144 User’s Manual INNOMAR Parametric Sub-Bottom Profilers / SESWIN
144 Innomar Technologie GmbH (2023-01)
are generated per time unit and distance travelled) and the signal to noise ratio of the received
echo signals (i.e. what is the weakest detectable reflector).
There are several relations of the above-mentioned parameters to be familiar with in order to
choose a proper parameter set for a specific survey task. Some of the relationships require a
trade-off, there is no perfect world in acoustics!
Frequency
The lower the frequency the more penetration could be achieved (due to less
attenuation of lower frequencies within the water column and sediments).
The higher the frequency the better resolution could be achieved.
However, the lower the frequency the lower the signal to noise ratio (i.e. low-frequent
sound pressure level is lower and more noise is picked up, such as vessel noise,
propeller noise, ambient noise etc.)
Therefore, not the lowest frequency is the best one for good penetration, but the frequency
which is as low as possible and provides an acceptable signal to noise ratio at the same time.
This would allow to detect weak reflectors with a high frequency which would be hidden behind
the noise at low frequency settings.
It is best to start in the middle of the given frequency range (8 – 10 kHz for the INNOMAR shallow-
water models) and then try to lower or increase frequency according the survey requirements.
Pulse Length
The shorter the pulse length the better the vertical layer to layer resolution.
The longer the pulse length the better the signal to noise ratio and thus the better the
penetration.
Therefore, often it will be necessary to choose a slightly longer pulse to improve the signal to
noise ratio of the data at the cost of vertical resolution, especially for lower frequency settings.
Use the shortest pulse length which is possible for the required penetration to ensure best
resolution.
Range
The range settings affect the ping rate. In normal transmission mode (shallow water) the
ping rate decreases with increasing range start and range length (the deeper the water
the lower the ping rate). In multi-ping mode (also called deep-sea pulse mode), ping rate
decreases with increasing range length, the range start has nearly no effect (the longer
the range length, the lower the ping rate).
The higher the ping rate the better the lateral resolution and the more stacking could be
applied during data processing for noise rejection.
However, sometimes it might be necessary to choose a longer range for easier
adjustment of the operating window (e.g. at steep slopes) or if deep penetration is
available and shall be recorded.
Important Note: All data outside the operating range (i.e. the range window visible on the screen)
will not be recorded! Tus make sure the “Range Length” is large enough to cover the entire depth
required for data logging.
The “Range Length” cannot be changed while logging data.
The “Range Start” can be changed with Mouse and Keyboard, the best way is to use the
[PageUp] and [PageDown] keys. Steps can be set up in the SESWIN software.
If possible, avoid the “Auto Range” function – this function will easily fail on slopes or when
you pass through other vessel’s keel water etc.
Gain
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