CN113155239B - Target detection and positioning method in layered medium without prior knowledge - Google Patents
Target detection and positioning method in layered medium without prior knowledge Download PDFInfo
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- G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
- G01F23/00—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm
- G01F23/22—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water
- G01F23/28—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water by measuring the variations of parameters of electromagnetic or acoustic waves applied directly to the liquid or fluent solid material
- G01F23/296—Acoustic waves
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
- G01F23/00—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm
- G01F23/22—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water
- G01F23/28—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water by measuring the variations of parameters of electromagnetic or acoustic waves applied directly to the liquid or fluent solid material
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- G01F23/2962—Measuring transit time of reflected waves
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Abstract
The invention discloses a target detection and positioning method in a layered medium without prior knowledge. The method comprises the following steps: arranging a plurality of array elements on the first surface of the first medium layer; sequentially transmitting acoustic pulses as forward acoustic beams by each array element, and respectively receiving signals generated by the acoustic pulses by a plurality of array elements; and determining the reverse sound beam corresponding to the received signal. Presetting a first trial sound velocity of a first medium layer; determining the position shape of the interface under the first tentative sound speed according to the n x n reflection signals; presetting a second trial sound velocity of a second medium layer; determining possible points of a target corresponding to the array elements according to the forward sound beams and the reverse sound beams; calculating the dispersion of the possible points; presetting a plurality of first trial sound speeds and second trial sound speeds to obtain the minimum dispersion of possible points; and determining a first trial sound velocity and a second trial sound velocity which approach to an actual value and the position form of an interface according to the minimum dispersion, and detecting and positioning the target in the layered medium by adopting a snapshot TR-RTM (TR-transfer molding) method.
Description
Technical Field
The invention relates to the field of ultrasonic detection, in particular to a target detection and positioning method in a layered medium without prior knowledge.
Background
Time reversal is a method of focusing through a medium (including heterogeneous media) without a priori knowledge. I.e. the acoustic pulses emitted by the individual acoustic sources, arrive at a point in space at different times. And time reversal operation is used, so that the time is fast, the time is transmitted later, the time is slow, the time is transmitted first, the target points are reached at the same time, and the target detection is realized. But the biggest disadvantage of this technique is that it cannot be positioned. For a layered medium, the signals of the interface and the signals scattered by the target overlap and cannot be distinguished after time reversal processing.
In physical flash (vol 36, No. 11), a hybrid TR-rtm (time reversal and reverse time migration) method of time reversal and reverse time migration of snapshots of target detection and positioning in layered media was proposed, which enables detection and positioning of targets in layered media. But this approach requires a priori knowledge of the layered media. Therefore, without prior knowledge (i.e., unknown configurations of the sound velocities of the upper and lower layer media and the interface), it is difficult to detect and locate the target in the layered media.
Disclosure of Invention
Aiming at solving the defects in the prior art.
The embodiment of the application provides a target detection and positioning method in a layered medium without prior knowledge, which can invert the sound velocity of upper and lower medium layers of the layered medium and the position and shape of an interface, and can realize more accurate detection and positioning of a target in the layered medium according to parameters in the layered medium obtained by inversion. The layered medium referred to herein is composed of a first medium layer and a second medium layer; the first medium layer comprises a first surface and an interface connected with the second medium layer. The target is located in the second dielectric layer.
The target detection and positioning method specifically comprises the following steps:
step S100: arranging n array elements on the first surface of the first medium layer; each array element is used as a transmitting array element to transmit acoustic pulse in sequence, and the acoustic pulse is used as a forward acoustic beam; the n array elements are used as receiving array elements, and each receiving array element receives n sound pulses which reach receiving signals of the receiving array elements after being reflected by an actual interface and scattered by a target respectively. And determining the reverse sound beam corresponding to each received signal according to the n x n received signals. Wherein the received signal comprises a reflected signal of the acoustic pulse passing through the actual interface and a scattered signal of the target.
Step S200: presetting first probing sound velocities of p first medium layers; determining the position and the shape of a tentative interface at the p-th first tentative sound speed according to the n-x-n reflection signals;
step S300: presetting second probing sound velocities of q second medium layers; determining possible points of n x (n-1) targets corresponding to n array elements under the p first tentative sound velocity and the q second tentative sound velocity according to the n forward sound beams and the n x n reverse sound beams; calculating the dispersion of n x (n-1) possible points;
step S400: determining the minimum dispersion in the dispersions corresponding to the q second trial sound speeds under the p first trial sound speed; wherein the minimum dispersion is the minimum of the q dispersions; determining minimum dispersion in the minimum dispersions corresponding to the p first probing sound speeds; the minimum dispersion is the minimum value of the p minimum dispersions;
step S500: and detecting and positioning the target in the layered medium by adopting a snapshot TR-RTM method according to the first probing sound speed and the second probing sound speed corresponding to the minimum dispersion and the position and the shape of the probing interface under the first probing sound speed corresponding to the minimum dispersion.
The first tentative sound velocity corresponding to the minimum dispersion obtained by the method approaches to the actual sound velocity of the first medium layer; the second tentative sound velocity corresponding to the minimum dispersion approaches the actual sound velocity of the second medium layer; the position and shape of the tentative sound interface at the first tentative sound speed corresponding to the infinitesimal dispersion approach to the actual position and shape of the actual interface.
In one embodiment, the step of obtaining the inverse sound beam by receiving the signal comprises: carrying out time reversal on the received signal to obtain a time reversal signal; the time reversal signal is used as a reverse sound beam after being advanced by a travel time difference; wherein, a travel time difference is the difference between the second travel time of the receiving array element to the target and the first travel time of the transmitting array element to the target.
In one possible embodiment, the layered media may be placed in a rectangular coordinate system to quantify the parameters of the various information therein. Specifically, the rectangular coordinate system includes an x-axis direction and a z-axis direction; the coordinate of the first surface in the rectangular coordinate system is (x, 0); the n array elements are arranged on the first surface, wherein the first array element is located at the origin of coordinates of the rectangular coordinate system, and the rest n-1 array elements are arranged at equal intervals along the x-axis direction.
In a further embodiment, the position and shape of the probing interface under the p-th first probing sound speed are determined, specifically the steps comprise:
determining n x n reflected signal travel time according to n forward sound beams and n reflected signals corresponding to each forward sound beam, wherein n x n reflected signals are total; under a first test sound velocity, when traveling according to n x n reflection signals, calculating to obtain n x n deviation ellipses, wherein the different deviation ellipses can generate aggregation, so that a plurality of dense points are formed, and partial tangents of the n x n deviation ellipses form a leading edge; the tangent points that make up the leading edge are fitted to a tentative boundary surface that approaches the position and shape of the actual boundary surface.
Obtaining a shift ellipse by using a kirchhoff shift method, wherein the shift ellipse is a set of detection points S of a probing interface, and coordinates (x) of the detection points Ss,zs) Calculated by the following formula:
wherein (a)i0) is the ith transmitting array element coordinate on the x axis, (a)j0) is the coordinates of the jth receiving array element on the x-axis, c1pFor the p-th first probe speed of sound,
after the acoustic pulse transmitted by the ith transmitting array element is reflected by the actual interface E, the reflected signal received by the jth receiving array element travels.
In a further embodiment, the possible points of the target at the first and second tentative sound speeds are determined, specifically the steps include:
firstly, under a first trial sound velocity and a second trial sound velocity, based on Snell's law, determining a first refraction point on a trial interface when a read forward sound beam reaches a first travel of the target; and calculating the wave front of the forward sound beam according to the first trial sound velocity, the second trial sound velocity, the first travel time and the first refraction point.
Secondly, under the first probing sound velocity and the second probing sound velocity, based on Snell's law, determining a second refraction point on a probing interface according to a second travel time when the reverse sound beam emitted by the receiving array element reaches the target; calculating the wave front of the reverse sound beam according to the first trial sound velocity, the second travel time and the second refraction point;
and finally, determining possible points of the target according to the wave front of the forward sound beam and the wave front of the reverse sound beam.
Wherein the wave front of the forward sound beam is a set of possible points of the target corresponding to the transmitting array element, and the coordinates (x) of the possible points of the target corresponding to the transmitting array elementipm,zipm) Calculated by the following formula:
wherein (a)i0) is the ith transmitting array element coordinate on the x axis, (x)is,zis) First refraction point coordinate corresponding to ith transmitting array element, c1pFor the p-th first tentative speed of sound, c2qFor the q-th second probe speed of sound,
the ith transmitting array element corresponds to the first travel time of the forward sound beam to reach the target O.
The wave front of the reverse beam is a set of possible points of the target corresponding to the receiving array elements, the coordinates (x) of the possible points of the target corresponding to the receiving array elementsjpm,zjpm) Calculated by the following formula:
wherein (a)j0) is the jth receiving array element coordinate on the x axis, (x)js,zjs) Second refraction point coordinates corresponding to jth receiving array element, c1pFor the p-th first tentative speed of sound, c2qFor the q-th second probe speed of sound,
the jth receiving array element corresponds to the second travel time of the reverse sound beam to reach the target O.
Wave front of the forward beam, i.e. coordinate (x)ipm,zipm) And the wave front of the reverse beam, i.e. the coordinate (x)jpm,zjpm) Coordinates (x) of the intersection of the sets of (2)pm，zpm) As possible points of the target.
In a further embodiment, the dispersion of n x (n-1) possible points is calculated by:
wherein v is the number of possible point pairs consisting of any two of the possible points in n x (n-1), v is n (n-1) (n (n-1) -1)/2, duIs the distance between two of the possible points in the u-th possible point pair, u e v.
The embodiment of the application has the advantages that: under the condition of no prior knowledge, the interface configuration of the layered medium is obtained by a transducer multiple-sending and multiple-receiving method; calculating corresponding dispersion through sound velocity of each medium in the trial layered medium, and determining trial sound velocity of each medium layer according to the dispersion; determining a corresponding tentative interface position shape according to the first tentative sound speed; finally, the TR-RTM method of the snapshot is utilized to invert the sound velocities of the upper medium and the lower medium of the layered medium and the position of the interface without prior knowledge, and the target in the layered medium can be accurately detected and positioned according to the parameters in the layered medium obtained by inversion.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings needed to be used in the description of the embodiments will be briefly introduced below, and it is apparent that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art that other drawings can be obtained according to the drawings without inventive work.
FIG. 1 is a schematic illustration of an acoustic propagation path according to an embodiment of the present invention;
FIG. 2 is a block diagram of a method for detecting and locating an object in a layered medium without prior knowledge according to an embodiment of the present invention;
FIG. 3 is a schematic diagram of a front intersection point of acoustic beams when a transducer is used for single transmission and multiple reception according to an embodiment of the present invention;
fig. 4 is a received signal diagram of signals transmitted by the 5 th array element and received by the 1 st to 20 th array elements according to the first embodiment of the present invention;
fig. 5 is a dispersion distribution diagram of target possible points corresponding to 3 second tentative sound speeds under the pth first tentative sound speed in the first embodiment of the present invention;
fig. 6 is a graph of target dispersion corresponding to q second tentative sound speeds at the pth first tentative sound speed in the first embodiment of the present invention;
FIG. 7 is a graph illustrating the minimum dispersion of the target for p first probing speeds of sound in accordance with an embodiment of the present invention;
FIG. 8 is a schematic diagram of the shape of the interface at the first probing speed of sound corresponding to the minimum dispersion in the first embodiment of the present invention;
FIG. 9 is a three-dimensional view of the results of target location in accordance with one embodiment of the present invention;
fig. 10 is a received signal diagram of signals transmitted by the 5 th array element and received by the 1 st to 20 th array elements according to the second embodiment of the present invention;
fig. 11 is a graph of the target minimum dispersion corresponding to p first tentative sound speeds in the second embodiment of the present invention;
fig. 12 is a schematic view of the shape of the interface at the first probing speed of sound corresponding to the minimum dispersion in the second embodiment of the present invention;
FIG. 13 is a three-dimensional view of the target positioning result in the second embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
The layered medium with different shape interfaces consists of the first medium layer and the second medium layerTwo dielectric layers. The first dielectric layer includes a first surface and an interface interfacing with the second dielectric layer. The longitudinal sound velocity of the first medium layer is a first sound velocity C1(ii) a The longitudinal wave velocity of the second medium layer is the second sound velocity C2. When a target O with an unknown accurate position exists in the second medium layer, the first sound velocity C1Second sound velocity C2And the position and the form of the actual interface are used as the prior knowledge condition for carrying out the detection and the positioning of the target O.
Fig. 1 is a schematic diagram of an acoustic propagation path according to an embodiment of the present invention. As shown in FIG. 1, the rectangular coordinate system includes an x-axis direction and a z-axis direction. The first surface of the layered medium is positioned on the x axis, and a target O exists in the second medium layer; first speed of sound C1Second sound velocity C2The position and shape of the actual interface, and the specific coordinates of the object O are unknown.
Fig. 2 is a flow chart of a method for detecting and positioning an object in a layered medium without prior knowledge according to an embodiment of the present invention. The specific coordinates of the object O in fig. 1 are detected and located by using the object detection and location method as shown in fig. 2. The method comprises the following specific steps:
step S100: n array elements are arranged on the first surface at equal intervals, wherein the first array element is positioned at the origin of coordinates of a rectangular coordinate system, and the rest n-1 array elements are arranged at equal intervals along the direction of an x axis; each array element is sequentially used as a transmitting array element to transmit acoustic pulses, n array elements are used as receiving array elements, and each receiving array element receives n acoustic pulses which reach receiving signals of the receiving array elements after being reflected by an actual interface and scattered by a target; determining a reverse sound beam corresponding to each received signal according to the n x n received signals; the receiving signals comprise reflection signals of the acoustic pulses passing through the actual interface and scattering signals of the target; the acoustic pulse is used as a forward acoustic beam, namely the number of the forward acoustic beam is determined by the number of the transmitting array elements, and the n transmitting array elements correspond to the n forward acoustic beams.
Specifically, the coordinates are (a)i0) as a transmitting array element T with the coordinate of (a)jAnd 0) as a reception array element R, and performs one-shot reception of an acoustic pulse. By array elementsT-transmit forward acoustic beam Fi(t) in the layered medium, the forward sound beam is received by the array element R after reflection of the actual interface and scattering of the target O. The received signals p (t) are mainly the reflected signals of the actual interface and the scattered signals of the target O.
Further, the received signal P (t) is time-reversed to obtain a time-reversed signal
The time reversal signal is sent out by the array element R after advancing a travel time difference, and the sent out signal is the reverse sound beam Rij(t) of (d). Wherein, one travel time difference is the second travel time from array element R to object O
First travel time with array element T to target O
Travel time difference of (2); the number of the reverse sound beams is determined by the number of the receiving array elements, when the n transmitting array elements transmit the sound pulse in sequence, each receiving array element receives n receiving signals, and the n receiving array elements correspond to n × n reverse sound beams.
Step S200: presetting first probing sound velocities of p first medium layers; and determining the position and the shape of the tentative interfaces at the p-th first tentative sound speed according to the n-x-n reflection signals.
Specifically, the coordinates are (a)i0) as a transmitting array element T with the coordinate of (a)jAnd 0) as a reception array element R, and performs one-shot reception of an acoustic pulse. After the acoustic pulse emitted by the reading array element T is reflected by the actual interface E, the array element R receives the reflected signal travel time of the reflected signal
Presetting the pth first probing sound velocity as c1pAnd a kirchhoff offset method is utilized, namely, an offset ellipse for single transmission and single reception of a transmitting and receiving pair consisting of an array element T and an array element R is obtained through calculation of a formula (1).
Wherein, the coordinate (x)s,zs) Is an offset ellipse, the coordinate (a) of the array element Ti0) and coordinates (a) of array element Rj0) is its setpoint, dijThe length is determined.
At a predetermined pth first tentative sound speed c1pWhen n array elements perform multiple transmission, n × n reflection signal travel times can be obtained according to n forward sound beams and n reflection signals corresponding to each forward sound beam. And then obtaining n x n offset ellipses according to the formula (1). n x n offset ellipses are interwoven with each other, and based on the theory that "different transmit-receive pairs of corresponding offset ellipses will gather to form some dense points" proposed in vol.65, p 1592 of Geophysics published in the journal of 2000, the most dense tangent points in n x n offset ellipses will form a leading edge. The tangent points forming the leading edge are fitted with a polynomial function (n x n tangent points are fitted with a polynomial expansion, the expansion coefficient is determined by least squares fitting) to form a curve which approaches the position and shape of the actual interface, i.e. the tentative interface.
Step S300: presetting second probing sound velocities of q second medium layers; determining possible points of n x (n-1) targets corresponding to n array elements under the p first tentative sound velocity and the q second tentative sound velocity according to the n forward sound beams and n reverse sound beams (n x n total reverse sound beams) corresponding to each forward sound beam; the dispersion of n x (n-1) possible points is calculated.
Specifically, the coordinates are (a)i0) as a transmitting array element T with the coordinate of (a)jAnd j th array element of 0) is used as a receiving array element R.
Reading the first travel time of the forward sound beam emitted by the array element T to reach the target O
Sound is explored at the first time based on Snell's lawSpeed c1pAnd a second probe speed of sound c2qAccording to the first travel time
Determining the coordinates (x) of a first refraction point located on a heuristic interfaceis,zis)。
The wavefront of the forward sound beam is calculated by formula (2).
Wherein, the coordinate (x)ipm,zipm) Is the wavefront of the forward acoustic beam.
A reverse sound beam determined according to the received signal of the array element R reaches a second travel time of the target O after being transmitted by the array element R
At a first tentative sound velocity c based on Snell's law1pAnd a second probe speed of sound c2qAccording to the second travel time
Determining the coordinates (x) of a second refraction point located on the test boundary surfacejs,zjs)。
The wave front of the reverse sound beam is obtained through calculation of formula (3).
Wherein, the coordinate (x)jpm,zjpm) Is the wave front of the reverse sound beam.
In fig. 3, the wavefront of the forward sound beam obtained by the above calculation is arc STThe wave front of the reverse sound beam is arc SR. Arc STAnd arc SRThe intersection point of (a) as a possible point of the target, and the coordinate is (x)pm,zpm)。
At the preset p-th firstHeuristic speed of sound c1pAnd a qth second probe speed of sound c2qThen, when n array elements are used for multiple transmission, n x (n-1) possible points (x) of the target can be obtained according to n forward sound beams and n reflected signals corresponding to each forward sound beampm,zpm). It should be noted that, for a self-generating and self-receiving array element, the forward sound beam and the reverse sound beam of the array element correspond to the same refraction point on the tentative interface. Therefore, the array elements of self-sending and self-receiving can not obtain the possible point (x) of the corresponding target through calculationpm,zpm). So that only n x (n-1) possible points (x) of the target are obtained when n array elements transmit and receive morepm,zpm)。
Calculating the p-th first trial sound velocity c through the formula (4)1pAnd a qth second probe speed of sound c2qNext, the spread of possible points for n x (n-1) targets.
Wherein v is the number of possible point pairs consisting of any two possible points in n x (n-1) possible points, v is n (n-1) (n (n-1) -1)/2, duIs the distance between two possible points in the u-th possible point pair, u ∈ v.
Step S400: determining the minimum dispersion in the dispersions corresponding to the q second trial sound speeds under the p first trial sound speed; wherein the minimum dispersion is the minimum of the q dispersions. Determining minimum dispersion in the minimum dispersions corresponding to the p first probing sound speeds; wherein the minimum dispersion is the minimum of the p minimum dispersions.
The first tentative sound velocity corresponding to the very small dispersion approaches the actual sound velocity of the first medium layer. The second tentative sound velocity corresponding to the very small dispersion approaches the actual sound velocity of the second medium layer. The position and shape of the tentative sound interface at the first tentative sound speed corresponding to the infinitesimal dispersion approach to the actual position and shape of the actual interface.
Step S500: and detecting and positioning the target in the layered medium by adopting a snapshot TR-RTM method according to the first probing sound speed and the second probing sound speed corresponding to the minimum dispersion and the position and the shape of the probing interface under the first probing sound speed corresponding to the minimum dispersion.
Specifically, for an arbitrary point X with coordinates (X, z) in the second medium layer, the convolution M of the forward sound beam and the backward sound beam0(t, X) is:
wherein t is a time variable, Fi(t, X) is a forward sound beam emitted from the ith array element to X point coordinates, RijAnd (t, X) is a reverse sound beam determined after the jth array element receives a forward sound beam emitted by the ith array element to the X point coordinate.
To reduce the computation time, the time-space domain is converted to the frequency-space domain, since the convolution of two functions in the time domain is equal to the product of the two function spectra in the frequency domain. Then when the time variable is the first trip
And the coherent and superposed sound field values of each array element are as follows:
where ω is the angular frequency, aijIs the interface reflection coefficient, bijF (ω) is the Fourier transform of the received signal P (t),
when the reflected signal of the ith array element reaches the jth array element after the acoustic pulse emitted by the ith array element is reflected by the actual interface E,
first travel time, t, for forward sound beam to reach target Oi(x) Is the travel time of the forward sound beam to reach any point X.
Second travel to target O at reverse speed of sound, tj(x) Travel time of the reverse sound velocity to any point X. The first term of equation (6) is the convolution of the forward beam with the target scattered portion of the backward beam. When the coordinates of the arbitrary point X and the target O are the same, the sound field value reaches a maximum value. Therefore, a peak-like sound field distribution is formed, and the position of the peak top is the position of the coordinate point of the target O. The second term of the formula (6) is the convolution of the forward sound beam and the reverse sound beam interface reflection part, the sound field distribution is the sound field distribution with smaller amplitude and non-uniformity, and the peak distribution of the target is not influenced, so that the method is utilized to invert the sound velocity of the upper medium and the lower medium of the layered medium and the position of the interface without prior knowledge, and the target in the layered medium can be accurately detected and positioned according to the parameters in the layered medium obtained by inversion.
Example 1
In the solid-liquid layered medium, the first medium layer is organic glass, and the second medium layer is water. A planar rectangular coordinate system was established for the solid-liquid layered medium, as shown in fig. 1. The rectangular coordinate system includes an x-axis direction and a z-axis direction. The first surface of the layered medium has coordinates (x,0) in a rectangular coordinate system. The solid-liquid layered medium is positioned in the quadrants corresponding to the positive direction of the x axis and the positive direction of the z axis. The longitudinal wave sound velocity of the first medium layer is 2696m/s, and the transverse wave sound velocity is 1285 m/s; the second medium layer is the sound velocity of the sound wave in water of 1500 m/s. The actual interface of the solid-liquid layered medium is a convex arc, the coordinates of the circle center are (14.4mm, 98.4mm), and the curvature radius is 70.0 mm. And placing a round steel bar with the radius of 1.5mm in the second medium layer as a target O in the solid-liquid layered medium, wherein the coordinate of a target O point is (18.0mm, 70.0 mm).
The first sound velocity, the second sound velocity, the actual interface configuration and the specific position of the target O of the solid-liquid layered medium are not known. By adopting the method for detecting and positioning the target in the layered medium without prior knowledge, provided by the embodiment of the invention, 20 array elements are placed at the position where z is 0 at equal intervals, the center of the first array element is positioned at the origin of coordinates (0, 0), the rest array elements are sequentially arranged at equal intervals along the direction of an x axis, and the center interval between the array elements is 2 mm.
The piezoelectric transducer with the frequency of 1MHz is adopted, 20 array elements are sequentially used as transmitting array elements to transmit acoustic pulse signals, and 20 array elements are used as receiving array elements to receive signals reflected by an actual interface and scattered by a target O, so that experimental data of the 20 array elements in a multi-transmitting and multi-receiving mode are obtained.
Because two waves, transverse wave and longitudinal wave, exist in the solid, four interface reflection signals (P-P wave, P-S wave, S-P wave and S-S wave respectively) and one target scattering longitudinal wave can be received for the solid-liquid layered medium.
Taking the example of the 5 th array element emitting acoustic pulses, and 20 array elements as receiving array elements, the signals received by the 1 st to 20 th array elements are as shown in fig. 4. As can be seen from the signals shown in fig. 4, the amplitude of the interface reflection signal of the longitudinal wave (P-P wave) and the target scattering signal is large, so that the longitudinal wave signal is selected as the research object, which corresponds to the experimental premise of the present application.
Presetting a first tentative sound velocity c1p2696m/s, and 3 different second probing sound velocities c2qAt 1000m/s, 1500m/s, and 2000m/s, a discrete point profile of different possible points of the target may be obtained, as shown in FIG. 5. In the figure c2qThe target discrete point distribution degree is obviously less than c when the target discrete point distribution degree is 1500m/s 2q1000m/s and c 2q2000 m/s.
Presetting more second trial sound speeds to obtain the first trial sound speed c1pMinimum dispersion D of 2696m/smin(c1p，c2q) Corresponding second tentative speed of sound c2q1527m/s, as shown in FIG. 6.
Presetting a plurality of different first tentative sound velocities c1pRepeatedly trying a plurality of second trial sound velocities c2qThe corresponding minimum dispersion results in a dispersion curve formed by a plurality of minimum dispersions, as shown in fig. 7. Obtaining minimum target dispersion D from the minimum target dispersionsmoremin(c1p，c2q) The minimum target dispersion DmoreminCorresponding scoreFirst probe speed of sound c in layer medium1p2670m/s, second tentative speed of sound c2q1521m/s, which respectively approaches to a first sound velocity (longitudinal wave sound velocity of the first medium layer) 2696m/s and a second sound velocity (sound velocity in water) 1500m/s of the solid-liquid layered medium; based on the first probe speed of sound of 2670m/s, the topography of the probe interface is obtained, which approximates the topography of the actual interface, as shown in fig. 8. The first tentative sound velocity, the second tentative sound velocity, and the location form of the tentative interface are taken into the TR-RTM method of the snapshot, and a sound field distribution diagram is obtained as shown in fig. 9, in which the position coordinates of the peak top are (17.2mm, 69.5mm), the position approaches the accurate position of the target O (18.0mm, 70.0mm), and the signal-to-interference ratio is 2.91.
The result shows that the relative error between the sound velocity of the medium on the upper layer and the lower layer obtained by inversion and the actual value is not more than 1%, the error between the target positioning result obtained by using TR-RTM processing of the snapshot and the target actual position is small, and the signal-to-interference ratio is improved to be more than 2 times. The sound velocities of the upper medium and the lower medium of the solid-liquid layered medium and the position and the shape of the interface are inverted without prior knowledge, and the target in the layered medium can be accurately detected and positioned according to the parameters in the layered medium obtained by inversion.
Example 2
In the liquid-solid layered medium, the first medium layer is water, and the second medium layer is organic glass. A planar rectangular coordinate system is established for the liquid-solid layered medium, as shown in fig. 1. The rectangular coordinate system includes an x-axis direction and a z-axis direction. The first surface of the layered medium has coordinates (x,0) in a rectangular coordinate system. The liquid-solid layered medium is positioned in the quadrants corresponding to the positive direction of the x axis and the positive direction of the z axis. The first medium layer is the sound velocity of sound wave in water of 1500 m/s; the longitudinal wave sound velocity of the second medium layer is 2696m/s, and the transverse wave sound velocity is 1285 m/s. The actual interface of the liquid-solid layered medium is a concave arc, the center coordinates of the arc are (23.4mm, -15.4mm), and the curvature radius is 70.1 mm. Cylindrical holes with the radius of 1.5mm are arranged in the second medium layer to serve as target O in the liquid-solid layered medium, and the coordinate of a target O point is (18.0nn, 70.0 mm). It should be noted that the target O, i.e. the cylindrical hole, in the second dielectric layer is obtained by drilling, parallel to the first surface.
The first sound velocity, the second sound velocity, the actual interface configuration and the specific position of the target O of the liquid-solid layered medium are not known. By adopting the method for detecting and positioning the target in the layered medium without prior knowledge, provided by the embodiment of the invention, 20 array elements are placed at the position where z is 0 at equal intervals, the center of the first array element is positioned at the origin of coordinates (0, 0), the rest array elements are sequentially arranged at equal intervals along the direction of an x axis, and the center interval between the array elements is 2 mm.
The piezoelectric transducer with the frequency of 1MHz is adopted, 20 array elements are sequentially used as transmitting array elements to transmit acoustic pulse signals, and 20 array elements are used as receiving array elements to receive signals reflected by an actual interface and scattered by a target O, so that experimental data of the 20 array elements in a multi-transmitting and multi-receiving mode are obtained.
Taking the example of the 5 th array element emitting the acoustic pulse, and 20 array elements as the receiving array elements, the 1 st to 20 th array elements receive the signal, and the waveform diagram of the single-transmission multi-reception signal is obtained, as shown in fig. 10. As can be seen from fig. 10, a real interface reflection signal and a target scatter signal are received. Since only longitudinal waves are present in the liquid, only one kind of interface reflection signal (P-P wave) is received.
Presetting a plurality of different first tentative sound velocities c1pRepeatedly trying a plurality of second trial sound velocities c2qThe corresponding minimum dispersion results in a dispersion curve formed by a plurality of minimum dispersions, as shown in fig. 11. From these minimum target dispersions, the minimum target dispersion is obtained, which is the minimum target dispersion DmoreminFirst probe speed of sound c in a corresponding layered medium1p1549m/s, second tentative speed of sound c2q2667m/s, a first sound velocity (sound velocity in water) 1500m/s and a second sound velocity (longitudinal sound velocity of the first medium layer) 2696m/s respectively approaching the liquid-solid layered medium; based on the first tentative sound speed of 1549m/s, the topography of the tentative interface is obtained, which approximates the topography of the actual interface, as shown in fig. 12. The first sound velocity, the second sound velocity and the shape of the probe interface are brought into the TR-RTM method of the snapshot to obtain the sound field distribution diagram as shown in FIG. 13, and the position of the peak top isThe set point is (17.4mm, 70.8mm), the exact position (18.0mm, 70.0mm) approaching the target O, and the signal-to-interference ratio is 5.97.
The result shows that the relative error between the sound velocity of the medium on the upper layer and the lower layer obtained by inversion and the actual value is not more than 3%, the error between the target positioning result obtained by using TR-RTM processing of the snapshot and the target actual position is small, and the signal-to-interference ratio is improved to be more than 2 times. The method realizes inversion of sound velocities of upper and lower-layer media and the position and shape of an interface of the liquid-solid layered medium without prior knowledge, and can realize more accurate detection and positioning of a target in the layered medium according to parameters in the layered medium obtained by inversion.
Therefore, the method realizes the detection and the positioning of the targets in the layered media with different interfaces in different shapes without prior knowledge, and inverts the sound velocity of the upper and lower media and the position and the shape of the interface.
It should be appreciated by those skilled in the art that, in the above example, the present invention provides a method for inverting the sound velocities of the upper and lower layers of the layered medium and the configuration of the interface without prior knowledge, and can achieve more accurate detection and positioning of the target in the layered medium according to the parameters in the layered medium obtained by the inversion.
The above-mentioned embodiments, objects, technical solutions and advantages of the present invention are further described in detail, it should be understood that the above-mentioned embodiments are only exemplary embodiments of the present invention, and are not intended to limit the scope of the present invention, and any modifications, equivalent substitutions, improvements and the like made on the basis of the technical solutions of the present invention should be included in the scope of the present invention.
Claims (9)
1. A method for detecting and positioning targets in a layered medium without prior knowledge, wherein the layered medium comprises a first medium layer and a second medium layer; the first medium layer comprises a first surface and an actual interface connected with the second medium layer, and the target is positioned in the second medium layer; characterized in that the method comprises the following steps:
step S100: arranging n array elements on the first surface; each array element is sequentially used as a transmitting array element to transmit acoustic pulses, the n array elements are used as receiving array elements, and each receiving array element receives n receiving signals which reach the receiving array elements after the acoustic pulses are respectively reflected by a practical interface and scattered by a target; determining a reverse sound beam corresponding to each received signal according to the n x n received signals; wherein the received signals comprise reflected signals of the acoustic pulses passing through the actual interface and scattered signals of the target; the acoustic pulse is taken as a forward acoustic beam;
step S200: presetting first probing sound speeds of p first medium layers; determining the position and the shape of a tentative interface at the p-th first tentative sound speed according to the n-x-n reflection signals;
step S300: presetting q second probing sound velocities of the second medium layers; determining n (n-1) possible points of the target corresponding to the n array elements under the p first tentative sound speed and the q second tentative sound speed according to the n forward sound beams and the n reverse sound beams; calculating the dispersion of n x (n-1) possible points;
step S400: determining a minimum dispersion of the dispersions corresponding to the q second tentative sound speeds at the p-th first tentative sound speed; wherein the minimum dispersion is a minimum of the q dispersions; determining a minimum dispersion of the minimum dispersions for the p first probing speeds of sound; wherein the minimum dispersion is the minimum of the p minimum dispersions;
step S500: and detecting and positioning the target in the layered medium by adopting a snapshot TR-RTM method according to the first tentative sound speed and the second tentative sound speed corresponding to the minimum dispersion and the position and the shape of the tentative interface under the first tentative sound speed corresponding to the minimum dispersion.
2. The method of claim 1, wherein determining the inverse sound beam corresponding to the received signal according to the received signal comprises:
carrying out time reversal on the received signal to obtain a time reversal signal; taking the time reversal signal as the reverse sound beam after advancing by a travel time difference; wherein the travel time difference is a difference between a second travel time of the receiving array element to the destination and a first travel time of the transmitting array element to the destination.
3. The method of claim 1, wherein determining the location and shape of the heuristic interface at the p-th first heuristic sound speed comprises:
determining n x n reflected signal travel times according to n forward sound beams and n reflected signals corresponding to each forward sound beam, wherein n x n reflected signals are total;
under the first tentative sound velocity, calculating n x n offset ellipses when traveling according to the n x n reflection signals, wherein partial tangent points of the n x n offset ellipses form a leading edge; fitting the tangent points that make up the leading edge to the tentative boundary surface that approaches the location and shape of the actual boundary surface.
4. The method of claim 1, wherein said determining possible points of said target at said first and second tentative sound speeds comprises:
determining a first refraction point located on the tentative interface according to the read first travel of the forward sound beam to the target based on Snell's law at the first tentative sound speed and the second tentative sound speed;
calculating the wavefront of the forward sound beam according to the first tentative sound velocity, the second tentative sound velocity, the first travel time and the first refraction point;
under the first and second probing sound speeds, based on Snell's law, determining a second refraction point on the probing interface according to a second travel of the reverse sound beam emitted by the receiving array element to the target;
calculating the wave front of the reverse sound beam according to the first trial sound velocity, the second travel time and the second refraction point;
and determining possible points of the target according to the wave front of the forward sound beam and the wave front of the reverse sound beam.
5. The method of claim 1,
the first tentative sound velocity corresponding to the minimum dispersion approaches to the actual sound velocity of the first medium layer;
the second tentative sound velocity corresponding to the minimum dispersion approaches the actual sound velocity of the second medium layer;
the position and the shape of the tentative boundary surface at the first tentative sound speed corresponding to the minimum dispersion approach to the position and the shape of the actual boundary surface.
6. The method of claim 3, further comprising establishing a rectangular coordinate system, the rectangular coordinate system comprising an x-axis direction and a z-axis direction; the coordinate of the first surface in the rectangular coordinate system is (x, 0); the n array elements are arranged on the first surface at equal intervals;
obtaining the offset ellipse by using a kirchhoff offset method, wherein the offset ellipse is a set of detection points S, and coordinates (x) of the detection points Ss,zs) Calculated by the following formula:
wherein (a)i0) is the ith coordinate of the transmitting array element on the x axis, (a)j0) is the coordinates of the jth receiving array element on the x axis, c1pFor the pth of the first trial sound speeds,
the acoustic pulse transmitted for the ith transmitting array element is transmitted through theAnd after the reflection of the actual interface E, the jth receiving array element receives the travel time of the reflected signal.
7. The method of claim 4, further comprising establishing a rectangular coordinate system, the rectangular coordinate system comprising an x-axis direction and a z-axis direction; the coordinate of the first surface in the rectangular coordinate system is (x, 0); the n array elements are arranged on the first surface at equal intervals;
the wave front of the forward sound beam is a set of possible points of the target corresponding to the transmitting array element, and the coordinates (x) of the possible points of the target corresponding to the transmitting array elementipm,zipm) Calculated by the following formula:
wherein (a)i0) is the ith coordinate of the transmitting array element on the x axis, (x)is,zis) A first refraction point coordinate corresponding to the ith transmission array element, c1pFor the pth of said first tentative sound speed, c2qFor the qth of the second trial sound speed,
corresponding to the first travel time of the forward sound beam to the target O for the ith transmitting array element;
the wave front of the reverse beam is a set of possible points of the target corresponding to the receiving array elements, the coordinates (x) of the possible points of the target corresponding to the receiving array elementsjpm,zjpm) Calculated by the following formula:
wherein (a)j0) is the coordinates of the jth receiving array element on the x axis, (x)js,zjs) Is the jthA second refraction point coordinate corresponding to the receiving array element, c1pFor the pth of said first tentative sound speed, c2qFor the qth of the second trial sound speed,
and the jth receiving array element corresponds to the second travel time of the reverse sound beam to reach the target O.
8. The method of claim 7, wherein determining the probable point of the target based on the wavefront of the forward acoustic beam and the wavefront of the backward acoustic beam comprises:
the wave front, i.e. the coordinate (x), of the forward sound beamipm,zipm) And the wave front of said reverse sound beam, i.e. the coordinate (x)jpm,zjpm) Coordinates (x) of the intersection of the sets of (2)pm,zpm) As possible points of the object.
9. The method of claim 1, further comprising establishing a rectangular coordinate system, the rectangular coordinate system comprising an x-axis direction and a z-axis direction; the coordinate of the first surface in the rectangular coordinate system is (x, 0); the n array elements are arranged on the first surface at equal intervals;
calculating the dispersion of n x (n-1) of said possible points by:
wherein v is the number of possible point pairs consisting of any two of the possible points in n x (n-1), v is n (n-1) (n (n-1) -1)/2, duIs the distance between two of the possible points in the u-th possible point pair, u e v.
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