Research On Key Technologies Of Underwater Pulsed Ultrasonic Imaging

Ultrasound has become a common method for underwater imaging and underwater detection due to its good directivity,easy focusing,small influence by water turbidity,and superior propagation characteristics than electromagnetic wave in water.It is widely used in marine resource detection,marine biological detection,seabed topography detection,underwater pipeline monitoring,underwater shipwreck salvage,underwater target detection and positioning.However,underwater ultrasonic imaging relies on piezoelectric transducers and pulse-echo detection.It is limited by size of transducer,performance of piezoelectric material and imaging mode.It has drawbacks of poor imaging resolution,poor signal-to-noise ratio(SNR)and low detection accuracy.So that,it is difficult to meet the needs of high-precision underwater detection.In this paper,the limitations of piezoelectric transducers and pulse-echo imaging are studied.The key techniques for improving underwater ultrasonic imaging performance are studied from the aspects of improving accuracy of time-of-flight(TOF),improving imaging resolution and improving spatial resolution.The main innovative work includes:The underwater optical detection method of pulse ultrasound is proposed.The model of acousto-optic effect of pulsed ultrasound in Raman-Nath regime is established.The laser is used as the sensor for the non-contact detection of ultrasonic sound field parameters.The test results show that the optical detection method of pulse ultrasound has good stability to sound pressure detection,high spatial resolution to sound field detection,wide frequency response,and accurate preservation of acoustic phase information.The imaging resolution is superior to conventional piezoelectric transducers.A directional acoustic energy correction model is proposed.The directivity function is optimized to make the amplitude superimposed imaging results conform to the actual situation of acoustic energy distribution and improve the SNR of imaging.The time complexity and imaging data characteristics of amplitude superposition imaging are analyzed.The triangular matrix data is used to replace the full matrix data to reduce the amount of imaging data.The parallel architecture and index matrix are used to accelerate the imaging calculation.The calculation time of single frame is shortened from 600 s to 130 ms.It improves the imaging accurancy and imaging speed of the amplitude superposition imaging algorithm.A cross-correlation TOF detection method is proposed.Ultrasonic optical detection is used to obtain a stable reference signal,which realizes TOF detection in phase level and improves the longitudinal resolution of imaging.A cross-correlation imaging correction method is proposed.The echo position is determined by the correlation coefficient of echoes,and the lateral edge position detection is realized,which improves the lateral resolution of imaging.Target and workpiece results show that,when using 1MHz ultrasonic transducer,the TOF resolution is 0.4ns,the repeated measurement error is less than 4ns,the ranging accuracy is better than0.3mm,and the thickness variation within 0.1mm can be detected.It improves the resolution and accurancy of the detection.Optical reconstruction of ultrasonic sound field is realized.The average value of sound pressure along the length of the acousto-optic effect is obtained by optical detection,and the two-dimensional and three-dimensional sound field reconstruction is performed by Radon transform.The optical reconstruction avoids the interference to the sound field by the conventional ultrasonic sensor,and has the advantages of good detection stability and high spatial resolution.The sound field reconstruction has a good consistency with the theoretical distribution of the sound field.