Teledyne 440, System Manual, Page: 225 / 256

A – Operating Theory
DPN 402196 Issue 4.1 © Teledyne TSS A- 7
complete lift curve will be analysed. In conditions of increased seawater conductivity, this scale may
not be sufficient to assess the whole curve, and must be increased.
When the vehicle has reached sufficient height, the “stop” button can be pressed. Then, the graph can
be assessed. It should show a smoothly rising curve which passes through the origin, as shown in
Figure A-5 . If the background compensation was not correctly carried out, then the graph will not meet
the origin, as shown in Figure A-6 A small y- intercept (50µV) can be tolerated, since it is the gradient
of the curve which is important.
Figure A-5: Perfect lift with the results passing through the origin
Figure A-6: Example of poor background compensation
If metal is present on the sea bed, then the graph will not move smoothly away from the origin as the
vehicle is lifted. This is shown in
Figure A-7 . This will have a detrimental affect on the accuracy of the
calibration values and could ultimately lead to incorrect survey data. More importantly, it indicates that
the background calibration will not be valid, and must be repeated.
Figure A-8 shows another effect
due to the coils moving as the vehicle was lifted. The vehicle was not lifted smoothly, resulting in
missing points in the middle of the graph.