Through-The-Sensors Concepts to Refresh the Environmental Picture

W.E. Avera, M.M. Harris, D.J. Walter, L.D. Bibee,and D.N. Lambert
Marine Geosciences Division

Introduction: The Naval Research Laboratory's (NRL) Marine Geosciences Division is advancing "Through-The-Sensor" (TTS) concepts that use ship and airborne sonar systems to obtain tactical environmental data and refresh the environmental picture in near real time. NRL's Acoustic Seafloor Classification System (ASCS) uses inversion and signal processing techniques developed under the NRL S&T program to determine acoustic impedance and seafloor properties. Classification of surficial seafloor sediments has been demonstrated using normal incidence beams from the AQS-20, UQN-4, BQN-17 acoustic systems, and backscatter from the EM-121 multibeam sonar. Signal and image processing algorithms were also developed to produce multibeam bathymetry and imagery from the AQS-20 mine-hunting sonar and the submarine Precision Underwater Mapping (PUMA) system. The UQN/Bottom Sediment Classifier (BSC) is currently installed on the USS Dextrous (MCM-13) in the Persian Gulf. The BQN-17/Submarine Sediment Classifier (SSC) was demonstrated aboard the USS Key West (SSN-722) off the California coast. Efforts include transitioning these capabilities for use with tactical decision aids (TDAs), environmental assessments, and integration into historical databases at the Naval Oceanographic Office (NAVOCEANO).

Surficial Sediment and Bathymetry: Two key environmental data types needed in the littoral are bottom type and bathymetry. The ASCS system displays a profile of the seismic stratigraphy and inferred bottom type beneath the vessel. Figure 1 shows two displays from the system. Classification of surficial seafloor sediments has been demonstrated using normal incidence beams from the UQN-4 fathometer on mine countermeasures (MCM) ships, the BQN-17 submarine fathometer, and the AQS-20 mine-hunting system. These data can be used in onboard decision aids and ultimately stored in historical databases.

Fig 1
Acoustic Sediment Classification System (ASCS) displays showing (left) bottom type and (right) 20-m penetration into the seafloor.

SEDIMAP™ is another system being developed at NRL for NAVOCEANO to determine bottom type using the angular dependence of backscatter from commercial multibeam sonars. SEDIMAP™ gives areal coverage by providing a swath of bottom type information below the vessel. Simulated annealing data inversion techniques are used to extract sediment grain size and roughness parameters.

Multibeam bathymetry and imagery are being extracted from the AQS-20 mine-hunting sonar (Fig. 2) and the submarine PUMA system. Data from the AQS-20 volume search sonar, designed to locate mines in the water column, is being used to generate multibeam bathymetry across swaths of the ocean bottom. The bathymetry can be imported into TDAs supporting mine warfare operations and used to augment the historical databases with bathymetry in areas where historical data are lacking, sparse, or perishable.

Fig 2
AQS-20 with the four sonars. The Volume Search Sonar is used to obtain multibeam bathymetry and sediment type.

PUMA bathymetry is sent to the Tactical Environmental Data Services (TEDS) system for distribution to onboard TDAs. Acoustic images of the seafloor are also extracted from the PUMA data. These images are combined with bathymetry from the system to produce an automated bottom feature extraction capability. The bottom feature extraction will support Navy databases with detailed bottom characteristics that are not otherwise available.

Ship Installations and Demonstrations: UQN-4 fathometers on MCM ships are being adapted to characterize bottom sediments. Real-time processing and display of seismic stratigraphy and inferred sediment parameters are used for situational awareness in real time. In the future, sediment parameters will automatically be provided to MCM tactical decision aids onboard ship. The UQN/Bottom Sediment Classifier (BSC) was installed in October 2001 for 1-year demonstration on the USS Dextrous (MCM-13), a forward deployed mine hunter in the Persian Gulf (Fig. 3). The demonstration was extended until Spring 2003 based on positive feedback.

Fig 3

UQN-4/Bottom Sediment Classifier (BSC) installed on the USS Dextrous (MCM-13). UQN-4 fathometer display is located in the Chart Room, and the BSC display is located in the Combat Information Center (CIC) for real-time visualization of bottom type.

The BQN-17 Submarine Sediment Classifier was demonstrated aboard the USS Key West off the California coast. Tests with the BQN-17 demonstrated mapping of subbottom sediment layers in water depths up to 3000 meters, with penetration up to 20 meters below the bottom. Dedicated tests with the AQS-20 sonar towed by an MH-53 helicopter (which can also be towed by the Remote Minehunting System) have been completed off the coast of Panama City, Florida, and modifications to record the data are being made to ensure this will be operational for the Fleet.

Summary: In summary, NRL's Marine Geosciences Division is advancing the TTS concept to produce ocean bottom type and bathymetry data from existing Fleet sonar systems. Recent research has focused on implementing and testing the technology. Concurrently, advances are being made toward combining data from multiple sonar systems and developing innovative ways to characterize the environmental battlespace.

[Sponsored by SPAWAR, ONR, NAVSEA]