1159 results about "Wave velocity" patented technology

A SAW transducer includes a pair of opposing bus bars formed on a surface of a piezoelectric substrate with a plurality of interdigital electrodes extending from each of the opposing bus bars. A center region and opposing edge regions are formed, wherein each edge region is proximate each bus bar and includes free end portions of the electrodes such that a wave velocity in each edge region is substantially lower than a wave velocity in the bus bars and in the center region, and wherein electro-acoustic sources extend into the edge regions for providing a SAW transducer having the piezoelectrically excited strongest mode is a piston mode with an amplitude distribution generally flat across most of the resonator area.

The invention discloses a time-domain failure distance measuring method with two asynchronous ends of a direct-current power line and self-adaptive parameters, which comprises the following steps of: setting up an asynchronous time-domain failure observation equation of the direct-current line, which includes quantities of fault distance, distributed resistances of the line, transmission wave velocity, wave impedance, asynchronous time difference of data at the two ends and the like to be observed; taking the time of initial failure traveling waves which arrive at the two ends of the line respectively as reference time of data at each end; introducing a traveling wave characteristic equation; and eliminating the quantities to be observed of asynchronous time difference in the time-domain failure observing equation, thus obtaining a novel time-domain failure distance measuring method of the direct-current line so as to realize self-adaption of the line parameter self-adapting without synchronizing with a clock. The time-domain failure distance measuring method has higher transition resistance durability and can effectively improve the accuracy and reliability of failure distance measurement at the two ends of the direct-current line.

The invention provides a natural gas hydrate deposit dynamic triaxial mechanic-acoustic-electrical synchronous test experimental device, and a test method. The natural gas hydrate deposit dynamic triaxial mechanic-acoustic-electrical synchronous test experimental device comprises an acoustic wave receiving device, an acoustic wave transmitting device, and a resistivity testing device. The natural gas hydrate deposit dynamic triaxial mechanic-acoustic-electrical synchronous test experiment method comprises the steps that: step A, a hydrate sample is synthesized; step B, the synthesized hydrate sample is subjected to dynamic loading, and electric-acoustic-mechanical property tests are simultaneously carried out, until the sample is subjected to shearing damage. The invention provides a mechanical response characteristic research of indoor hydrate deposit under dynamic load. With the device and the method, mechanical, electrical, and acoustic parameter synchronous testing of hydrate deposit under dynamic load can be realized. With the test data, a relationship between mechanical parameters and wave velocity can be obtained by fitting.

A method for tomographic modeling of seismic wave travel times from well drilling records generated by drill string vibrations during drilling and identifying in the recorded vibration at least one of direct, reflected, refracted, converted and diving waves so as to measure wave velocity in near surface layers.

The invention discloses a wavelet difference algorithm-based cable fault localization method. The method comprises the following steps of: 1, detecting a signal in real time and synchronously uploading the signal; 2, performing signal acquisition; 3, performing cable fault localization by steps of performing signal pre-treatment which comprises normalization treatment, de-noising treatment and fault signal modulus transformation, judging a wavelet packet entropy fault region, determining the fault point by selecting a one-dimensional signal, performing wavelet transformation, solving a wavelet coefficient, detecting a modulus maximum point, sorting singular points after the maximum point, performing first-order difference operation and difference comparison, and performing localization operation on the cable fault position by using a single-end fault localization method free from influence of the wave velocity; and 4, synchronously outputting the treatment result. The method has the advantages of reasonable design, simple operation, convenient implementation and high localization precision, and effectively solves the practical problems that the fault region cannot be identified and the localization precision is relatively low and easily influenced by various factors in the conventional cable fault detection method.

The invention discloses a non-invasive multipath pulse wave detection device, which comprises at least two sensors, each of the sensors is provided with a fixing device thereon and connected to a signal processing unit, and the signals detected by the sensors are processed and stored by the signal processing unit. The invention synchronously collects pulse waves and wave velocities in a multipathmanner and uses a wave form processing technique for the pulse wave collecting points of the different sites of a human body, is capable of analyzing the wave velocities and the wave forms aiming at the different sites of the human body, and provides more specific information reflecting cardiovascular system states for clinic real-time monitor.

The invention discloses a non-contact type automatic blood pressure measuring system. Blood pressure of a measured person is measured by video recording. The non-contact type automatic blood pressure measuring system comprises a video acquiring module, an ROI extracting module, a pulse wave signal generating module, a filtering and noise reduction module, a waveform analyzing module and a blood pressure value calculating module. An automatic blood pressure measuring method comprises the following steps of acquiring a video of a finger tip; positioning the position of a tracked finger by using contour tracing of the finger tip; acquiring images of the finger; performing primary color separation and ROI segmentation on the images of the finger; respectively taking all pixel mean values of the filtered images as feature values to generate two time-varying pulse wave signal waveforms; performing comparison analysis on the two pulse wave waveforms to obtain transmission time of pulse; calculating the wave velocity of the pulse waves along an artery according to an image center distance d; and calculating a blood pressure value by using a conversion relation between pulse wave velocity and blood pressure. By using a noninvasive and non-contact type remote physiological signal detecting method, interference of motion artifact in a video record is overcome, the measuring precision is high, and blood pressure of a plurality of persons can be measured simultaneously.

The invention relates to a method for hybrid electric power circuit fault distance measurement of a power distribution network, in particular to a method for hybrid circuit combined distance measurement of the power distribution network based on single-ended fault information. The method for hybrid circuit combined distance measurement comprises the steps that firstly, high-frequency traveling wave components of a fault current signal and a fault voltage signal of a hybrid circuit are decomposed; secondly, the wave velocity of traveling waves transmitted in an overhead line and a cable is measured offline; thirdly, preliminary fault distance measurement is conducted according to a correlation coefficient analytical method; fourthly, final fault distance measurement is conducted according to a wave tip combining method; fifthly, a fault distance measurement result is verified. According to the method, only the fault information measured at one end of the circuit is used, initial wave tips and various reflecting wave tips of the traveling waves can be effectively extracted when a fault occurs, the method can be applied to the occasions without the double-ended traveling wave distance measurement function, accurate fault distance measurement is achieved, and the defects of fault distance measurement in the hybrid circuit of an existing power distribution network are overcome. According to the method, on the basis that fault distance measurement is achieved through the current traveling waves, a voltage traveling wave distance measurement result is comprehensively considered, and the reliability and the accuracy of distance measurement judgment are improved.

The invention discloses a test method and a system for grouting compactness of a pre-stressed duct of a bridge. The test method includes determining the center line position of the pre-stressed duct to be tested and fixing an acceleration transducer at the center line position; performing knocking and excitation around the acceleration transducer through an excitation hammer, and collecting elastic wave data produced by excitation through the acceleration transducer; and calculating the equivalent reflection wave velocity according to the elastic wave data and determining the grouting compactness of the pre-stressed duct of the bridge according to the equivalent reflection wave velocity. According to an embodiment of the test method and the system, the non-destructive testing technology is adopted to detect the whole grouting quality of the duct with a pre-stressed structure; and the test method and the system have the advantages of high accuracy and efficiency and low cost, have great significance for objective evaluation of quality conditions of the pre-stress structure, and are suitable for being widely applied to engineering.

The invention belongs to seismic data processing in petroleum geophysical exploration and geological engineering survey, and relates to near-surface velocity modeling and static correction in seismic exploration aiming at research and development of a shallow velocity model required for combined post static correction in a detector chamber of single-point high-density seism. A method for establishing a near-surface velocity model in high-density seismic static correction processing comprises the following steps of: supposing that the near-surface velocity is linearly increased along with the depth, calculating refracted wave velocity of different geophone offset from the travel time difference of primary waves of adjacent detection points of the high-density seism according to the physical property of the primary wave of the near-surface velocity field, establishing a near-surface ramp velocity field parameter according to the relationship between the geophone offset and the refracted wave, and further establishing the near-surface velocity model which can be directly used for static correction quantity calculation in seismic processing and also can be used as an initial model for tomographic inversion of near-surface velocity. The method has high calculation speed, and does not depend on the change of the initial position caused by a seismic focus.

The invention provides a method and a device for determining a gas cut position in the drilling process based on pressure wave time difference. The method comprises the following steps: enabling the device to take a pressure wave generated by the action of a hydraulic throttle valve as a wave source and judging whether the gas cut occurs or not by detecting the time difference between pressure waves received by a stand pipe pressure sensor and a sleeve pressure sensor; and according to the detected wellhead gas and liquid flow rate, the detected casing pressure and the detected transmission time difference between the pressure waves in an annulus and a drill rod, carrying out Runge-kutta iteration on all parameters by adopting an annulus hydraulic model, obtaining a corresponding relation existing among the distribution of void rates in different grids in the annulus, the wave velocity response time, the gas cut position and the pressure wave response time, determining an accurate position where the gas cut occurs. Compared with a traditional gas cut detection method, the method and the device provided by the invention have the beneficial effects that not only is the conventional gas cut detection function realized, but also the gas cut position can be accurately and quickly determined; and the method and the device have the advantages of accuracy in detection, strong real time, low equipment cost, simplicity in operation, wide application scope and the like.

The invention discloses a vehicle queuing overflow anticipation method for a city bottleneck road section, and is used for judging whether the road section suffers from or is about to suffer from vehicle queuing overflow. The vehicle queuing overflow anticipation method for the city bottleneck road section mainly comprises two parts of a maximum generalized queuing length determination method and a vehicle queuing overflow anticipation method. Firstly, the allowable vehicle queuing length value, i.e. the overflow safe distance, of the bottleneck road section is determined; then, a traffic shock wave model is combined to calculate the wave velocity of the queuing accumulation shock wave and the queuing dissipation shock wave; the layout position of a shock wave detector is determined; finally, according to the layout position of the detector, the maximum generalized queuing length is calculated; if the maximum generalized queuing length is larger than the overflow safe distance, the vehicle queuing overflow phenomenon is judged to happen or to be about to happen. According to the vehicle queuing overflow anticipation method for the bottleneck road section, which is provided by the invention, the premise and the quality guarantee are provided for judging whether the intersection bottleneck road section vehicle queuing overflow control is implemented in time.

The invention discloses a channel concrete lining non-destructive testing system, which includes a computer, and is characterized in that: the computer is connected to a dedicated ground-penetrating radar subsystem, and the computer includes a radar data processing subsystem and an ultrasonic data processing subsystem, and the radar The data processing subsystem includes a file management module, a profile data processing module, a lining disease simulation analysis module, a layer tracking and disease identification module, and a lining physical property analysis module. The ultrasonic data processing subsystem includes an ultrasonic wave velocity calculation module and a coagulation intensity Judgment module. This system improves the data processing efficiency and accuracy of the construction quality inspection of canal concrete lining, and provides scientific, accurate, advanced and effective technical support for the construction quality inspection of canal concrete lining.

Disclosed is a new method for providing accurate real time predictions of pore pressure and fracture gradient, at the rig site by determining the wave velocity from drill cuttings by a portable continuous wave technology (CWT) tool that measures drill cuttings at high resonant frequency and then using the velocity obtained in combination with the novel method of the present invention to arrive at accurate predictions for pore pressure and fracture gradient. The new technique offers real time pore pressure prediction at the rig site with small error margin that is not otherwise available using seismic, VSP, or check shot velocities in exploration.

The invention relates to the technical field of non-invasive blood pressure detection, in particular to a non-invasive blood pressure detection method based on mixing of photoelectric green-light pulses and an electrocardiogram. A multi-mode bioelectricity sensor is adopted in the non-invasive blood pressure detection method. The non-invasive blood pressure detection method includes the following steps that a conventional method based on photoelectric plethysmography (PPG) is adopted, the characteristic parameters in PPG signals are extracted, a measurement model of the blood pressure of the human body is built, blood pressure calibration is carried out, the calibration parameters closely related to the blood pressure value of the human body are obtained, and then the calibration parameters are used to carry out blood pressure measurement of pulse waves based on PPG waveform and ECG waveform; a multi-parameter blood-pressure estimation model is built with the wave velocity measurement method of the pulse waves based on the PPG waveform and the ECG waveform, and the final measured data is compared, analyzed and corrected. According to the non-invasive blood pressure detection method, as the mixing mode of the PPG waveform and the ECG waveform is used for detection, the detection accuracy is effectively increased; the non-invasive blood pressure detection method and the multi-mode bioelectricity sensor have the extremely-high hardware integrating degree, and good conditions are created for convenience and miniaturization of products.

The invention provides a method for predicting the transversal wave velocity of a carbonatite rock reservoir and relates to the technical field of geophysical exploration. The method comprises the following steps: determining the mineral component, the reservoir porosity as well as the fluid saturation of the reservoir; dividing the reservoir porosity into a rigid porosity, a fracture porosity and a clay porosity; obtaining the parameters of each mineral in the reservoir; calculating the bulk modulus Km and the shear modulus mum of a rock matrix through a Voigt-Reuss-Hill average model; calculating rhom; generating kfl and rhofl; generating the equivalent density rhodry of a rock dry skeleton; sequentially adding the porosities into the rock matrix through a DEM model, and calculating Kdry and mudry; carrying out fluid replacement on the rock dry skeleton through a low-frequency Gassmann equation so as to generate the density rho of a saturated fluid rock; calculating Ksat and musat; calculating the longitudinal wave speed (as shown in the description) and the transversal wave speed (as shown in the description) of the saturated fluid rock, inverting a virtual fracture porosity phi'f by a simulated annealing method according to a first relative error (as shown in the description) or a second relative error of rho and rhoo; replacing phif with phi'f, returning and calculating to generate the transversal wave speed V1 of the carbonatite rock reservoir.

The invention discloses a method for utilizing a laser ultrasonic wave to precisely measure a third order elastic constant of metal. The method comprises the following steps: respectively measuring wave velocities of a longitudinal wave, a transverse wave and a surface wave excited by a laser under stress-free state and stress state; utilizing the wave velocities of the longitudinal wave, the transverse wave and the surface wave measured under stress-free state to calculate a second order elastic constant and a density of metal according to a sonic elasticity theory and a Rayleigh equation; utilizing the ultrasonic wave velocities of the longitudinal wave, the transverse wave and the surface wave measured under stress state to introduce an equivalent second order elastic constant and an independently measured linear coefficient of thermal expansion; and lastly, calculating the third order elastic constant according to the sonic elasticity theory. According to the method, a pulse laser source is utilized to excite a sound surface wave, non-contact exciting is performed under a thermal elastic system and an overheated phenomenon of materials is avoided, so as to realize nondestructive measurement. A large amount of sound surface wave data spread for different distances is collected and a correlation function is utilized to calculate a wave velocity of the sound surface wave and a spread distance of sound wave, thereby greatly reducing error of arriving time value of the sound surface wave and promoting the measuring precision of a sound wave velocity.

The invention discloses a three-dimensional wave velocity imaging measurement-while-drilling method and device for rock in front of the tunnel face. Before an advanced geological drilling rig works, earthquake wave receiving sensors are rapidly arranged on the tunnel face and the tunnel lateral face; a loop-line combined three-dimensional observation mode is adopted, and according to the data fed back through a drilling depth sensor in real time, the running mode of a drill rod of the advanced geological drilling rig and the drilling mode of a drill bit are automatically switched at the right moment through a drilling rig control system; earthquake source signals are collected in real time through a pivot sensor, noise signals are collected in real time through a noise sensor, and earthquake waves are collected through the earthquake wave receiving sensors in real time. By means of the device, the distribution condition of unfavorable geologic bodies in front of the drilling portion of the advanced geological drilling rig and the distribution condition of unfavorable geologic bodies around a tunnel can be obtained, the rock in front of the drilling portion can be accurately assessed, the quality level of the rock in front of the drilling portion can be predicted, the unfavorable geologic bodies in front of the tunnel face can be found in time, a construction scheme can be optimized accordingly, and use reference can be provided for guaranteeing the tunnel construction safety.

The invention discloses an oscillation wave partial discharge identifying and positioning method for an asynchronous double-end power cable. The method disclosed by the invention comprises the following specific steps: respectively installing oscillation wave partial discharge signal acquisition devices at the two ends of the cable; obtaining the voltage and partial discharge capacity signals at the two ends of the detected cable through data acquisition; recording and converting the corresponding data to obtain a waveform file which is needed for failure positioning; calling a partial discharge positioning algorithm to carry out the failure positioning; and computing the accurate position of a failure point generating partial discharge signals. The positioning analysis result obtained by the invention is more accurate; the error rate is small; the wave velocity does not need to be known, thereby reducing the errors caused by computing the wave velocity; the excessive manual intervention is needed; the problems of large data quantity, inaccuracy in positioning and the like because different pulses under the voltage of each experiment are matched are avoided; and the rapid and accurate failure positioning course of the method is beneficial to the timely repair of faulty lines and the reduction of loss caused by power failure.

A non-invasive array-based hemodynamic monitoring system on chip is disclosed. The non-invasive array-based hemodynamic monitoring system on chip comprises a CMOS MEMS pressure sensor array, a readout circuit, and a signal control system. The CMOS MEMS pressure sensor array is configured to sense a pulse wave of a blood vessel. The readout circuit is coupled with each of the CMOS compatible MEMS pressure sensors and is configured to read the pulse wave and transformed the pulse wave into a voltage signal. The signal control system is coupled with each of the readout circuit, and is configured to estimate a wave velocity according to the voltage signal.

The invention belongs to the field of application of geophysical wave technology and relates to a method of inversing rock fracture parameters under different pressures by rock ultrasonic measurement data. The method includes the steps of firstly, respectively measuring longitudinal wave velocity and transverse wave velocity of a rock sample in dry and fluid-saturated states under different pressures; secondly, forward-modeling and calculating the longitudinal wave velocity and transverse wave velocity by a model of 'general theory of porous and fissured medium elastic wave motion; thirdly, joint-establishing an inversion target function; fourthly, solving a global minimum of the target function by optimal GA (genetic algorithm); and fifthly, outputting corresponding model target parameters when the global minimum of the target function is obtained. By the method, the change of different-scale fractures or fissures in rock can be reflected, and the fracture density and facture aspect ratio of the rock under different pressures can be acquired conveniently.

The invention relates to a method for measuring fault location of a HVDC transmission line under consideration of measured wave velocity; the method is characterized by comprising the following steps: considering the variation characteristic of the wave velocity on the basis of the traditional dual-end location measuring method, and performing the calibration of a wave tip and the determination of the wave velocity in the same process; respectively acquiring the positive and negative data of the voltage of the HVDC transmission line by means of devices at two ends of the line; after the data is subjected to filtering processing and line-mode conversion, analyzing the obtained line-mode traveling wave, obtaining a time-frequency oscillogram of signals and determining the arrival time of the fault traveling wave to two ends of the line as well as a corresponding instantaneous frequency value; obtaining the corresponding line-mode wave velocity by using frequency-wave velocity curves; and finally, obtaining the fault distance by means of a dual-end distance measurement formula.

The invention belongs to the field of geotechnical engineering, and provides a layered speed positioning method for a regional rock microseismic source. The method comprises the following steps: (1), dividing the rock region of a to-be-detected seismic source into different wave velocity layers, building a three-dimensional rectangular coordinate system, and calculating an analytic equation of interfaces of all wave velocity layers; (2), installing sensors in the rock region, and measuring the spatial coordinates of all sensors; (3) carrying out an explosion test in the rock region, and calculating the wave velocities of all wave velocity layers; (4) calculating the spatial coordinates (x0, y0, z0) of the microseismic source. The method provided by the invention achieves high positioning precision of the microseismic source, and can prompt the technology of microseismic monitoring to play a good role in prediction and early warning in engineering practice.

The invention discloses a method for improving the location precision of micro seismic sources for mining. Through wave velocity zoning, a wave velocity model is more in line with the velocity structure characteristics of the complex rock masses of mines in a micro seismic monitoring area (such as a mine roadway, a goaf, an ore body or a surrounding rock), the extraction precision of the actual arrival time of seismic wave is higher, and thus, the precision of seismic source inversion and location is improved. In the process of seismic source inversion and location, an initial iteration seismic source parameter depth threshold is set through the layout and monitoring range of the space envelope of wave detectors, and a weighting factor is introduced during inversion according to the waveform quality. Therefore, the location precision of seismic sources is improved.

The invention discloses a direct current (DC) travelling wave fault location method based on wave velocity optimization, which comprises the following steps: obtaining the fault distance via a ranging formula according to the determined initial wave velocity and the time of transferring the fault travelling wave to a rectification side and an inversion side, and marking the fault distance as x (0); performing interpolation of a fault distance-wave velocity curve according to the obtained fault distance, and obtaining a wave velocity v1 of transmitting the fault travelling wave from the fault point to the rectification side of a line and a wave velocity v2 of transmitting the fault travelling wave from the fault point to the inversion side of the line respectively; calculating the fault distance again by use of the obtained wave velocities v1 and v2, and marking the fault distance as x (i); comparing the nth ranging result and the (n+1)th ranging result, and judging whether the error of the two is less than a set value or greater than the maximum iteration times; if the error is less than the set value or greater than the maximum iteration times, outputting the fault distance; and if the error is greater than the set value, performing iteration again by use of x (i) and circulating to calculate the fault distance. By adopting the method disclosed by the invention, the travelling wave fault location precision of a DC line can be greatly improved in the premise of not changing the hardware facility of the existing ranging device.

The invention provides a core three-dimensional multi-parameter measuring instrument based on visual observation of crack. The instrument comprises a controller and a core holder. The controller is used for sending and receiving signal; and measurement of all core parameters is finished in the core holder. The instrument uses nosean core as a main subject, and is provided with upper plungers with three different functions comprising a visualization plunger, an electroacoustic plunger and a porosity permeability plunger, and a public lower plunger. For experiments, nosean core is placed into the holder which is loaded with plungers at both ends, and parameters of the system are set, so that the instrument can automatically complete the corresponding parameter tests. Under conditions of heating and pressurization, the visualization plunger is used to realize real-time observation on crack changes and measurement of crack width through an internally configured camera; the electro-acoustic plunger and the porosity permeability plunger can measure parameters of core, such as porosity, permeability velocity, wave velocity and resistivity, and observe a displacement process and changes of core cracks; and measurement of parameters at three directions X,Y and Z of the core can be realized by changing loading direction of nosean core.

A measurement-while-drilling method and device assesses outburst risk and evaluates gas drainage performance of a coal seam. The device includes a compartment, a pressure sensor, a temperature sensor, a flow sensor, an electromagnetic sensor, an acoustic sensor, a wave velocity measurement module, a monitoring and control module, a power supply and a communication interface. These are installed between a drill bit and an inclinometer or a first drill pipe. Measurements are performed while drilling to obtain a real time gas parameter, lithologic and coal seam information, for on-site assessment of outburst risk. During drill bit replacement, gas pressure, temperature, flow velocity, wave velocity, electromagnetic radiation, and an acoustic signal are recorded in real time, to calculate stress of the coal seam and coal seam outburst risk. On-site measurement is done while drilling without sampling and can use multiple parameters to perform synchronous measurement to obtain a comprehensive evaluation.

Provided is a traveling wave fault locating method based on multi-measuring-information. Various transformer station measuring units acquire a ground mode component and a line mode component by performing phase-mode transformation decoupling on a received fault initial wave head, solve a ground mode wave velocity in combination with a trained neural network, and preliminarily estimate fault distance according to a difference of the time when the ground mode component and the line mode component reach the same measuring unit. A traveling wave positioning main station determines a suspected fault line set in view of the fault distance estimated by each measuring unit, and positions a fault line in combination with a real-time network topologic structure and a fault traveling wave characteristic. A non-fault line closest to the fault line is selected to compute real-time line mode wave velocity. A fault point distance is accurately computed according to a dual-end or expanded dual-end range finding method. The traveling wave fault locating method based on multi-measuring-information measures the fault distance by using multi-point data, prevents range finding failure caused by the fault of a single device, and increases positioning precision and reliability.

The invention relates to a cable single-ended travelling wave distance measurement method using fault signature frequency bands and TT conversion and belongs to the technical field of electrical power system relay protection. Wavelet decomposition is carried out on single-ended travelling wave transient state signals which are then reconfigured into time-domain signals of a plurality of frequency bands, attenuation coefficients and wave velocity of a cable line in each frequency band are relatively fixed, correlation coefficients of initial travelling wave heads (initial wave heads) and fault point reflected wave heads under each frequency band after faults are calculated by adopting relative analysis, the frequency band with maximum correlation coefficients is designed to be a characteristic frequency band, and travelling wave velocity is calculated according to center frequency of the characteristic frequency band. Moment of the travelling wave heads to reach the measurement end is accurately demarcated by using a TT algorithm of time domain conversion in the characteristic frequency band. The cable single-ended travelling wave distance measurement method uses the center frequency of the characteristic frequency band to calculate the travelling wave velocity, reduces errors caused by selection randomicity of the travelling wave velocity in a traditional cable single-ended travelling wave distance measurement technique, is applied to the TT conversion algorithm to demarcate reaching moment of the travelling wave heads, and improves distance measurement accuracy.