The primary objective of this study was to identify and examine the design features of an effective underwater dynamic classification electromagnetic induction (EMI) sensor. This examination included electromagnetic and hydrodynamic analyses to determine a sensor head configuration that will enable effective classification of unexploded ordnance (UXO) when operated in a hydrodynamically stable survey mode. The analyses applied a combination of modeling and laboratory/test-stand data collection to identify key design features and verify classification performance of an underwater towed system concept.
It should be noted that this design study leverages the work of previous and ongoing SERDP & ESTCP-funded underwater phenomenology studies (e.g., MR-2728, Principal Investigator (PI): Shubitidze; MR-2409, PI: Bell; MR-2412, PI: Billings; and MR-1714, PI: Schultz) and is, therefore, not intended as a comprehensive phenomenology assessment. Instead, the specific objectives focused on evaluating the requirements for adapting land-based dynamic classification methods to the underwater environment, assessing the hydrodynamic factors critical to a towed array configuration, and developing the design features that fully exploit the information gleaned from these analyses. The project team did include some analysis of seawater/field interactions, but in the context of applying land-based classification methods, namely background removal techniques, to the dynamic underwater environment.
The concept of this project is to employ an advanced EMI design that will enable dynamic classification from a hydrodynamically stable towed configuration. This concept requires two significant adaptations to the land-based dynamic system:
1. A modified form factor. Aside from the hydrodynamic factors that influence sensor form, the underwater environment also presents operational factors that require modifications to land-based designs. For example, due to the large areas and lack of vegetation, a wider swath (i.e., larger transect spacing) is typically desired for underwater survey systems.
2. A modified sensing range. While land-based sensors can be placed at ground level, underwater towed sensors must operate above the seafloor, typically at a minimum standoff of 0.5-1.0 meters or more. Therefore, the sensor range must be modified to detect and classify at ranges that may exceed those required on land.
The figure below shows a drawing of the underwater sensor concept. The basic components include seven transmitters (four in the X-direction, two in the Y-direction, and one Z-directed), 15 (three-axis) receivers, and transmit and data acquisition electronics. The design features a four-point two bridle configurations as well as lifting surfaces along the leading and trailing edges of the array to assist roll and sway stabilization. The concept also includes a pair of electronics bottles that house transmit and analog to digital converter electronics located on top of the sensor to elevate the center of buoyancy, which also increases stability.
Overall dimensions of the array are 3.0m x 1.5m x 1.0m (Width x Length x Height or X x Y x Z). The array frame comprises 2.5-inch square tubing, which will serve two purposes: 1) to act as rigid structural members; and 2) to provide insulated formers for the transmitter wire. Previous underwater phenomenology studies have demonstrated that insufficient transmitter wire insulation against seawater can increase the coil capacitance enough to reduce current damping after shutoff and increase noise levels (e.g., Bell et al., 2016; Billings et al., 2016). The project team has selected frame members that will be appropriately sized to provide structural support and electrical insulation against seawater.
The performance assessment consisted of two stages: 1) verification of EMI classification performance; and 2) verification of towed operation feasibility. The classification performance assessment comprised test stand data collection to verify the EMI modeling process, followed by simulations to determine expected classification performance as a function of target type and range. The towed operation assessment included simulations of the hydrodynamic load cases in the ProteusDS environment. Throughout both stages the project team applied the performance metrics to determine whether the specific performance objectives were met.
Considerations for Underwater Dynamic Classification - Throughout the design study, the project team assessed requirements for adapting land-based dynamic classification methods to the underwater environment. Results from the analyses indicate that the basic principles of the classification approach will be transferrable to underwater operation. The study demonstrated that the towed array configuration can provide reliable classification features at the targeted operational standoff. The project team assessed implementation of processing steps, such as background compensation, that will be critical for effective classification. Results indicate that the standard dynamic processing flow used on land should be applicable to underwater data, except for possibly the earliest time gates (i.e., <200-500 microseconds) where seawater interaction with the target may be significant.
Additionally, the single shot classification method the project team has used successfully for land surveys may be particularly beneficial for underwater surveys where degraded positioning quality will be a factor. This approach appears robust against sample to sample positioning error, which may be encountered during tow point surge. While cumulative errors over large tow distances may be relatively small for constant tow speeds, towline surge due to wave action may produce relatively large errors in sample to sample positioning.
The greatest risk for classification quality during implementation of this system is most likely due to the unknown noise characteristics of the dynamic underwater environment. The characteristics of noise in the data have a significant impact on the reliability of classification features (i.e., whether the polarizabilities are well constrained). There was not a substantial amount of data acquired dynamically in the underwater environment to determine, with certainty, the expected noise characteristics. The project team applied dynamic noise characteristics captured on land to create synthetic data. The project team believes this noise is relevant because it captures motion induced changes in receiver flux that should be encountered underwater as well; however, there may be additional noise features, particularly in the early time gates, that are not present in data acquired on land.
To mitigate this risk, the project team believes it will be possible to increase the effective transmitter power to improve signal-to-noise ratio (SNR) as needed for reliable classification. The design demonstrates the uniform field characteristics at target depth that are required for high quality classification. If noise levels prove higher than anticipated, it may be possible to overcome these SNR limits with greater transmitter power. For example, the land-based one pass time domain EMI array system operates at an effective transmitter power of approximately 100 A-turns. For the simulations, the project team increased this level to 200 A-turns for the underwater system. This level is still fairly modest and could most likely be increased further if necessary to improve SNR. The primary limitation in the hardware design is to ensure that the transmitter components can handle the higher kickback voltages at greater power levels.
Considerations for Towed Operation - The hydrodynamic analysis included a comprehensive assessment of the key factors related to the operational performance of the system. The results indicate that the system design offers a hydrodynamically stable configuration that should meet the positioning and stability objectives for enabling effective classification. The three-dimensional (3D) transmitter design provides a framework that enables vertical separation of the center of gravity and center of mass in a way that maximizes righting moment when a roll or pitch perturbation is encountered. Results from the simulations demonstrated that the system should enable operation in relative proximity to the seafloor (i.e., 1- 2m standoff).
Specific design considerations related to operational feasibility of the system include:
- Sway response of the system was notably slow for large sway offsets. This slow response is due to the lack of surface area exposed to the flow at small yaw angles. This effect may be most noticeable when turning. The system may require several minutes to obtain alignment with the tow point after large turns. This effect can be mitigated through pay-in of the towline during turns. Additionally, increased frontal drag area will improve the sway response; however, more drag results in greater layback and clump weight.
- Tow point heave due to surface conditions should have a minimal impact on standoff variability (+/- 15cm in worst case operating conditions); however, the simulations indicated that it may cause a net offset in standoff. This offset can be compensated for with towline length adjustment.
- Depth control responsiveness varies considerably with towline angle. Higher tow speeds (more drag) produce more layback and a lower towline angle, which reduces the vertical component of the winch pay-in velocity. The simulations indicate that responsiveness to inclines of 10 degrees should be obtainable under typical survey conditions. For greater responsiveness to seafloor variability, lower speeds or larger clump weights should be applied.
- Sensor position error will most likely be the greatest contributor to overall (global) target localization error. Accurate measurement of the sensor depth will be critical. Additionally, if cable catenary is significant, towline tension will need to be monitored to adjust for these line length offsets.
This concept feasibility study culminated with the design of an underwater dynamic classification sensor based on advanced EMI principles. This design has been optimized specifically for meeting the increased standoff requirements for underwater towed operation. The electromagnetic analysis results demonstrate that the dynamic classification methods used on land are applicable to underwater operations and can achieve high quality classification at the increased standoff range. The hydrodynamic analysis results demonstrate that the 3D transmitter configuration provides a hydrodynamically stable design due to the increased metacentric height. Drag forces will not be significant or prohibitive for towed operation at the targeted survey speed (2-4 knots). Overall target localization error can be minimized (i.e., ~0.5m or less) through accurate measurement of the towline depth and line length.