Making sense of underwater acoustics4 January 2010
Kenneth J LaBry explains how Fenstermaker has applied underwater acoustic remote sensing to underwater substructure inspection and spatial element mapping
The inspection of the underwater structural components of dams, locks, flood-gates and other water control structures is crucial if such components are to maintain dependable performance and reach the maximum number of years expected for operation. Until recently the technology to perform such inspections was limited to the use of divers and robotic vehicles that perform largely visual and tactile surveys. This is problematic under inclement environmental conditions such as high current flow, extensive structural surfaces, hazardous high flow dynamics through small aperture openings on dams, and low to no visibility.
Due to the significant impact of these conditions on results and findings, these surveys tend to be extremely subjective and provide only very generalised comparison information. These typical cursory surveys cannot be used to establish a baseline of structure condition due to the limited information provided.
With the development of underwater acoustic, steered beam sonar and profiling, remote sensing systems which have the ability to produce very high definition sonar imagery, as well as precise acoustic profile measurement, there is now a cost effective alternative to conventional methods of underwater substructure inspections. These can provide a baseline reference for future surveys as well as a non-subjective data set that can be used for comparison with previously recorded structure conditions. This methodology can be used to provide a comprehensive spatial element map of underwater structure surfaces and their interface with the water bottom.
Underwater acoustic imaging and measurement systems contain an emitter and a receiver, and rely on the emission of a sound wave and measurements of the time required for a reflection of the emitted sound pulse to return to the receiver, as well as measurement of the intensity or amplitude of the reflected sound pulse. In many systems the emitter and receiver are the same physical sensor or transducer. The physical characteristics of the acoustic pulse wave are critical to the resolution of the sonar imagery and precision of the acoustic measurement.
The main critical physical characteristics are the frequency of the acoustic wave form, pulse length of the acoustic transmission and the beam width of the acoustic wave form. Generally speaking, the higher the frequency the greater the resolution – but there is a tradeoff in effective range. A narrower beam width also produces higher definition but reaches a tradeoff limit with scan coverage requirements increasing the time required to complete a scan. The best compromise to optimize the physical beam forming characteristics are a fairly high frequency in the range between 500kHz and 900kHz, a horizontal beam width of less than 1°, and the shortest pulse length that can effectively produce the desired detection range. For acoustic profiling measurement the smallest practical acoustic footprint for the beam produces the greatest range of measurement accuracy, and reduces the noise from multi-path reflections due to nearby structural components.
The configuration of the instrumentation that Fenstermaker has utilised in multiple dam evaluation surveys is a dual element, fan beam/conical beam, 360° mechanically scanned imaging and profiling sonar. The system operates at a frequency above 500 kHz and is configured for a horizontal beam width of less than 1° and a conical beam of less than 2°. The system is also configured for beam steerability in both the horizontal and vertical planes with redundant steerability in one of the axes, the redundancy axis being defined by the operator. The system is largely based on a variation of the Kongsberg Mesotech MS1000 digital, high resolution, mechanically scanned sonar. The system scan patterns are reflected in Figures 1 and 2.
The inspection of underwater structural components with high definition acoustic imaging techniques requires significant considerations in the deployment methodology. The sensor must be maintained in a stable position during the scan segment and the sensor orientation must be such that the grazing angle of the acoustic beam across the structure surface provides for visualisation of surface undulations, projections and abnormalities in the plane of observation. Because of this aspect of the methodology, most scenarios will require the ability to vary the sensor orientation in order to remain normal to a specific observational plane that corresponds to a structure surface.
The deployment methodology can be problematic in environments where high water currents or close proximity heavy shipping traffic exists. Fenstermaker has overcome these problems by designing and constructing several self-contained deployment mechanisms, which are portable and can be quickly deployed.
The quantitative measurement of profiles of structural abnormalities and the interface between the substructure and the water bottom requires the use of steered beam profiling. By utilising dual axis steerability we can gather quantitative measurement data over areas where abnormalities were observed utilising underwater acoustic imaging. The steered beam profiling with a narrow conical beam provides for acoustic measurement accuracies of 0.1” at ranges of 75’ on absolute acoustic boundaries and provides the capability to collect profile measurements between narrowly spaced structural members.
The steered beam profiling methodology is also utilised to generate water bottom elevation models to evaluate scour and deposition. It maps scour depressions, sediment accretion, and erosion patterns and correlates these to the corresponding hydrodynamic flow characteristics that would produce these water bottom characteristics.
The profiling methodology involves the incorporation of a compendium of remote sensing instruments to resolve angular motion, providing heading reference, positional reference and acoustic measurement of submerged features.
The example shown in the figures above is a result of data rendering from a survey performed for a Louisiana Department of Transportation and Development project at D’Arbonne dam in Union Parish Louisiana. In this case terrestrial lidar (HDS) laser scanning was utilised to scan the superstructure of the dam and surrounding environment and Fenstermaker’s underwater acoustic system was use to scan the submerged structural components of the dam as well as the water bottom upstream and downstream of the concrete spillway structure. The data sets were then integrated to construct a comprehensive ‘as-found’ 3D model of the dam system from which geographic position and metrology can be extracted. This provided a comprehensive baseline, assisted in identifying areas of concern that were in need of repair or remediation and provided volumetric and measurement data for determining extents of repair and material needs (see Figure 4 and Figure 5).
The resulting model identified the complex erosion pattern and scour characteristics downstream as well as providing data to qualify any differential movement in the spillway apron slabs and concrete dam structure. The survey also defined possible problematic characteristics at the toe of the concrete structure on the reservoir side of the dam that potentially indicated a piping condition. This prompted a recommendation for more intensive diver aided investigation.
When this investigation occurred approximately 60 days later, a dynamic situation was observed whereby the depressions in the silt at the toe of the structure were considerably larger and had grown together. The acoustic imaging system was utilized to guide a pipe probe into the depression and inject dye to define any significant flow prior to sending a diver into the environment. Flow was not observed, however the dye did not pool either. Upon close inspection by divers the hole in the sediment was caused by a compromised joint and seal at the base of the structure allowing water and sediment to be transported through the structure and into the drainage system.
This case study shows the multi-faceted role of the acoustic remote sensing system to map and identify problems, then assist in defining a safe working environment and finally assist in directing diving efforts expediently to problem sites in conditions of low to no visibility.
A matter of cost
Incorporating both underwater acoustic imaging and underwater acoustic profiling allows for the qualification and quantification of structural and water bottom abnormalities in a cost effective manner and adds an element of safety by providing a visual rendering of the working environment for dive team direction. The production rate of area covered relative to other methods provides for a significant cost savings. It provides a much more extensive and non-subjective data set to be used for subsequent comparative analysis with future surveys that are performed in the same manner. The geo-referencing of all acoustic data provides for real world displacement measurement along the water bottom and structural surfaces.
Subsequent to the D’ Arbonne dam evaluation the Louisiana Department of Transportation and Development, Public Works and Water Resources Division selected Fenstermaker for the inspection of 19 other dams maintained by the State of Louisiana.
Kenneth LaBry is a physicist with over 20 years of experience in underwater acoustics remote sensing. www.fenstermaker.com