Chau-Chang Wang

Seafloor Roughness Measured by a Laser Line Scanner and a Conductivity Probe

To support modeling acoustic backscatter from the seafloor, a conductivity probe and a laser line scanner were deployed jointly to measure bottom roughness during an experiment off the New Jersey coast in summer 2006. The conductivity probe in situ measurement of porosity (IMP2) is impervious to water turbidity and yields a 1-D profile with 10-mm horizontal spacing and 1-mm resolution in the vertical direction. The laser line scanner is limited by water visibility but it provides 2-D grid points with resolutions 0.3 mm across track, 0.5 mm along track, and 0.3 mm in the vertical direction. Two sets of data, suitable to model mid- to high-frequency acoustic backscatter, were collected from two sites 900 m apart on August 14 and 17, 2006. The roughness spectra obtained from the laser scanning were compared to those measured by the IMP2. The spectra from the two methods are consistent over wave number range 0.0188-3 rad/cm, which are the wave number range common to both methods. The efficacy of the laser scanner is also confirmed by showing the spectral line created by the IMP2s periodic probing marks. The 2-D spectra generated from the laser scan data show that the bottom roughness at these sites is azimuthally isotropic, but significant spatial heterogeneity is observed.

Evaluation of Laser Scanning and Stereo Photography Roughness Measurement Systems Using a Realistic Model Seabed Surface

The topography of the seabed is influenced by sediment transport due to wave motion, current disturbance, and biological activities. The bottom roughness generated by these processes can substantially alter acoustic wave penetration into and scattering from the bottom, and therefore, it is essential to make accurate measurements of the bottom roughness for such acoustic applications. Methods to make direct measurements of bottom roughness include stereo photography, laser line scanning, and sediment conductivity. Roughness can also be measured indirectly using high-frequency sound backscatter. For optically-based methods, the accuracy of these measurements is typically evaluated using the elevations, lengths, or diameters of simple surface features of known dimensions. However, for acoustic applications, the statistical characteristics of the surface, e.g., the roughness spectrum, are more meaningful. In this paper, we present a fabricated rough surface milled into a 40 times 60 cm2 plastic block for use as a benchmark in the assessment of two in situ roughness measurement systems: a laser scanning system and a digital stereo photography system. The surface has a realistic roughness power spectrum that is derived from the bottom roughness measured during the 1999 Sediment Acoustics Experiment (SAX99) and was fabricated by a computer numerical controlled milling machine. By comparing the fabricated surface spectrum to the measured spectrum, a determination of the accuracy of the roughness measurement is evaluated, which is of direct relevance to acoustic applications.

Design and Application of Autonomous Underwater Acoustic Recorder

The goal of this work is to design and fabricate a autonomous acoustic recording system which is low cost, reconfigurable and portable for acoustic research. In the commercial market, some off-the-shelf audio systems are taken as a core unit and modified into underwater acoustic logger. Generally, these consuming products have some built-in filters and compression algorithms such that the recorded signals have some unknown distortion. It is not recoverable either. Our system consists of four components, i.e., a PC-104 single-board computer (Celeron 1G), a 12-bit A/D converter (PCM-3718HO), an 20/40 dB amplifier (provided by APL, University of Washington) and power management circuit board. Currently, the throughput of data stream attains to 69 kHz. Six scalable A/D channels are available to share the total bandwidth. A C program with multiple threads is developed to control the I/Os, digitize the underwater acoustic signals and stream data to the hard disk continuously. At present, two ITC-6050C hydrophones are connected to the system. One hydrophone is sampled with zero gain,the other is sampled with zero and 20 dB gain. Running on 35.2 AH, 16 V lithium cells,the system can operate about 4.5 hours before the depletion of power. The system was tested with compressed air nozzle jetting noise (broad-band white noise) to calibrate the hydrophones in the experimental sink. In order to verify its performance, this system works with an autonomously recording unit Bioprobe simultaneously for comparison. We can verify whether the results are coincided with each other. Further, A field test was conducted in shallow bay area. A fishing boat (37 tons, 6-cylinder diesel engine) traveling at constant speed was used as the sound source, and ran both parallel and perpendicular to shore. Bubble noise generated by the ship propeller propagated in the very shallow water waveguide (5 to 13 meters).Thus, these cases show results for both range-independent and range-dependent scenarios. The source track were recorded by using a GPS recorder on the boat, data processing resulted in range-frequency plot shows that the interference pattern reported in the literature was captured. Aided by numerical simulations, it is able to investigate the phenomenon of propagation of sound for wedge with lossy bottom. This system, connected with multiple hydrophones, will be applied in the research for target detection, environmental measurement and monitor, the harbor protection, and so on.

Measurement of Seabed Roughness with Laser Scanning System

Knowing the temporal change of the seabed, we can understand the nature of the undersea environment in more details. The scale of the features on the seabed varies from meters for large sandwaves, and down to less than one millimeter for the prints left by marine creatures. So far, there is not a single instrument that can cover the whole range and still preserve the resolution. For measuring small-scale roughness of seabed, laser scanning is an alternative. In this work, we report the integration of Seabed Laser Scanner (SLS), developed by Institute of Undersea Technology, Sun Yat-sen University, on the linear stage of In Situ Measurement of Porosity 2 (IMP2), developed by Applied Physics Laboratory, University of Washington to carry out deep sea sediment 2D roughness measurement and comparison. IMP2 consists of a 4 meter long scaffold and a linear stage. SLS is mounted on the linear stage to carry out the scanning. To simplify the integration and avoid the possible failure coming from the additional underwater connections, SLS is designed to be a self-contained system. It runs on a PC104 with Windows XP and feeds on its own battery pack. The only interaction between the two systems is achieved by the proximity of a set of magnets and a relay. As the linear stage starts, the first magnet triggers SLS to power up and standby. As the stage comes to the end of the rack, the second magnet triggers the image acquisition. The integrated system was deployed three times during the Shallow Acoustics Experiment 2006, about eighty miles off the coast of New Jersey. We successfully retrieved data from SLS for the first two trials; the third trial failed due to the aborted mission of IMP2. A 300 cm times 30 cm and a 350 cm times 25 cm seafloor were mapped at 80-meter water depth. From the reconstructed 3D surfaces, we found that the seafloor is full of shell debris and covered by mud-like sediment. The 2D spectra were estimated from the 3D surface. The results indicated that the seafloor roughness follows a power-law spectrum and isotropic. These spectra estimated provide the boundary condition for modeling the acoustic propagation and scattering.

Development of a short-period ocean bottom seismometer in Taiwan

Taiwan is located near the edge of the continental Eurasia plate and the Philippine Sea plate, a quake-prone zone. Averagely speaking, out of 1000 sensible quakes detected per year, more than 50% of the epicenters are located in the surrounding waters. Therefore, there is a great need from our seismology community to deploy ocean bottom seismometer (OBS) to have high fidelity data, and widen the aperture of the observation as well. In this paper, we report the development of a “midweight” OBS called Yardbird which has one year deployment duration. We utilized the newly available ultra-low power MCU (micro control unit) and SD card to design a compact data logger. The response of 4.5-Hz geophones was extended to 3 s, and the modified geophone was used as the seismic sensors. With the compactness of the sensor module, it enables us to design a compact dual-axis motor-driven stage to level the vertical component to be less than 0.1 degree with respect to the gravity. Currently, the total power consumption of Yardbird OBS is less than 0.2 mW for three channels. A pilot experiment was conducted in the south western Okinawa trough in 2010. During the three months experiment, an array of five Yardbirds captured tele-seismic event from Chile, and deep quakes from the subducting oceanic plate. The results prove that the performance of Yardbird OBS suffices the basic needs of seismic research.

Evaluation of using a CCD camera as a versatile measurement system for flume tank observation

A plane-based camera calibration method which substantially reduces the complexity of the procedure is presented in this paper. With the proposed method, camera array can be installed as a basic facility for wave flume tank experiment. Wave flume tank experiment is a methodology commonly used for ocean engineering research. Various parameters, such as wave properties on the surface (height, wave form, average level), current in the water (velocity field, vorticity and particle trace), bottom profile and sub-bottom layer thickness are measured with different apparatuses. Still image camera is one of the sensors widely used to extract information, such as waveform, bedform and velocity field (particle image velocimetry). The accuracy of image-based measurements mainly relies on the quality of the camera calibration which is a time-consuming process especially in field work. For the phenomena of interest observable on the flume tank window glasses, the features are planar in nature. Therefore, the camera calibration can be simplified as a map projection problem such that the time and efforts needed are reduced significantly while maintaining the desired accuracy. We studied the performance of the plane-based method by comparing the measurements with the theoretical values of a known curve which mimics a sandslope. The robustness of the method is examined by comparing the results acquired under the condition that the camera tripod and the quick-release are dissembled, assembled and repositioned during the experiments. According to the error analysis, the measurement can be accurate to less than 1. mm for static feature and 1 to 2 mm for dynamic feature.

On the estimation of roll alignment error of an ultra short baseline navigation system

This study proposes an algorithm to estimate the alignment errors of an ultra short baseline (USBL) navigation system. This algorithm is based on positioning errors caused by heading, pitch, and roll misalignments, respectively, when running a straight-line USBL transponder positioning. The positioning errors arising from each of the angular misalignments were derived. The differences of positioning error arising from each of the angular misalignments were identified and, accordingly, these differences outlined a simple and intuitive iterative scheme to calibrate each misalignment angle in turn. In addition, the effect of measurement error on the estimation of roll misalignments was investigated. The criterion to obtain a robust estimation of the roll alignment error was provided. Two practical problems associated with vessel motion along a straight-line path were considered. These two problems encountered in field operations refer to course deviation and cross-track error, and they were analyzed and solved to improve the accuracy of estimation. As a result, a vessel without the capability of dynamic positioning (DP) can be run to collect USBL observations for misalignment calibration. A field experiment was conducted and the experimental results demonstrate the validity and high efficiency of the proposed estimation approach. The experimental results also show that an inaccurate estimation of roll alignment error will significantly degrade the quality of estimations on heading and pitch alignment errors.

Evaluation of an off-the-shelf acoustic system for precision positioning of seabed transponders

In this study, a commercial off-the-shelf acoustic system with ranging accuracy of 20 centimeters is employed to conduct an experiment of positioning seabed transponders. The feasibility of using a GPS/acoustic system with off-the-shelf transponders to achieve centimeter-level positioning accuracy was evaluated. In order to verify the relative positioning accuracy between seabed transponders, a seafloor acoustic transponder (SAT) system was developed. Three transponders were mounted on the SAT system to form a triangle with known baselines. In addition, a data acquisition and logging system was developed to integrate with a motion reference unit (MRU) and a fiber optic gyro (FOG) for collecting the heading information of the SAT system. A field experiment to verify the relative positioning accuracy between transponders was carried out. The collected GPS/acoustic observations and sensor data were then used to estimate the positions of the three transponders based on an optimization technique combined with ray trace calculations. The transponder positioning result shows that the root mean square slant range residuals are in good agreement with the inherent ranging accuracy of the acoustic system. The experimental results also show that the mean value of the acoustic slant range residuals is about 3 centimeters and the relative positioning error between transponders is less than 3 centimeters. Furthermore, the heading of the SAT system derived from the position estimates of the three transponders is in good agreement with the measurement obtained by the MRU and FOG on the SAT system.

Applying Point-Based Principal Component Analysis on Environment, Ships and Cetaceans Whistles Signal Classification

For many undersea research application scenarios, instruments need to be deployed for more than one month which is the basic time interval for many phenomena. With limited power supply and memory, management strategies are crucial for the success of data collection. For acoustic recording of undersea activities, in general, either preprogrammed duty cycle is configured to log partial time series, or spectrogram of signal is derived and stored, to utilize the available memory storage efficiently. To overcome this limitation, we come up with an algorithm to classify different sound patterns and store only the sound data of interest. Conventionally, pattern recognition for acoustic signal is done in spectral level. Features like characteristic frequencies, large amplitude of selected frequencies or intensity threshold are used to identify or classify different patterns. On main limitation for this type of approaches is that the algorithm is generally range-dependent, as a result, also sound-level-dependent. This type of algorithms will be less robust to the change of the environment. One the other hand, one interesting observation is that when human beings look at the spectrogram, they will immediately tell the difference between two patterns. Even though no knowledge about the nature of the source, human beings still can discern the tiny dissimilarity and group them accordingly. This suggests that the recognition and classification can be done in spectrogram as an image processing and recognition problem. In this work, we propose to modify PCA (Principal Component Analysis), a popular technique used in face recognition, to classify sounds of interest in the ocean. Among all different sound sources in the ocean, we focus on three categories of our interest, i.e., rain, ship and whale and dolphin. The sound data were recorded with an instrument called PAL (Passive Acoustic Listener) developed by Nystuen [1], Applied Physics Lab, University of Washington. The recording strategy is to turn on the system and picks up 4.5 seconds at the beginning of the pre-set duty cycle. Spectrogram is derived to check if certain frequency band with significant intensity exists. If so, additional 4.5 seconds clips will be recorded until the conditions cease to exist. Otherwise, the system goes to sleep mode and waits for the next duty cycle. Therefore, the final data file consists of intermittent recording of 4.5 seconds clips for most of the time. Conventional PCA takes the whole m times n face image (in our case the spectrogram) and stretch it into a long mn times 1 vector. M training images, represented by these mn times 1 vectors, are used to obtain eigen vectors. The vectors corresponding to the largest-k eigenvalues are used to construct the recognition space, thus all the images are points in this space. Face recognition is then reformulated as finding the shortest Euclidian distance between points. For our case, unlike human faces, there are no well-defined features like eyes, ears, mouth and nose. For this reason, the original spectrogram is not suitable to be used directly as the input for PCA. In stead, the invariant moments of a spectrogram image serves this purpose better. Our modified PCA uses the first seven invariant moments (all scalars) of each spectrogram image frame to constitute the 7times1 feature vector. The rest of the procedure follows exactly as the conventional PCA. Among all the data, we manually identify twenty frames for each cases, and use them as the base training set. Feed several unknown clips for classification experiments, we suggest that both point-based feature extraction are effective ways to describe whistle vocalizations and believe that this algorithm would be useful for extracting features from noisy recordings of the callings of a wide variety of species.