Appendix B Test Overview
Io
SENSOR
96
SMT-200 Operations and Technical Manual 2.00/R1
NOTE
: The SEG standard for geophones states that the upward motion of the geophone case should
produce negative numbers on tape. This normally means geophones are wired so that an upward
motion of the case will result in a negative going signal. This is a recommendation only and every
geophysicist is obviously free to choose SEG or Non-SEG polarity
B.3 Noise
Test
The SMT-200 assumes that there are no other signals that are influencing the geophone under test. If
other signals are acting upon the geophone then these signals will be added to the test signal being
applied and the unit will incorrectly identify this an error signal generated by the phone. To avoid this
situation the ambient noise can be monitored. There are two types of noise that can disturb the
SMT-200 readings.
Mechanical Noise
This can be random or periodic noise produced by objects such as trucks
passing near the phone under test or generators running nearby. The geophone
is physically moving to produce an error signal. To avoid this run the tests away
from heavy machinery and avoid people walking near the geophone while the
unit is collecting data.
Electrical Noise
Can be less obvious to detect. The error signal is being induced into the leads
that make up the test system even though there is no physical connection
between the two. This process is know as induction and is the principle
transformers use. Normally this is caused by running tests under or over high
voltage power lines. If there is no obvious reason for tests failing suspect
induced noise, possibly from underground power lines.
TEST PRINCIPLE
………
The SMT-200 monitors the output of the geophone for 800 samples over 96 ms, the average level of
noise is then computed. The lower the noise valve the better. In practice (depending on the geophone
string confoguration) a result of less than 30mV ambient noise ng picked up will not effect the results.
B.4 Leakage
Test
In the perfect geophone string, the current flowing out of one terminal of the string is exactly equal to
the current flowing into the other terminal of the string. If there is another path the current can take
then this is known as leakage.
The figure above shows the current has found another path, in this to case to ground. The main problem
here is the effect it has on the recording systems input stages. Most modern recording systems have
differential inputs. That means any signal common to both wires will not be seen on the output of the
preamplifier. This is know as common mode rejection. This will only work if the current into and out of the
preamp are equal and balanced . A typical example is 50 Hz mains pickup. If there is a current imbalance
some of the 50 Hz common mode signal will be converted to a differential mode signal and appear on the
output of the preamplifier. This is typically seen when the geophone strings protective jacket has been
damaged and the string is used in water.
Leakage path due to damaged leader cable
