IMAGENEX DT101Xi, Technical Manual

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Doc no: 430-041-01 16 of 64
Where:
LS = Line spacing (m)
A = overlap required (e.g. 75% coverage; A = 0.75, 100% coverage A=1)
Z = Approximate depth below transducer (m)
Ø = Set Beam Angle - (e.g. 120) (°)
3.2.5. Ping Rate and Survey Speed
There is a relationship between the survey speed, water depth, fore / aft beam width, overall swath width and
ping rate. The ping rate in the DT101
Xi
and DT102
Xi
is automatically set and is based on the range setting,
number of beams selected and specification of the computer system used to run the control software. The
real-time ping rate can be viewed in the Sonar Status window (Real Time PRF)
– Options > Sonar Status.
Both the two way turn around time (in ms) and frequency (Hz) are displayed.
The maximum speed of the vehicle should be planned and adjusted to ensure 100% forward overlap of the
beam footprint. The maximum speed for a MBES can be calculated using the following equation (Eq 3):
𝑣 = 𝑆 × 𝑑 × tan(𝛽/2)
(Eq 3)
Where:
v = speed (m/s)
S = MBES ping rate (ping/sec)
d = depth (m)
β = fore aft beam-width (3° for DT101
Xi
and DT102
Xi
)
Survey speed should also be such that:
•
The sonar head is not subjected to excessive force from water flow
•
Water flow over the transducers does not cause aeration and a loss of performance
•
The transducer pole (if applicable) does not experience excessive vibration. Vibration of the pole (if
fitted) is a big contributing factor to data artefacts
•
If data is being logged during turns, it should be remembered that outer beams will have less
coverage during a turn. Speed should therefore be reduced to allow full coverage of the seabed
3.2.6. Sounding Grid Size
A sounding grid (also called a sounding matrix) is a series of adjoining squares that cover the area being
surveyed and is used in data processing to have a bathymetrically representative model of the survey area
but with significantly less data points than were originally logged. A grid size is generally defined and is a
function of required resolution. If, for example, a 0.5 m grid size is defined, then the processing package will
calculate a representative depth (e.g. median depth) for all soundings that were measured within those spatial
parameters (i.e. within a 0.5 m square). A balance should be sought between size (and therefore
manageability) of the model and required resolution and detail required to be reported. The grid size is
generally defined by the deliverable specification, but as a rule of thumb, the grid size should not exceed the
average footprint size of the MBES on the seabed. Given a flat seabed, the following equations (Eq 4, Eq 5,
Eq 6, Eq 7, Eq 8, and Eq 9) can be used to calculate the footprint size (the shape of which approximates an
ellipse):