IMAGENEX DT101Xi, Техническое руководство

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The angular deflection of the sound impulse as a result of the
temperature gradient is given by Snell’s Law:
sin ∅
2
sin ∅
1
=
𝑉2
𝑉1
(Eq 12)
Or
∅
2
= arcsin (
𝑉2 × sin ∅
1
𝑉1
)
(Eq 13)
Where
Ø
2
= Angle of refracted beam (rad)
Ø
1
= Angle of incident beam (rad)
V2 = Sound velocity second layer
V1 = Sound velocity second layer
As sated above, it is recommended that a sound velocity cast be done prior to the start of the survey, at least
every 12 hours thereafter, and at the very end of data collection.
Ideally; and for surveys of large areas, there should be a comprehensive distribution of sound velocity profile
casts, both temporally and spatially, to maintain a representative currency of the sound velocity profiles for
the survey area. These should be recorded in an organised way for application later.
One SV cast should as a minimum, be done at the deepest location of the survey.
4.2. MBES Patch Test
The alignment of any MBES with the motion reference unit and gyro is critical to the accuracy of the
bathymetry system, both in terms of depth and position of acquired soundings. It is seldom possible to exactly
align the MBES head with the sensors measuring heading, pitch and roll, so an alignment error (residual
bias) must be determined. In addition, there may be a latency in the positioning system that impacts the
positional accuracy of acquired data.
Before calibration, it is necessary to check the installation parameters and to perform a sound velocity cast
to update the conversion of two-way travel time to range and calculation of the refraction solution. It is advised
to verify that the SV sensor of the MBES system (if fitted) correlates well with the surface SV of the
independent SV sensor used in the cast. Any discrepancies should be investigated.
In addition, accurate positioning is essential. An RTK GPS or inertially aided DGPS is recommended.
Good weather is required for the patch test.
The patch test should be performed in the deepest water available in the survey area.
The length of survey line for each aspect of the patch test are dependent on the size of the vessel / vehicle.
It is recommended that the survey lines be no shorter than 100 m and that the vessel / vehicle has sufficiently
completed any turn to come on line and that motion and heading reference units have settled.
The speed for the patch test is determined by the vessel in question’s characteristics. It should be conducted
at survey speed, such that the trim of the vessel be similar to that expected during survey.
The vessel should be in the ballast configuration expected during the survey. The Surveyor should for
example be aware that with a small vessel, the movement of personnel will affect the vessel trim and therefore
the patch test results. With a larger vessel, movement of fuel and water for example, may affect the trim and
render the measured values incorrect.