Field Experience with a New Sub-Bottom Investigation Tool Acoustic 3-D Imaging of the Sub-seabed

Gary Dinn, P.Eng. Vice President Technology Development

PanGeo Subsea Inc. St. John’s, Canada

gdinn@pangeosubsea.com

Abstract— A new means to interrogate and image the sub-seabed has been developed comprising a conventional linear hydrophone array (beamforming) together with Inertial Navigation System (INS) stabilized Synthetic Aperture Sonar (SAS) processing. The system, called the Sub-Bottom Imager, completed its first commercial work during 2011 in decommissioning, high voltage DC (HVDC) cable surveying, and marine archaeology applications with unprecedented results.

Index Terms—sonar, sub-bottom, beamforming, INS, SAS, voxel, AUV, WROV. (key words)

I. INTRODUCTION The production of high quality sub-bottom imagery

showing accurate detail of the sub-seabed is becoming essential to support offshore construction and related fields including oil and gas production and the burial of underwater cables and pipelines, decommissioning of older subsea infrastructure, and the detection of geohazards such as unexploded ordnance and buried boulders.

In conventional sub-seabed mapping, it is important to

discriminate layers that correspond to geological units (stratigraphy) as well as provide the ability to penetrate into the seabed as far as possible to achieve data to useful depth with a single pass. Sub-bottom profilers have long been valued for their ability to provide this information as a 2D line display of the traversed bottom. In sub-bottom profiling however, the normal inverse relationship between the frequency of the sonar and its spatial resolution is further complicated by the exponential absorption of energy by the seabed sediments as a function of increasing frequency, typically quoted in dB/m/kHz. To achieve useful penetration in the seabed sediments then, low frequencies (<10kHz) by sonar standards are required and yield a correspondingly lower spatial resolution. As a result, conventional sub-bottom profilers cannot both penetrate to useful depths and also reliably image buried man-made objects that in many cases have sub-wavelength dimensions for the frequencies employed.

II. LIMITS TO PENETRATION The peak power output of a sub-bottom profiler is a key

variable in generating source levels that aid in achieving penetration into the seabed. Increasing power levels, however,

have both engineering and physical limits in their ability to positively affect this equation. For example, employing very high-power drive levels as a means of generating high acoustic source levels can cause cavitation and attendant bubble formation on the active face of the transducer, drastically lowering its effectiveness. One means of overcoming this is to form arrays of multiple transducers firing together in phase, at acceptable source levels to increase effective power delivered into the seabed. While this can achieve somewhat better depth of penetration, it is cumbersome and does nothing to increase resolution.

While delivering energy at depth is a key requirement in

order to image into the seabed, it is not sufficient. A key limitation in attempting to discriminate buried objects in the seabed is not only achieving signal penetration or even acceptable signal-to-noise ratios for reflections from targets at depth, but rather also achieving sufficient signal-to-reverberation ratio that permits a target to stand out against the backdrop of other energy that arrives at the receiver. This is especially important in complex seabeds containing gravels, cobbles and boulders, the presence of which may be of key interest. Without a sufficient signal-to-reverberation ratio, only major specular reflectors are typically resolvable in the data.

One means of achieving sufficient signal-to-reverberation

ratios is to avoid acoustically illuminating large areas of the seabed with the source transducer. This implies achieving tight directivity of the source energy consistent with a narrow beam. However, this can be at odds with the requirement to use a low frequency to achieve penetration, as the directivity of a transducer is a function of the physical size (i.e. diameter) versus wavelength (frequency). A conventional low frequency transducer would have to be impractically large in comparison to its wavelength to achieve a suitably narrow beam, and even then would be subject to the physical constraints of side-lobe generation in the transmit source pattern complicating the subsequent data analysis.

III. FOCUSING APPROACH A more elegant means of achieving this directivity is with a

type of transducer known as a parametric array. A parametric array transmits at a much higher frequency (typically 100kHz), benefiting from the inherent directivity of the shorter

978-1-4673-0831-1/12/$31.00 ©2012 IEEE This is a DRAFT. As such it may not be cited in other works. The citable Proceedings of the Conference will be published in IEEE Xplore shortly after the conclusion of the conference.

wavelength (vs. transducer dimensions), but the 100kHz transmission levels are modulated at a desired lower frequency (i.e. 8kHz), intended for sub-bottom penetration. Analysis of this type of transducer indicates that the confined beam of acoustic energy impacts a small area of the seabed and imparts the lower frequency energy from that point, resulting in the creation of very little volume reverberation in the seabed. Hence, parametric sonar achieves acceptable signal to reverberation levels in this way. However, since source levels of the low secondary frequency generated by parametric sonar are somewhat weak, signal-to-noise ratio can be a concern if these systems are deployed from a platform containing other noise generating equipment such as a Work Class Remotely Operated Vehicle (WROV).

The alternative to achieving the required signal to

reverberation ratios by focusing the energy of the sub-bottom sonar on transmit, as accomplished with the parametric array, is to evenly illuminate the entire seabed of interest, and then focus or listen for energy only from a very narrow, specific direction. To be effective, such an approach must form an extremely narrow and focused listening “beam” (see Fig. 1). Based on the relationships between frequency, wavelength and transducer size, this beam forming approach implies a very large listening array and that the target of interest is in the near-field of the array. To be in the near-field is to say that a point of interest anywhere in the target volume of seabed is a discernibly different distance from each of the elements that make up the listening array, and that the individual elements are spaced less than a wavelength from each other. Such a phase coherent array of the required size would be physically large and somewhat impractical to wield.

IV. SYNTHETIC APERTURE APPROACH Synthetic aperture sonar (SAS) is a method of combining

the energy from many successive sonar pings and through the use of sophisticated processing, form in software a virtual coherent array with a very narrow effective beam. For sub-bottom interrogation, this approach can yield the required signal-to-reverberation ratio to discern faint target echoes in a reverberant seabed, while also providing excellent signal-to-noise ratios. The larger effective “virtual” sonar array allows for high-resolution sub-bottom imagery at the lower frequencies required for sub-bottom penetration making full 3D

imaging of a continuous volume of seabed not only possible, but practical.

In terms of seabed construction activity (other than

foundation pile driving), the zone of greatest interest is the first 5m of Depth Below Seafloor (DBS). This zone encompasses the target burial depth of pipelines and cables for protection from fishing gear, anchoring and even ice encroachment/scouring, and surpasses the depth at which archaeological artifacts and Unexploded Ordnance (UXO) is typically found. To reliably penetrate the seabed to this depth implies an appropriate frequency range for the acoustic energy to be generated, and together with the requirement to resolve the time of arrival of the signal to sufficient precision, defines a necessary time-bandwidth product. When combined with the requirement to resolve objects as small as 10cm in diameter typical of buried power cables, and to also cover a reasonable swath of seabed, these inputs begin to define a solution space for a practical system, and have implications for its operational implementation.

The subsea industry must know the depth of burial of a

cable or pipeline to ensure that the required degree of protection has been achieved. The degree of accuracy and resolution of this knowledge must be better than 10% of the target depth of burial, which could be a shallow as 1m. To resolve the time of arrival to better than this requires a pulse length (time) and bandwidth (chirp frequency modulation) that are sufficient to be decoded with the required arrival time precision. Practical transducer bandwidth limitations further imply a pulse duration and physical length in the water column (i.e at 1500m/s sound velocity) which defines the minimum distance that the array must be above the seabed during operation to avoid the first seabed return arriving at the receiver while the chirp pulse is still being transmitted. If the array transport vehicle is a WROV, then this platform defines both forward survey speeds and local generated noise levels, primarily from the Hydraulic Power Unit (HPU), that must be accommodated. Together, the resulting problem definition is both a challenging and interesting one, but possible to be satisfied with a practical system design.

The synthesis of an achievable realization of the

requirements and approaches outlined above can be described as follows:

A modular linear array of multiple hydrophone elements, with sub-wavelength inter-element spacing, and having an overall length not greater than can be accommodated by typical WROV launch and recovery systems, together with appropriate chirp source transducers, and a suitable Inertial Navigation System (INS) to determine the exact position and orientation of the hydrophone array at the time of each chirp transmission. When flown on a standard Work Class ROV, and together with the ability to upload and process in real time the entire data flow from the acoustics, the system described here can continuously image a 5m wide x 5m deep swath of the seabed, at forward speeds exceeding 1m/sec. (Fig. 2)

Fig. 1. Typical Beamforming and Steering

Fig. 2. Complete SBI Equipment set with ROV skid mount

The new technology being described here is referred to as a “Sub-Bottom Imager” (SBI) rather than a sub-bottom profiler because instead of providing a two dimensional “profile” of the seabed along the line of traverse, it has the ability to interrogate the seabed to provide a “volumetric” resolution of 10cm x 10cm x 10cm. Thus the seabed is rendered in a 3D mosaic of volume pixels or “voxels”.

The SBI combines the benefits of a near-field coherent-phase linear array, with SAS synthesis to yield a unique ability to “image” buried objects in the seabed. This paper describes the technology behind the system with particular reference to the Sub-Bottom Imager as developed by PanGeo Subsea, and outlines the sonar design and data processing challenges, while providing operational examples from the use of the system in detecting and imaging a variety of buried subsea targets.

V. VOLUMETRIC IMAGING The Sub-Bottom Imager repetitively ensonifies the volume

of seabed under the hydrophone array and continuously receives the resulting acoustic reflections from within the seabed, as the array moves forward on the vehicle. This “raw” sonar data is digitized and transmitted via Ethernet over the vehicles fibre optic umbilical to the surface support vessel.

The high-volume, continuous flow of data is “rendered” on board by computers employing parallel processing architectures to produce a 3D volumetric data set of the seabed. (Fig. 3) The seabed under the array can be thought of as a three dimensional cube, encompassing an array of voxels, each comprising x, y, z position, signal intensity, and contribution count. The data is referenced to INS position and each cell contains the average signal intensity of the number of times that voxel was contributed to as the array traversed over top.

Fig. 3. SBI 3D Volumetric Rendered Data Set

Figure 3 shows the ensonified seabed volume after rendering limits are applied, visualized in the 3D survey software environment NaviModel. Signal intensity is represented by color with blue representing quieter areas and red higher intensity reflections. The upper blue layer is the water column below the transducer array with the adjacent red layer the seabed interface. The acoustic beam spreads out as the signal penetrates into the seabed, giving a volume width of 3.5m at the transducer, increasing to approximately 8m at the lower extent of the vertical penetration into the seabed of about 8m.

Although not evident in this image, the “contribution

count” data is an indication of the over-sampling that occurs due to the high ping repetition frequency (prf) required to satisfy the SAS requirements, combined with the vehicle forward speed, which effectively causes the re-illumination of the same seabed scene many times as the vehicle advances. This is key to the system’s ability to provide a multi-aspect view of objects, as they are detected on approach, while over top, and as the system moves beyond.

A. Synthetic Aperture Sonar Synthetic aperture sonar (SAS) coherently combines the

data from many chirps to emulate a longer sonar array, increasing spatial resolution in the direction of motion. The SBI uses this effect, in combination with conventional physical array beam forming, to achieve its imaging results. The physical array dimension for the SBI is adjustable as the hydrophone is a combination of modular 8 channel sections that can be assembled in 3, 5 or 7 unit lengths yielding a 24, 40 or 56 channel array. The imagery provided in this paper was collected with a 40-channel array, having an overall length of 3.5m. Since the SAS effective length is a software construct, in effect using software and processing to replace costly and unwieldy sonar hardware, it is a programmable variable. To provide a comparable resolution to that achievable with the physical 3.5 m long array, the SBI processing shown here employed a 4m SAS length in the direction of motion.

Fig. 4. Synthetic Aperture Sonar and Arraying Focusing

The key determiner of the effectiveness of the SAS is the quality of the Inertial Navigation Data from the INS system. Effectively, the INS must position the array accurately and precisely at all points along the duration of the SAS length. This is a somewhat different requirement to the traditional use of INS to provide accurate position over long durations of time. With typical forward vehicle speeds of 1m/s, the 4m SAS

length is only 4 seconds in duration, but within these 4 seconds the INS must provide an accuracy and precision of much less than the wavelength of the sonar frequencies involved while also exhibiting very low jitter.

During development of the SBI, controlled INS testing was conducted in the large towing tank at the National Research Council Institute for Ocean Technology in St John’s, Canada, with the objective of quantifying INS performance and its effect on image clarity. The fully instrumented tow carriage, which runs the length of the 200m long, 8m wide and 7m deep tank (Fig. 5), was used as a motion reference while the hydrophone array was transited over a bottom scene comprising active sonar targets. (Active targets were used as a means of mitigating the effect of sonar bounces in the confined tank.) A combination INS and Doppler Velocity Log (DVL) was co-located with the hydrophone array on the same mounting frame during the testing. The resulting sonar image from each run was rendered with the INS data as motion reference and separately with the carriage instrumentation as motion reference, and subsequently compared. Only when a high-resolution INS system, such as an IXBLUE PHINS combined with a 1200 kHz DVL was employed did the results yield satisfactory SAS array formation and image resolution. Testing with lower quality INS systems failed to yield acceptable results.

The SBI system’s 5m depth below seabed specification indicates that a short recording window per ping is sufficient to receive all the data necessary to form the final rendered image. This in turn allows prf rates that can be as high as 75Hz. In practice, network latencies tend to limit typical achievable prf’s to about 50Hz. To ensure that the seabed below the array is fully illuminated, the SBI uses three chirp projectors that fire in sequence. Each projector covers about half of the swath, providing redundant coverage and contributing to the resulting robust oversampling. The data flow generated by the system includes 40 channels of 24 bit sonar data, INS data, DVL data, altimeter data, sound velocity data, system status info and related metadata. Together, the continuous data flow from the system can exceed 120Mb/s, requiring a Gb/s connection or a spare optical fibre that can be driven independently of other networks. This data volume is on par with more sophisticated multibeam systems, and during operation is continuously being recorded to disk.

During the early development of the system, the recorded data volume was post processed using a batch approach and reviewed in a delayed session after collection. The data processing required is quite intensive and includes: bandpass filtering, cross correlation, and spatial rendering. While batch processing was acceptable during development, it was not considered a viable commercial approach given the cost of vessel time and the need to confirm imaging results in real time. Additionally, many applications require that “live” feedback be provided to the ROV pilot to permit a buried target, such as a pipeline or power cable, to be “tracked” in real time.

To achieve the processing speed required, a parallel processing approach was adopted based on implementing the algorithms using processors designed for video gaming applications. The nVidia processor family was selected due to the suitability of its hardware capabilities and also because of the availability of the CUDA operating system that provided a convenient software development platform. The final implementation of the rendering hardware consists of multiple nVidia CUDA video processing cards in a cutting edge PC chassis with up to 96G of RAM.

The rendering processing effort is greatly dependent on the fineness of resolution of the final image render. The frequencies employed in the system range from 4-16kHz which when combined with the array dimensions indicate focusing that supports a final rendered resolution of 10cm3. However, practical experience has indicated that although this resolution is appropriate according to theory, image improvements are evident at finer 5cm3 rendered resolutions. The processing demand increases greatly to achieve this finer 5cm resolution as it implies 8 times (2x2x2) more processing.

The current parallel processing hardware implementation

can support real time image rendering to a maximum fineness of 7cm3 under ideal controlled conditions. In practice in the field with achievable network performance limits this is not realizable and 10cm3 is employed. When finer resolutions are required, post processing is performed on board the vessel on a separate rendering machine. As computing hardware speeds are continually improving, it is expected that without implementing any additional software optimizations, real time 5cm3 render resolutions will be achievable within the near future.

VI. OPERATING CONSIDERATIONS

A. Velocity Model The SBI is a near-field coherent sonar imaging system that

relies on the difference in arrival time of a reflection from a target in the seabed to each element of the hydrophone array being resolvable in time. To determine this, the SBI rendering software uses a two-layer velocity model and this requires knowledge of the sound velocity in both media. While the SBI unit is fitted with a sound velocity sensor for determining the speed of sound in the surrounding water column, the sound

Fig. 5. Tow tank where INS testing was conducted

velocity in the sediment must be estimated. In many areas, good information exists to baseline the sound velocity estimate for the sediment. But this is not the only information that can be applied.

In many cases, a survey is undertaken to image and locate a man-made object in the seabed, such as a buried pipeline or cable, for which the engineering specifications are known. Initially, an estimated speed of sound in the sediment is used to form the image and the results are reviewed. If the image has converged and the object is clearly visible, this implies that the estimate is within 5% and this may be entirely acceptable as is. If it is desired to look for a sharper image, a re-render of the already collected data can be performed with the speed of sound in the sediment perturbed in one direction. This may improve the image or not, and a decision can be made to re-render with a perturbation of the sound velocity in the other direction.

Although this is a post processed, iterative approach, it can be performed as an initial “calibration” step before a survey to improve the real time results, or if the initial results are acceptable for real time objectives (i.e. following the pipe in the seabed), such fine tuning can be done off the vessel at a later date. Since the SBI is intended for shallow seabed penetration to a limit of 5m, small errors in sediment velocity do not have a chance to accumulate significantly and experience has shown that a good initial estimate combined with a confirmed target image provides a generally acceptable level of confidence.

B. Ambient Noise The SBI was designed to work with the level of noise

generated by the standard systems onboard a typical work class ROV. The primary noise generator in this case is the Hydraulic Power Unit (HPU), which can develop 150hp or more. During development, noise measurements were made on a WROV to baseline the noise levels that were present across the frequency band of interest. Without the addition of other noise, the SBI accommodates the ROV noise level quite well. Additional noise, however, can be contributed by the vessel operating in Dynamic Positioning (DP) mode from a position too close to the ROV.

Fig. 6. SBI on a WROV (bottom skid) with multibeam also onboard

Vessel position relative to the ROV during operations is driven by a variety of factors. The ROV itself must be geo-referenced to the vessel to provide a means to correct any long-term INS drift. This is normally accomplished with an ultra-short baseline (USBL) positioning system such as a Kongsberg HiPAP system. USBL systems operate best when the angle from the hydrophone array to the remote beacon is more vertical than horizontal. Therefore, to optimize the position accuracy of the USBL, the vessel needs to be close to the ROV location. In deeper water (the SBI is rated to 1000m), this does not present a problem, but in shallower water depths (i.e. <50m) this places the noise source of the vessel in relatively close proximity to the SBI system. If the DP system is working against strong winds or surface currents, the level of engine activity can interfere with the SBI image quality. This has been noted only under extreme conditions and has been easily rectified by maintaining the vessel at a minimum range of about 100m. At the resulting angle from the acoustic positioning receiver to the target beacon, good performance can be obtained, especially with the latest USBL positioning systems such as the IXBLUE GAPS system.

Multibeam systems provide the seabed surface reference data of choice for buried pipelines and power cables as the broad swath can indicate undisturbed seabed to the left and right of the disturbed trench area. In Fig. 6, the SBI is shown skid mounted on a WROV together with a dual head multibeam. The multibeam system can be seen frame mounted at the top left of the ROV, cantilevered to have a clear view of the seabed. The SBI and the multibeam operate well together and provide the capability for a one-pass survey that collects both data sets simultaneously.

C. Shallow Water An important seabed for buried subsea cable survey is the

shallow water and shore approach area, which constitutes water depths of 3m to 10m. These are water depths that are not considered navigable with larger vessels suitable for ROV operations and must be handled by smaller craft. Deployment considerations are key to successfully applying the SBI in these shallow waters, as the acoustic energy transmitted by the system bounces off both the seabed and water surface to create a multiplicity of signal arrivals at the hydrophone. In ROV operations it is preferred to fly at a height of 3.5m above the seabed. In shallow water operations this guideline cannot be used, as the primary consideration is to avoid the surface multiple contaminating the portion of the data set that is consistent with the expected target depth below seabed.

Fig. 7. Shallow Water Vessel Deployment

In practice, for very shallow waters this is handled by maintaining the hydrophone array at a fly height just below the sea surface, which effectively makes the array co-planer with the water surface (Fig. 7). This eliminates the immediate multiple that arises from the unavoidable acoustic back-radiation from the source transducers and leaves only the desired forward radiation to illuminate the seabed.

For very shallow water (3m) this is analogous to normal ROV fly heights, but as the water depth increases, so does the distance from the array to the seabed.

The increased distance to the seabed effectively reduces the path length differences from a target in the seabed to the individual hydrophone array elements, eventually compromising the image detail. A pole mount that can be lowered provides a means to accommodate this. When fly heights of about 7-8m are reached, the array is repositioned to 3.5m off the seabed by lowering the pole. With this adjustment, seabed surveying can continue into deeper waters.

The vessel mount shown in Fig. 7 has been used to successfully survey an energized, buried power cable in water depths up to 11m. The maximum pole extension employed was 7m below water surface, under essentially flat calm sea conditions, operating from the near-shore catamaran vessel “Coastal Liner”. When sea conditions increased above 30cm wave height, it was necessary to retreat to port. Fortunately, these water depths and cable shore approaches are often found in more sheltered areas, making this approach practical in months when weather conditions are favorable.

Fig. 8. Subsea Power Cable Buried 2m DBS

Seabed conditions can also have effects on the SBI image quality. Specifically, very hard, reflective seabeds provide a strong initial reflection that appears as a glare line at the seabed interface. This can reduce the ability to clearly image objects that are below the array center in top 30cm of the seabed in the rendered volume. When this occurs the A to D converters show evidence that they have saturated and this can be reduced by lowering the minimum gain setting through software control of the receiver circuitry. Lower depths in the seabed volume are unaffected when this occurs. Figure 8 shows an energized 25cm diameter power cable imaged with an extended pole mount in 10m water depth. The blue swath is the path of the vessel, with the red line showing the cable itself. The volumetric data has been sliced horizontally here to the 2m DBS level to show the cable.

D. Survey Software Once rendered, the SBI data set is a true 3D volume that is

most appropriately handled in visualization software specifically configured for 3D representation of survey data sets. The in-house development of such a software solution was considered and rejected in favor of a more efficient solution. While full 3D survey software suites for handling sonar data are not common (as many systems handle the data only as surfaces, which was not consistent with the nature of the SBI data), there were some existing systems identified that handle sonar data in the required manner. Usability considerations meant that it was important to identify existing survey software that could import and handle the data, but also provide a significant customer user base as well as developers willing to work to add the desired and necessary features. The software solution identified and adopted was NaviModel by EIVA in Denmark. The NaviModel 3D environment not only imports the rendered SBI data set but can also filter and merge it with other relevant data sets from the area of interest such as multibeam data of the seafloor.

Fig. 9. NaviModel View of SBI 3D data set, multibeam seafloor and HVDC cable model in seabed

Figure 9 shows a section of the rendered SBI 3D data set imported into NaviModel together with the multibeam seabed data. The vertical arrows indicate undisturbed seabed reference left and right, and the actual cable position, center.

Fig. 10. NaviModel view with SBI data sliced at level of cable to show match

with engineered cable position

An important additional capability implemented in the NaviModel software is the ability to filter using the “contribution count” for each voxel. This makes it possible to display only those voxels that have had their contents reconfirmed through oversampling by more than a user

selectable threshold. A statistical histogram distribution can also be displayed and used as a guide for setting the threshold. A typical well-sampled voxel may have a contribution count of 50 or more, resulting from the interaction of forward speed and ping repetition frequency. The imaging effect of applying this filter is to remove “noise” and provide a cleaner image.

Figure 11 shows a side view along the length of a buried HVDC cable. The upper run line is the seabed and the lower red line is the cable. The cable can be seen entering the picture at the left and rising to very close to the seabed, then continuing to descend to the right and exiting the frame at about 2m DBS. This failure to maintain target burial depth is examined in Fig. 12.

Fig. 11. HVDC Cable plan view slice showing cable riding over hard layers

(red indicating high signal intensity) in the seabed

Fig. 12. SBI 3D data volume sliced vertically along path of HVDC cable

In Fig. 12, the SBI 3D data set has been sliced horizontally to a depth into the seabed appropriate to show the HVDC power cable of Fig. 11 traversing the top of the shallow arc over a hard layer. The trenching machine likely encountered the harder area and could not attain the design depth of burial. The ability of the SBI to provide such geotechnical clues while surveying adds considerable value to the data set.

VII. AUV-BASED SURVEYING To date, the applications for the SBI have all involved the

need to access real time data for guiding the survey of buried subsea assets. However, the cost of operating the fully equipped WROV with fibre-optic umbilical and associated launch vessel are significant, even if entirely appropriate for uses such as oil and gas, subsea cabling etc. For other applications, such as pipeline pre-route survey or marine mining investigation, a more cost-effective platform is desired. The Autonomous Underwater Vehicle (AUV) & associated launch vessel has the potential to fulfill this role at an operational cost that can be as little as 10% of that of the WROV/large vessel combination. An AUV platform to carry the SBI is therefore of interest and a program of investigation has been initiated in cooperation with Memorial University of Newfoundland (MUN) in St. John’s.

In terms of survey cost and efficiency, an important advantage of using an AUV as the host platform for the SBI is that it allows fully autonomous operations. This frees the deployment vessel to perform other survey tasks, and can substantially increase the survey productivity rate. AUV’s are also quieter and more stable than either surface platforms or WROVs and have the potential to produce higher quality data.

MUN owns and operates an International Submarine Engineering (ISE) Explorer vehicle that is one of the larger AUV’s in commercial operation. The size of the platform is an important factor in assessing an AUV’s potential to carry the large SBI hydrophone array and source transducers, making the ISE Explorer an ideal choice.

The SBI hydrophone is 3.5m in length and must move forward over the seabed to be surveyed while mounted athwart the AUV much like an aircraft wing in relationship to the fuselage. The configuration adopted for the wing mount is shown in Fig. 13.

Fig. 13. SBI Hydrophone mounted in wing on Explorer AUV

Fig. 14. Hydrodynamic testing in Marine Institute Flume Tank

Extensive hydrodynamic testing was conducted on the AUV/wing hybrid to assess fly-ability and control-ability prior to open water testing. The moving water flume tank at the Marine Institute was used to simulate a variety of forward flight speeds for this testing (Fig. 14).

Successful control algorithms and strategies were devised based on the results of the controlled testing and were subsequently employed in open water tests in Conception Bay, Newfoundland. Full sonar integration testing is currently slated for spring 2013.

VIII. SOME REAL WORLD APPLICATIONS OF SBI DATA There are a number of applications where the unique data

sets, high-resolution and high survey rates provided by the SBI are very useful. Locating and confirming depth of burial for subsea power cables is an obvious example. This application has been driven by the fact that the SBI is unaffected by the magnetic fields generated by functioning power cables, and can detect cables at much deeper burial depths below the seafloor than magnetometer technology.

Decommissioning of old seabed infrastructure is another

application where the SBI has the ability to aid efficiencies. Buried seabed assets installed during the development of many oil and gas fields of 30 years ago are now located only by lines on a chart with pre-GPS survey accuracy. Excavation for removal can often become an expensive search effort as well as a removal effort. The SBI has been used to unambiguously identify buried pipelines, pipe and cable bundles to depths of up to 4.5m DBS for excavation and removal.

The SBI effectively images by detecting variations in

acoustic impedance in the seabed and as a result can detect non-metallic targets such as stainless and synthetic flowlines, as well as geohazards such as boulders and hard layers. Work is ongoing to correlate specific imagery with geological and geotechnical features in areas where experience and other data exist.

Marine archaeology is another area that holds promise for SBI application. The imagery produced can be used to identify

buried objects; especially those that are manmade and have regular overall shape and size, such as cannons. The images below show bronze cannons from an 18th century ship wreck off the UK coast. Since these objects are similar in size and shape to Unexploded Ordnance (UXO), this would represent another possible application for the technology.

Fig. 15. Archaeological site showing cannons buried in the seabed

IX. CONCLUSION The Sub-Bottom Imager is a new sonar configuration that

provides a unique 3D data set of the sub-seabed. Its application of conventional array beamforming together with SAS processing, stabilized by state-of-the-art INS technology has never before been commercially offered. The results from the initial survey experience with the system have shown specific benefits in sectors such as the power industry and oil and gas production.

The refinement of the system continues as more and more

data is collected over varying targets and seabed. With more applications being piloted with the technology, it is possible that this surveying approach will be as common as multibeam for specific seabed applications in future.