1. PARAMETRIC SUB-BOTTOM PROFILER: A NEW APPROACH FOR AN OLD PROBLEM
N° MS06
PARAMETRIC SUB-BOTTOM PROFILER:
A NEW APPROACH FOR AN OLD PROBLEM
Geoff Holland
EXTERNAL THREATS TO CABLES & BURIAL
The greatest threats to submarine cables are fishing and shipping in shallow water areas, and burial is the prime tool for cable protection.
TRADITIONAL SUB-BOTTOM PROFILERS
Sub-bottom profilers identify the thickness and internal structure of sediments, which helps burial assessment.
years.
The resolution of sub- bottom profilers has historically been fairly poor—layering cannot be adequately seen in the upper seabed.
The technology hasn’t seen many advances in the last thirty years, although CHIRP was an improvement in some seabed types.
It is therefore vital to understand the upper 1-2m of the seabed, in order to accurately predict burial at the survey stage, and also to optimise burial depth achieved during the installation.
THE PROBLEM WITH TRADITIONAL SYSTEMS
THE PARAMETRIC SOLUTION
As mentioned above, the main issue is resolution in the upper seabed
Systems are typically low frequency (<10kHz). Pingers have higher frequency, which improves resolution, but low power, which can lead to rapid acoustic attenuation and poor penetration. Boomers have higher power, but this leads to ringing in the upper seabed, especially in shallow water.
Transducer ‘receive’ mode typically looks for a return of a single frequency; reflectors sitting close together can be seen as a single reflector, because their individual returns occur in the same receive pulse. CHIRP technology helps in this regard, because the frequency sweeps through a range and individual reflections of different frequency can be identified within a single receive pulse.
Systems typically have a wide beam angle, with side lobes—data from side lobes can create anomalies that can mask true data.
Two geophysical phenomena exacerbate poor resolution in the upper seabed:
RINGING/REVERBERATION
Ringing is the result of high intensity acoustic energy causing the sediment grains in the upper seabed to reverberate. This causes a chaotic energy return to the transceiver, obscuring resolution in the upper seabed. This is a particular problem in coarse sediments in shallow water, because the sub-bottom profiler’s energy source is so close to the seabed. It is also made worse by wide beam angles, which cause blurring laterally.
MULTIPLES/GHOST REFLECTIONS
This is a result of the high energy acoustic wave reflecting off the seabed and then again off the sea surface, before it is eventually seen by the transceiver. It results in a mirror image of the seabed at double the depth on the sub-bottom profile. This ghost reflector can obscure data, especially in shallow water.
Seabed Multiple
From L3-Klein Website
FREQUENCY
The Parametric transducer simultaneously transmits two Primary Frequency signals of slightly different high frequency energy.
Parametric systems utilise the non-linear property of acoustic energy propagation in water—sound speed varies as a function of pressure (i.e., increased depth)—to mix the two primary acoustic signals, achieving ‘sum’ and ‘difference’ frequency components.
The Difference Frequency is the one of greater interest, as it is a lower frequency; also, it is generated in a narrow beam associated with the original higher frequency:
Beam width from the transducer is a function of the wavelength of the source signal and the diameter of the transducer. Given a fixed transducer diameter, as the frequency increases, the beam width decreases, which focuses the energy on the seabed.
Thus a small transducer is capable of producing reasonable bottom penetration with very good spatial resolution.
NARROW BEAM WIDTH WITHOUT SIGNIFICANT SIDE LOBES
A conventional system’s beam has a main acoustic energy lobe, with half-energy side lobes. This means that the sound at the border of the beam has a longer two-way travel time than at the center of the beam. Thus the reflected signal is longer than the transmitted signal, and this affects lateral spatial resolution. Refraction, particularly in deeper water, can make this worse. Slower survey speeds are required so as not to impair spatial resolution.
Parametric systems have a narrow beam width, which means better spatial resolution and higher survey speed.
The narrow beam width also reduces reverberation, signal to noise ratio is reduced, and ambiguous reflections are avoided.
From Innomar Technologie GmbH
CONCLUSIONS
Current research indicates the potential for clearer seabed images of the upper seabed with Parametric technology, which could improve the identification of sediment structure and composition. This would lead to more accurate to Burial Assessment Studies and better protected cable systems.
Improved data quality and survey speed could be achieved: sub-bottom profile transducers are usually co-housed in the side scan sonar towfish; modern side scan sonars can survey at ~8 knots, but survey speed is currently low (~3.5 knots), due to impaired spatial (lateral) sub-bottom profile quality.
Transducers are lightweight and more easily mobilised onto smaller vessels. This could make the technology a particularly good solution for inshore surveys, where the very shallow water often leads to reverberation and multiple data issues anyway.
Side by side comparisons of standard vs. parametric sub-bottom profile data could be undertaken, particularly in deeper water, where acoustic power might be an issue, with a view to improving cable survey Scope of Work specifications for the future.