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A – Operating Theory
A.2.4.4 Rejection Parameters
The rejection values, once determined, should last the length of the survey. However, it is sensible to
check that seawater signals are correctly rejected in the way described above once every half hour.
The SDC software will generate a reminder every half hour. The background compensation must be
repeated, but is sufficient merely to check that the active compensation is still correct.
The rejection values are determined by many factors, but the most important of contributions are due
to the conductivity of the seawater and the overall water depth. The relations between the values and
water depth is non linear, and changes more steeply at shallow water depths. For this reason, if the
survey begins in deep water but moves into shallow water (e.g. when following a pipeline inshore) the
calibration must be more frequent. However, at water depths of 50m or greater, the values will no
longer be affected by depth.
The effect of seawater conductivity is more difficult to compensate for, since it can vary in an
unpredictable way. In deep water (>100m) the conductivity (determined by salinity and temperature)
will be nearly constant, and should have little effect. However, if the survey crosses the thermocline or
will approach areas of different conductivity such as vents or fresh water outfalls, then the active
compensation will need to be checked more frequently.
If a conductivity probe is available on the vehicle, then any changes will be apparent to the users. A
change of more than a few mS/cm may necessitate recompensation. If it believed that the conductivity
has changed, then the rejection can be checked as described above.
A.2.4.5 Background Noise Profile
The SDC software provides a "Background Noise Profile" dialog. This facility displays traces of the
three coil voltages along with additional displays to determine spurious signals within the surveying
vicinity.
Figure A-9
illustrates the display during a Background Noise Profile.
Figure A-9: Background Noise Profile Window
The Port voltage is displayed in red, the Centre voltage in amber and the Starboard voltage in green.
The dotted red line displays the signal threshold currently set for the system.
The voltages displayed during the Background Noise Profile can be logged. This data is timestamped
allowing for comparison with other survey data to assist post-survey analysis. The format of the
Background Noise Profile log file is shown in
section 6.4.1.3 "Background Noise Profile Logging
Format" on page
6-32.
DPN 402196 Issue 4.1
© Teledyne TSS
A- 9
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