Exploiting nonlinear propagation in echo sounders and sonar

The 10th European Conference on Underwater Acoustics (ECUA). 2010, Istanbul, Turkey. Mainstream sonars transmit and receive signals at the same frequency. As water is a nonlinear medium, a propagating signal generates harmonics at multiples of the transmitted frequency. For sonar applications, energy transferred to higher harmonics is seen as a disturbance. To satisfy requirements for calibration of echo sounders in fishery research, input power has to be limited to avoid energy loss to harmonics generation. Can these harmonics be used in sonar imaging? The frequency dependency of target echos, and the different spatial distribution of higher harmonics can contribute to additional information on detected targets in fish classification, ocean bathymetry, or bottom classification. Our starting point was the sonar equation adapted for the second harmonic. We have simulated nonlinear propagation of sound in water, and obtained estimates of received pressure levels of harmonics for a calibration sphere, or a fish as reflector. These pressure profiles were used in the sonar equation to compare harmonics to fundamental signal budget. Our results show that a 200 kHz thermal noise limited echo sounder, with a range of 800 m will reach around 300 m for the second harmonic. This means the second harmonic is useful in many applications

Validation of temporal parameters within the skating sub-techniques when roller skiing on a treadmill, using inertial measurement units

The aim of this study was to develop and validate a method using inertial measurements units (IMUs) to determine inner-cycle parameters (e.g., cycle, poles and skis contact, and swing time) and the main sub-techniques (i.e., G2, G3 and G4) in cross-country roller ski skating on a treadmill. The developed method is based on the detection of poles and skis initial and final contacts with the ground during the cyclic movements. Thirteen well-trained athletes skied at different combinations of speed (6–24 km∙h-1) and incline (2–14%) on a treadmill using the three different sub-techniques. They were equipped with IMUs attached to their wrists and skis. Their movements were tracked using reflective markers and a multiple camera infrared system. The IMU-based method was able to detect more than 99% of the temporal events. It calculated the inner-cycle temporal parameters with a precision ranging from 19 to 66 ms, corresponding to 3.0% to 7.8% of the corresponding inner-cycle duration. The obtained precision would likely allow differentiation of skiers on different performance levels and detection of technique changes due to fatigue. Overall, this laboratory validation provides interesting possibilities also for outdoor applications.publishedVersio

Inner-Cycle Phases Can Be Estimated from a Single Inertial Sensor by Long Short-Term Memory Neural Network in Roller-Ski Skating

Objective: The aim of this study was to provide a new machine learning method to determine temporal events and inner-cycle parameters (e.g., cycle, pole and ski contact and swing time) in cross-country roller-ski skating on the field, using a single inertial measurement unit (IMU). Methods: The developed method is based on long short-term memory neural networks to detect the initial and final contact of the poles and skis with the ground during the cyclic movements. Eleven athletes skied four laps of 2.5 km at a low and high intensity using skis with two different rolling coefficients. They were equipped with IMUs attached to the upper back, lower back and to the sternum. Data from force insoles and force poles were used as the reference system. Results: The IMU placed on the upper back provided the best results, as the LSTM network was able to determine the temporal events with a mean error ranging from −1 to 11 ms and had a standard deviation (SD) of the error between 64 and 70 ms. The corresponding inner-cycle parameters were calculated with a mean error ranging from −11 to 12 ms and an SD between 66 and 74 ms. The method detected 95% of the events for the poles and 87% of the events for the skis. Conclusion: The proposed LSTM method provides a promising tool for assessing temporal events and inner-cycle phases in roller-ski skating, showing the potential of using a single IMU to estimate different spatiotemporal parameters of human locomotion.publishedVersio

Worst-case analysis of array beampatterns using interval arithmetic

Over the past decade, interval arithmetic (IA) has been utilized to determine tolerance bounds of phased array beampatterns. IA only requires that the errors of the array elements are bounded, and can provide reliable beampattern bounds even when a statistical model is missing. However, previous research has not explored the use of IA to find the error realizations responsible for achieving specific bounds. In this study, the capabilities of IA are extended by introducing the concept of ``backtracking'', which provides a direct way of addressing how specific bounds can be attained. Backtracking allows for the recovery of both the specific error realization and the corresponding beampattern, enabling the study and verification of which errors result in the worst-case array performance in terms of the peak sidelobe level. Moreover, IA is made applicable to a wider range of arrays by adding support for arbitrary array geometries with directive elements and mutual coupling, in addition to element amplitude, phase, and positioning errors. Lastly, a simple formula for approximate bounds of uniformly bounded errors is derived and numerically verified. This formula gives insights into how array size and apodization cannot reduce the worst-case peak sidelobe level beyond a certain limit.Comment: This article may be downloaded for personal use only. Any other use requires author and AIP Publishing prior permission. This article appears in The Journal of the Acoustical Society of America and may be found at https://doi.org/10.1121/10.0019715. The current e-print was typeset by the authors and can differ in, e.g., pagination, reference numbering, and typographic detai

APES Beamforming Applied to Medical Ultrasound Imaging

As of today, sonar imaging is the most effective means of documenting the subsea environment. Existing sonar imaging systems generally rely on conventional beamforming methods to form the image. While conventional beamformers are robust and simple, they leave many of the challenges of sonar imaging unresolved. Sonar images are often degraded by noise, and the image resolution as well as the range at which useful images can be obtained is limited. This thesis addresses the use of adaptive beamforming and imaging methods applied to active sonar. The goal of an adaptive beamformer in this context is to improve the quality of the sonar image by allowing the beamformer to adapt to the situation, recognizing sources of noise and interference and suppressing them before they have the chance to contaminate the image. The desired result is an image containing more useful and correct information, less noise, and improved image resolution. Focus has been on investigating how different adaptive methods can be implemented in a practical setting, and analyzing the performance of each method. Key challenges that are addressed include coherent signals, arbitrary array geometries, computational load, and robustness. Two of the most common adaptive beamforming methods, the minimum variance distortionless response (MVDR) and the amplitude and phase estimation (APES) beamformers, are considered, as well as a low complexity variant of the adaptive MVDR beamformer. Adaptive imaging methods based on aperture coherence represent a promising class of adaptive methods, and are also considered. We conclude that in many cases, improved image quality is obtained by using adaptive beamforming methods

CEReS Newsletter No.62, January 2011

[掲載記事] 「＜調印式報告＞千葉大学・パジャジャラン大学との大学間交流協定」ほ

Hydrological Characteristics of the Mae Klong River Basin in Thailand

[ABSTRACT] This paper discusses hydrological characteristics in the Mae Klong River Basin in Thailand. Firstly, rainfall distribution characteristics are examined by using daily rainfall data in sixteen rainfall observatories. Secondly, hydrological events of the two dam basins were qualitatively analyzed. It was found that, there was a relationship between the Western Monsoon and the rain occurrences; Si Sawat (Sri Nagarind) zone was draughtier in the Mae Klong River Basin. Si Sawat Dam Basin and Khao Laem Dam Basin, although they are close to each other (50 Km in distance), their differences concerning hydrological characteristics are high

Sensor-based gait analyses of the six-minute walk test identify qualitative improvement in gait parameters of people with multiple sclerosis after rehabilitation

The aim of this work was to determine whether wearable inertial measurement units (IMUs) could detect gait improvements across different disability groups of people with Multiple Sclerosis (pwMS) by the six-minute walk test (6MWT) during a rehabilitation stay in a specialized rehabilitation center. Forty-six pwMS and 20 healthy controls (HC) were included in the study. They performed the 6MWT with two inertial measurement units (IMUs) placed on the feet. Thirty-two of the pwMS were retested at the end of the stay. PwMS were divided in a mild-disability and a moderate-disability group. The 6MWT was divided in six sections of 1 min each for technical analysis, and linear mixed models were used for statistical analyses. The comparison between the two disability groups and HC highlighted significant differences for each gait parameter (all p < 0.001). The crossing effect between the test–retest and the two disability groups showed greater improvement for the moderate-disability group. Finally, the gait parameter with the higher effect size, allowing the best differentiation between the disability groups, was the foot flat ratio (R2 = 0.53). Gait analyses from wearable sensors identified different evolutions of gait patterns during the 6MWT in pwMS with different physical disability. The measured effect of a short-time rehabilitation on gait with 6MWT was higher for pwMS with higher degree of disability. Using IMUs in a clinical setting allowed to identify significant changes in inter-stride gait patterns. Wearable sensors and key parameters have the potential as useful clinical tools for focusing on gait in pwMS.publishedVersio

Air versus Sulfur Hexafluoride Gas Tamponade for Small and Medium-Sized Macular Holes: A Randomized Noninferiority Trial

Purpose: To investigate whether air tamponade is noninferior to sulfur hexafluoride (SF6) gas tamponade for small (≤ 250 μm) and medium-sized (> 250 μm and ≤ 400 μm) macular holes (MHs). Design: Multicenter, randomized controlled, noninferiority trial. Participants: Patients aged ≥ 18 years undergoing surgery for primary MHs of ≤ 400 μm in diameter. Methods: The patients in both groups underwent conventional pars plana vitrectomy with peeling of the internal limiting membrane. At the end of the surgery, the patients were randomized to receive either air or SF6 gas tamponades, stratified by MH size. Postoperatively, the patients followed a nonsupine positioning regimen for 3 days. Main Outcome Measures: The primary end point was the MH closure rate after a single surgery, confirmed by OCT after 2 to 8 weeks. The noninferiority margin was set at a 10–percentage-point difference in the closure rate. Results: In total, 150 patients were included (75 in each group). In the intention-to-treat (ITT) analysis, 65 of 75 patients in the air group achieved primary closure. All 75 MHs in the SF6 group closed after a single surgery. Six patients were excluded from the per-protocol (PP) analysis. In the PP analysis, 63 of 70 patients in the air group and all 74 patients in the SF6 group achieved MH closure after a single surgery, resulting in closure rates of 90% (95% confidence interval [CI], 79.9%–95.5%) and 100% (95% CI, 93.9%–100%), respectively. For the difference in closure rates, the lower bound of a 2-sided 95% CI exceeded the noninferiority margin of 10% in both ITT and PP analyses. In the subgroups of small MHs, all 20 patients in the air group and all 28 patients in the SF6 group achieved primary closure. Conclusions: This prospective randomized controlled trial proved that air tamponade is inferior to SF6 tamponade for MHs of ≤ 400 μm in diameter.publishedVersio