number of cycles have been acquired. The output will be
in milliseconds or millivolt-seconds per volt.
This equation was originally given to Zonge by Newmont as the "Newmont Standard"
chargeability. Since that time it has been determined that this is not really the Newmont
standard, but it can be obtained by multiplying this "Zonge Standard" by 1.53. In order to reduce
confusion, we have retained the original chargeability definition, and convert to the Newmont
Standard (if desired) in our data processing programs.
For the "Zonge" standard at 0.125 Hz (8 second period):
Where T is the cycle period of 8 seconds and the integral of the secondary (Vs) or off-time
voltage is from 0.45 sec to 1.1 sec.
With 1024 points sampled per cycle, Vs is summed over 83 counts out of 256 per quarter-cycle.
The 13 windows defining the off-time decay waveform are obtained on 150 ms intervals at 0.125
Hz. The closest combination of windows to get an approximation of the chargeability is a sum
of windows 4, 5, 6, and 7. At 0.125 Hz this effectively integrates from 500 ms to 1100 ms,
which is 50 ms shorter than the standard window, so this approximation will always be slightly
lower than the Zonge Standard chargeability.
With W
i
= Normalized decay point value in 10's of milliunits
= (Sum of Vs over the 150 ms intervals)/(Vp x 19)
the chargeability, M = T/1024 x 1.87 x 19 x 6W
i
/10
where: 1.87 is the Swift constant
19 is the number of counts per 150 ms window
T/1024 is 'W, the digitization interval
The following formula for M is used for the approximation of the Zonge Standard at 0.125 Hz (8
second period):
M = (1.87 x 19 x 8) x (W4 + W5 + W6 + W7) / (1024 x 10)
For frequencies up to and including 0.5 Hz (2 second period), 1024 points are sampled per cycle
(256 during each on-time and 256 during each off-time). At 1 Hz the sample rate is 512. The
windows measured are proportional to those taken at 0.125 Hz, and the results will be printed out
in identical format.
Since the number of samples per cycle at 1 Hz is 512 rather than 1024, the equation for
chargeability for 1 Hz is as follows:
M = (1.87 x 9 x 1) x (W4 + W5 + W6 + W7) / (512 x 10)
Reference: Swift, C.M., Jr, 1973, The L/M parameter of time domain IP measurements --- a
computational analysis, Geophysics, v 38, p 61-67.
³
uu Vs
Vp
T
M
87.1
1024
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