98 User’s Manual Innomar SES Parametric Sub-bottom Profilers
98 Innomar Technologie GmbH (2020-09)
6.6 How to avoid interference with other acoustic systems
Often it is required to operate several acoustic systems at the same time to increase survey
efficiency, for instance collecting multi-beam data and sub-bottom data together. To avoid inter-
ference and data quality issues you should generally try to use different frequencies for the dif-
ferent acoustic systems. If similar frequencies are used by the systems, interferences may occur
and affect the data quality obtained seriously.
Sometimes even acoustic systems using higher frequencies than the INNOMAR SBP are affected,
due to the harmonics of the primary frequencies of the SES system that are generated within the
water.
There are different strategies to avoid or reduce interferences:
Place the transducers at different locations and make the distance as large as possible.
Free run all systems (i.e. without synchronizing them). This will result in uncorrelated inter-
ferences that may be removed by digital signal processing like stacking and spike removing
(e.g. median filter).
Synchronize all systems to a fixed ping cycle and give each system a fixed time slot for
pinging. This results in determined interference that may not harm data quality or may be
removed more easily than random interference.
A typical survey operation requires collecting multi-beam data and sub-bottom data at the same
time. If both systems are working in a similar frequency range, interferences may cause wrong
bottom detections in the multi-beam data sets. The INNOMAR data usually is not affected seriously
by the MBES. Since the INNOMAR systems use a very narrow sound beam, in most cases only
the centre (nadir and near-nadir) MBES beams are affected. To increase the signal-to-interfer-
ence ratio and thus get better bottom detection for the MBES it is recommended to use the high-
est possible transmit energy (the longest pulse length) for the MBES.
If the systems are not synchronized, the wrong MBES depth values are mostly at random depths
and can be edited or post-processed only with time consuming work.
To avoid random interferences and to put the spikes coming from the INNOMAR SBP to a certain
depth level, the MBES and the INNOMAR system should be synchronized. For this either the
INNOMAR or the MBES can be used as master to trigger the other system.
In most cases the multibeam data are of higher priority than the sub-bottom data. Therefore, the
MBES runs a master device at full ping rate and the SBP has to fit into the given time slots. This
is a perfect solution for shallow water applications (or low altitudes for ROV based operation) and
will be discussed in section 6.6.2 on page 99.
6.6.1 Synchronizing / Triggering – General Remarks
It is possible to synchronize two or more systems via trigger pulses in order to reduce acoustic
interference. There are “Sync IN” and “Sync OUT” BNC-connectors placed at the front panel of
the INNOMAR SBP transceiver unit that can be used to get TTL compatible trigger pulses to/from
other systems.
In the case of triggering another system, the “Sync OUT” has to be used. The triggered (slave)
system has to be able to handle the (rather high) ping rates that are produced by the INNOMAR
system.
It is also possible to trigger the INNOMAR system by another system using the “Sync IN” connector.
Every time a trigger pulse is detected at Sync IN, the SES system is forced to transmit a sound
ping (or a burst of sound pings). Please keep in mind that the ping rate of the INNOMAR SBP may
be decreased significantly and therefore the lateral resolution and the data quality may be de-
creased, too.
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