IMAGENEX DT101Xi, Technical Manual

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Doc no: 430-041-01 20 of 64
4. Pre-survey Observations
Prior to commencement of the survey operation, two very important tasks should be undertaken, namely, a
sound velocity cast and a patch test:
4.1. Sound Velocity Cast
A sound velocity measurement is essential for the operation of any MBES. Both DT101
Xi
and DT102
X
i have
the option of an integrated AML OEM SV sensor. This sensor however is for the measurement of SV at the
transducer, and not as a means of determining an SV profile from the surface to the seabed.
Note: The time of flight sensor’s calibration certificate is supplied as part of the MBES system and is valid for
1 year. The Surveyor should ensure the calibration certificate is in date, and send the sensor away for re-
calibration if the certification has expired. Removal of the OEM SV sensor from the sonar head is not
complicated.
The required temporal and spatial distribution of casts is determined by several environmental factors and
varies significantly. It is recommended that an SV cast be at least every 12 hours and more frequently if large
variance is observed between casts.
There are two methods of determining SV through the water column namely:
•
Time of flight sensor
•
Conductivity, temperature and depth sensor (CTD)
A modern time of flight sensor is calibrated under stringent laboratory conditions and is thought to be more
accurate than a CTD, which relies on formulae based on curve fitted empirical coefficients.
Most SV sensors can be set to log at set depths or continuously. The set depth is a better option as the profile
is generally smoother.
Imagenex has free software to process CTD casts and collate the output as set intervals. There are various
formulae (for example Del Grosso, Chen and Millero, Mackenzie etc.) for calculating sound velocity and are
maximum depth and region specific. If the region of operation is oceanographically significant (e.g. the Arctic
Ocean or Black Sea), local formulae may need to be applied. In order to operate correctly, a CTD will normally
need to have a correct atmospheric offset applied. This should be taken from a reliable and calibrated
atmospheric pressure sensor.
It is recommended that after a SV cast has been done, the SV at the surface (of the cast) be checked against
the integrated SV sensor (if fitted), and any discrepancies investigated.
In addition to the calculation for converting two-way travel time (TWTT) to a range (Eq 11), ray bending due
to refraction from the differences in sound velocity through the water column should also be applied. Most
MBES processing software has the ability to read an SV profile and apply SV changes to compensate for ray
bending (Eq 12, Eq 13) and overall sound propagation. The Surveyor should check that the software in
question applies a ray bending algorithm. It is strongly recommended that an SV cast be done and checked
prior to any patch test.
Note: The ray bending application within the Imagenex control software has been disabled, so ray bending
needs to be applied within the MBES logging or processing software.
𝑅𝑎𝑛𝑔𝑒 =
SV × T
2
(Eq 11)
Where
SV = Sound velocity (ms
-1
)
T = Two-way travel time