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FDU test functions

Sensor tests > Sensor Leakage test

428XL Users Manual Vol. 3 197

June 11, 2013

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Sensor Leakage test

This test is used to measure the global leakage resistance between theseismic channel and the earth ground.

Configuration

Figure 7-15

ADC input: connected to both the input circuitry from the sensor and

to the internal test network.

Pre-amplifier gain: 1600 mV (0dB) or 400 mV (12 dB), user-selected.

DAC: connected to the internal test network.

Filter type: user-selected; Sample Rate: user-selected (defaults to

2 ms if automation).

Note The Sensor Leakage test is irrelevant if the Input of the FDU is

left unconnected (or if the resistance connected exceeds

9999.

DSPADC

Test

network

24 bits

Pre-amp.

Signal ground

Channel input

Testgenerator

(DAC)

Test data from LAUL

Earth ground

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Two test sequences are used (T1, T2):

For Beginning and End times (Tband Te), see page 165.

Test principle

The principle behind this test consists of applying a voltage across the

FDU's ground and the earth reference. The DAC supplies two different current levels (with known

amplitude) to the internal network.

The ADC input is connected to the sensor channel and the voltage at

the outputs is measured.

The measured output voltage, mean1and mean2 is the value after

scaling the DSPs output (x1.62 or x0.42).

Filter type Sample Rate (ms) T1 (ms T2 (ms)

0.25 128 128

0.5 128 128

0.8 LIN 1 128 128

2 128 128

4 128 128

0.25 128 128

0.5 128 128

0.8 MIN 1 128 1282 128 128

4 128 128

0 %

50 %

Tb TeT1 Tb TeT2

Input level

(% of generator

full scale)

http://-/?-http://-/?-

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FDU test functions

Sensor tests > Sensor Leakage test

428XL Users Manual Vol. 3 199

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Knowing the output voltage and the current level, the system

computes the leak resistance value of the input sensor channel,

expressed in Ohms.

The test returns the leak resistance seen by the FDU, that is the global

leakage resistance between the input conductors of the receiver link and

the earth. Naturally this is an equivalent resistance, which may result

from a complicated network of leakage resistances.

Note The lower the resistance of the geophone, the more accurate the

the leakage measurement.

With no leakage between the conductors of geophone arrays, theimpedances, with respect to the ground (earth), of the two conductors

connecting a geophone array to an FDU are equal. The unwanted

signals picked up (atmospheric interferences, earth potential, etc.) are

then sensed in common mode and thus rejected by the FDU.

If any leakage takes place (due to water penetration in cables, or

connectors or geophones, etc.) then the links exhibit unbalanced

impedances. As a result, the common-mode signals are somewhat

converted into differential signals and therefore added to the seismicsignal.

Leakage may give rise to other faults:

Leakage between two conductors in the same geophone array will

result in a difference in the response to a pulse (gain and damping).

The discrepancy will be detected by the check for similarity in a Tilt

test.

Leakage between a conductor in a geophone array and one in anotherarray will give rise to crosstalk.

Leakage between a power supply conductor and a receiver link

conductor will give rise to noise which will be detected by a Sensor

Noise test.