CN105259557A - Multi-frequency emission beam formation method and application - Google Patents

Abstract

The invention discloses a multi-frequency emission beam formation method. The method includes: (1) the beam direction of an emission array of a crossed array is divided into a plurality of sectors, a series of sector sonar beam signals of different frequencies are emitted in sequence in each sector, and the sector sonar beam signals with each frequency point to a beam direction in the corresponding sector; and (2) after the sector sonar beam signals with all the frequencies in each sector are emitted, sonar echo signals are received by employing a reception array in the crossed array, frequency information corresponding to all the sector sonar beam signals in each sector is extracted via discrete Fourier transform, beam formation calculation is performed in frequency domains corresponding to the frequency information, and a beam intensity result is obtained. According to the method, the emission time of the crossed array can be reduced, the beam performance similar to a two-dimensional plane reception array is obtained, and the complexity of an underwater real-time three-dimensional acoustic imaging system is reduced. The invention also discloses the application of the method in the reconstruction of three-dimensional acoustic images.

Description

A kind of multi-frequency launching beam formation method and application

Technical field

The present invention relates to three-dimensional acoustics imaging field, particularly relate to a kind of multi-frequency launching beam formation method and application.

Background technology

Real-time three-dimensional acoustic imaging techniques is a kind of underwater Detection Techniques being subject to extensive concern in recent years under water, and the various fields such as its engineering construction at sea, sea floor surreying, underwater facility inspection, under water archaeology, great oceanographic engineering and military engineering protection is with a wide range of applications.

Three kinds of main at present three-dimensional acoustics formation methods are: focusing acoustic imaging/sound lens focusing imaging, Wave beam forming acoustics imaging and holographic acoustics imaging.Wherein, digital beam forming technology has the advantages such as flexible, efficient, therefore obtains applying more and more widely in acoustic imaging system.

Under water in real-time three-dimensional acoustic imaging system, for obtaining the three-dimensional information of scene, usually needing a two dimensional surface transducer array for receiving sonar echo signal, and carrying out Wave beam forming calculating based on this signal, building three-dimensional acoustics image.And two dimensional surface receives the use of battle array, usually along with huge array element quantity, thus result in difficult problems for following two restriction underwater 3 D acoustic imaging techniques development: 1) hardware system is complicated, and the huge array element quantity of two dimensional surface transducer array causes the hardware circuits such as relative signal filtering, amplification, sampling and digital signal processing huge; 2) calculated amount is huge, and the beamforming algorithm computational requirements simultaneously forming ten hundreds of beam intensity signals is too high.

In order to overcome an above-mentioned difficult problem, a part of scholar adopts the mode of thinned array and beamforming algorithm optimization to reduce system hardware complexity and algorithm calculated amount.And the Wave beam forming effect that another part scholar utilizes transmitting and receiving process common, replace plane to receive battle array with multiple linear array, thus reach reduction array dimension, reduce the object of array element quantity.Wherein most typical application and cross array (Mill ' scross).But cross array is when carrying out the structure of three-dimensional acoustics image, and need to scan successively observation scene according to vertical beam direction, this causes its imaging time long, and image update frame per second is too low.Therefore, traditional cross array is only applicable to some near fields or the scene without real-time demand.

In the tradition application of cross array, in order to build the three-dimensional acoustics image of whole observation scene, emission array needs to launch fan-shaped beam signal to all vertical beam directions, and each transmitting all needs to wait for that sonar echo returns from BURN-THROUGH RANGE.Therefore, cross array needs considerable time to scan whole observation scene, and it generates a hardwood three-dimensional acoustics image required time and is proportional to emitting times (vertical beam direction number Q) and BURN-THROUGH RANGE (Rmax).

Summary of the invention

The invention provides a kind of multi-frequency launching beam formation method, be applied to cross array beams and formed.The method for underwater 3 D acoustic imaging system hardware spending and calculated amount huge, and the not enough difficult problem of the real-time of cross array, to reduce the emitting times of cross array for starting point, effectively the emitting times of cross array is reduced to the covering of the fan number of division from vertical beam direction number, significantly shorten the time needed for structure one frame three-dimensional acoustics image, to some extent solve the problem of cross array real-time deficiency, and the wave beam performance index (main lobe width close with two dimensional surface receiving array DM algorithm can be obtained, side lobe peak), significantly reduce computational requirements simultaneously.

Concrete technical scheme of the present invention is as follows:

A kind of multi-frequency launching beam formation method, comprises step:

(1) beam direction of emission array in cross array is divided into multiple covering of the fan, in each covering of the fan, launch the fan-shaped sonar wave beams signal of a series of different frequency successively, the fan-shaped sonar wave beams signal of each frequency points to a beam direction in corresponding covering of the fan;

(2) after in each covering of the fan, the fan-shaped sonar wave beams signal transmitting of all frequencies terminates, the receiving array in cross array is utilized to receive sonar echo signal, the frequency information that in each covering of the fan, all fan-shaped sonar wave beams signals are corresponding is extracted by discrete Fourier transformation, and Wave beam forming calculating is carried out in the frequency domain that frequency information is corresponding, obtain the beam intensity result of respective numbers.

In the present invention, the receiving array of the horizontal direction of cross array comprises M array element, and the emission array of vertical direction comprises N number of array element, and the array element distance receiving battle array and transmitting battle array is respectively dr and dt.Vertical Launch beam direction number is Q, default vertical beam direction is divided into K covering of the fan by multi-frequency launching beam formation method, in each covering of the fan, the fan-shaped sonar wave beams signal of different frequency is launched successively, the corresponding vertical beam direction of signal of each frequency to J the vertical beam direction (Q=K × J) of presetting.Firing Sonar beam signal frequency adopts and increases progressively form, and increasing frequency is Δ f.In receiving course, after the sonar wave beams signal transmitting of launching all frequencies in covering of the fan terminates, receiving array receives sonar echo signal, by discrete Fourier transformation (discretefouriertransform, DFT) computing, extract the frequency information that in echo, J Firing Sonar beam signal is corresponding simultaneously, on J frequency domain, carry out Wave beam forming calculating concurrently, generate P × J beam intensity result.After completing the transmitting and receiving process of a covering of the fan, similar process is carried out to all the other covering of the fans.After all covering of the fans complete aforesaid operations, then can obtain the beam intensity result in complete P × Q direction.

Preferably, in each covering of the fan, be the fan-shaped sonar wave beams signal of τ successively to different beam direction fire pulse width, the time-domain expression launching fan-shaped sonar wave beams signal is:

$\begin{array}{c}{s}_{nMFT}\left(t\right)=s_n(t,f_1,{\mathrm{\β}}_{q(k_S,1)})+s_n\left(\right(t-\mathrm{\τ}),f_2,{\mathrm{\β}}_{q(k_S,2)})\\ +\mathrm{...}+s_n\left(\right(t-J\mathrm{\τ}),f_J,{\mathrm{\β}}_{q(k_S,J)})\end{array}$

Wherein,

$s_n(t,f_j,{\mathrm{\β}}_q)=\left\{\begin{array}{cc}Asin(2{\mathrm{\πf}}_{j}t+{\mathrm{\θ}}_n(f_j,{\mathrm{\β}}_q));& 0\≤t\≤\mathrm{\τ}\\ 0;& otherwise\end{array}\right.$

θ _n(f _j,β _q)＝2πf _j·(n-1)dt·sinβ _q/c

In formula: s _nMFTt () is time-domain function, A is signal amplitude, θ _n(f _j, β _q) be phase offset parameter, dt is for launching array element distance, f ₁, f ₂... f _jfor the frequency of each fan-shaped sonar wave beams signal, β _qfor vertical beam deflection, vertical beam call number q is about division covering of the fan number k _swith the function of frequency indices j, k _s=1,2 ..., K, j=1,2 ..., J, n are for launching array element call number, and J is the number of beam direction, and c is sonar velocity of propagation under water.

Preferably, number frequency of each fan-shaped sonar wave beams signal increases progressively according to the step frequency of Δ f, i.e. f _j+1-f _j=Δ f, for ensureing can to extract complete frequency information in Wave beam forming calculates, should meet 1/ Δ f≤τ between step frequency Δ f and pulse width τ.

Preferably, the beam pattern of emission array is expressed as:

$\left|B_{TrMFT}(f_j,{\mathrm{\β}}_q)\right|=|\underset{n=1}{\overset{N}{\Σ}}{S}_n\left(k_j\right)\·\exp (-{\mathrm{j\θ}}_n(f_j,{\mathrm{\β}}_q)\left)\right|$

Wherein:

$S_n\left(k_j\right)=\underset{l=1}{\overset{L}{\Σ}}{s}_n(l,f_j,{\mathrm{\β}}_q)\·\exp (-j\frac{2\mathrm{\π}}{L}l\·k_j)$

$k_j=\frac{f_j}{f_s}L$

In formula: N is the element number of array of emission array, L is that DFT computing is counted, and l is its call number, f _sfor sample frequency, k _jfor frequency f _jline spectrum number.

Preferably, the beam pattern expression formula of receiving array is:

$\left|B_{\mathrm{Re}MFT}(f_j,{\mathrm{\α}}_p)\right|=|\underset{m=1}{\overset{M}{\Σ}}{S}_m\left(k_j\right)\·\exp (-{\mathrm{j\θ}}_m(f_j,{\mathrm{\α}}_p)\left)\right|$

Wherein:

$S_m\left(k_j\right)=\underset{l=1}{\overset{L}{\Σ}}{s}_m\left(l\right)\·\exp (-j\frac{2\mathrm{\π}}{L}l\·k_j)$

${\mathrm{\θ}}_m(f_j,{\mathrm{\α}}_p)=2{\mathrm{\πf}}_j\·(m-\frac{M+1}{2})dr\·{\mathrm{sin\α}}_p/c$

In formula: M is the element number of array of receiving array, k _jfor frequency f _jline spectrum number, S _m(k _j) be echo signal sample data s _mleaf transformation in the L point discrete Fourier of (l), θ _m(f _j, α _p) be the phase offset parameter of receiving array, dr is for receiving array element distance, α _pfor horizontal beam deflection.

Present invention also offers a kind of method for reconstructing of three-dimensional acoustics image, utilize above-mentioned multi-frequency launching beam formation method to obtain beam intensity result (x, y, z, i), wherein i is beam intensity value, (x, y, z) be three dimensional space coordinate corresponding to beam intensity result, adopt distance value z ' to compensate transmitting time delay, and adopt the three dimensional space coordinate (x after compensating, y, z ') rebuild, obtain correct real-time three-dimensional acoustic picture;

The distance value z ' of described compensation is:

$z^{\′}=z-\frac{{\mathrm{c\Γ}}_j}{2}=z-\frac{c\mathrm{\τ}\[q-(k_S-1)J-1\]}{2}$

In formula, Γ _jfor the time delay value of each frequency signal, τ is pulse width, and q is beam direction call number, k _sfor dividing covering of the fan sequence number, j is signal frequency index.

The multi-frequency launching beam formation method being applied to cross array that the present invention proposes, significantly shorten it and build time needed for a frame three-dimensional acoustics image, improve the real-time of cross array, with the array element quantity of (M+N), obtain the wave beam performance similar to two dimensional surface receiving array (M × N), effectively reduce the hardware complexity of real-time three-dimensional acoustic imaging system.In addition, compared with existing two dimensional surface receiving array beamforming algorithm, computational requirements reduces by 3 orders of magnitude.

Accompanying drawing explanation

Fig. 1 is the cross array junctions composition of the present invention;

Fig. 2 is the cross array beams energy profile of the present invention;

Fig. 3 is that multi-frequency launching beam of the present invention forms method flow schematic diagram;

Fig. 4 is transponder pulse signal time-domain diagram of the present invention;

Fig. 5 is the present invention's cross array beams deflection definition figure.

Embodiment

In order to describe the present invention in more detail, below in conjunction with the drawings and specific embodiments, method of the present invention is described in detail.

As shown in Figure 1, cross array is made up of two orthogonal linear arraies.Wherein, the receiving array of horizontal direction comprises M array element, and the emission array of vertical direction comprises N number of array element, and the array element distance receiving battle array and transmitting battle array is respectively dr and dt.Cross array is in xOy plane, using receiving array and emission array intersection as true origin.

Cross array has the three-dimensional acoustics imaging capability identical with two-dimensional planar array, its main cause is that cross array utilizes transmitting and receiving array to carry out Wave beam forming in the vertical and horizontal direction respectively, by its acting in conjunction, realize the structure of three-dimensional acoustics image.Under identical sonar signal frequency and array aperture condition, compared to two-dimensional planar array (M × N number of array element), cross array can obtain the angular resolution identical with it with M+N array element, therefore quantitatively has very large advantage in array element.Its beam energy distribution plan as shown in Figure 2.

Suppose that Vertical Launch beam direction number is Q, horizontal received beam direction number is P, the idiographic flow of multi-frequency launching beam formation method is: first, default vertical beam direction is divided into K covering of the fan, in each covering of the fan, transmitting transducer arrays, by the phase shift compensation between each array element, launches the fan-shaped sonar wave beams signal of different frequency successively to J the vertical beam direction (Q=K × J) of presetting, the corresponding vertical beam direction of signal of each frequency; Then, after the sonar wave beams signal transmitting of frequencies all in this covering of the fan terminates, receiving array receives sonar echo signal, by DFT computing, extract the frequency information that in echo, J Firing Sonar beam signal is corresponding simultaneously, on J frequency domain, carry out Wave beam forming calculating concurrently, generate P × J beam intensity result; Afterwards, similar process is carried out to all the other covering of the fans.After all covering of the fans complete aforesaid operations, then can obtain the beam intensity result in complete P × Q direction.The emission process schematic diagram of multi-frequency launching beam formation method as shown in Figure 3.

In a covering of the fan, each transmission frequency f _jthe vertical beam direction that correspondence one is default, emission array is in order successively to the fan-shaped sonar wave beams signal that J direction fire pulse width is τ, then the time-domain diagram of transponder pulse signal as shown in Figure 4.

It can thus be appreciated that the time-domain expression of Firing Sonar beam signal is as follows:

$\begin{array}{c}{s}_{nMFT}\left(t\right)=s_n(t,f_1,{\mathrm{\β}}_{q(k_S,1)})+s_n\left(\right(t-\mathrm{\τ}),f_2,{\mathrm{\β}}_{q(k_S,2)})\\ +\mathrm{...}+s_n\left(\right(t-J\mathrm{\τ}),f_J,{\mathrm{\β}}_{q(k_S,J)})\end{array}$

Wherein:

$s_n(t,f_j,{\mathrm{\β}}_q)=\left\{\begin{array}{cc}Asin(2{\mathrm{\πf}}_{j}t+{\mathrm{\θ}}_n(f_j,{\mathrm{\β}}_q));& 0\≤t\≤\mathrm{\τ}\\ 0;& otherwise\end{array}\right.$

In formula, A is signal amplitude, θ _n(f _j, β _q) be phase offset parameter, can be expressed as:

θ _n(f _j,β _q)＝2πf _j·(n-1)dt·sinβ _q/c

Vertical Launch beam direction index q is about division covering of the fan sequence number k _swith the function of signal frequency index j, between three, relation is as follows:

q(k _S,j)＝(k _S-1)J+j

In addition, adjacent transmission signal frequency increases progressively according to the step frequency of Δ f, that is:

f _j+1-f _j＝Δf

For ensureing that DFT computing can extract complete frequency information in received beam is formed, and should meet following constraint between step frequency Δ f and pulse width τ:

1/Δf≤τ

From above-mentioned expression formula, in multi-frequency launching beam formation method, Firing Sonar signal and parameter are all about frequency f _jfunction.Therefore, the beam pattern of emission array can be expressed as:

$\left|B_{TrMFT}(f_j,{\mathrm{\β}}_q)\right|=|\underset{n=1}{\overset{N}{\Σ}}{S}_n\left(k_j\right)\·\exp (-{\mathrm{j\θ}}_n(f_j,{\mathrm{\β}}_q)\left)\right|$

In formula, S _n(k _j) be sonar wave beams signal sampling result s _n(l, f _j, β _q) L point DFT convert, its expression formula is as follows:

$S_n\left(k_j\right)=\underset{l=1}{\overset{L}{\Σ}}{s}_n(l,f_j,{\mathrm{\β}}_q)\·\exp (-j\frac{2\mathrm{\π}}{L}l\·k_j)$

Wherein, k _jfor frequency f _jline spectrum number, if sample frequency is f _s, then k _jcan be expressed from the next:

$k_j=\frac{f_j}{f_s}L$

After the sonar wave beams signal transmitting of frequencies all in this covering of the fan terminates, receiving array receives sonar echo signal, by DFT computing, extract the frequency information that in echo, J Firing Sonar beam signal is corresponding simultaneously, Wave beam forming calculating is carried out concurrently on J frequency domain, generate P × J beam intensity result, its beam pattern expression formula is as follows:

$\left|B_{\mathrm{Re}MFT}(f_j,{\mathrm{\α}}_p)\right|=|\underset{m=1}{\overset{M}{\Σ}}{S}_m\left(k_j\right)\·\exp (-{\mathrm{j\θ}}_m(f_j,{\mathrm{\α}}_p)\left)\right|$

In formula, S _m(k _j) be echo signal sample data s _ml the L point DFT of () converts, θ _m(f _j, α _p) be the phase offset parameter of receiving array, can be expressed as follows:

$S_m\left(k_j\right)=\underset{l=1}{\overset{L}{\Σ}}{s}_m\left(l\right)\·\exp (-j\frac{2\mathrm{\π}}{L}l\·k_j)$

${\mathrm{\θ}}_m(f_j,{\mathrm{\α}}_p)=2{\mathrm{\πf}}_j\·(m-\frac{M+1}{2})dr\·{\mathrm{sin\α}}_p/c$

From above-mentioned transmitting and receiving beam forming process, be different from two dimensional surface receiving array, the beam direction angle (α of cross array _p, β _q) independently define based on transmitting and receiving linear array, as shown in Figure 5.

Therefore, adopt the cross array of multi-frequency launching beam formation method about beam direction angle (α _p, β _q) overall beam pattern as follows:

|B(f _j,α _p,β _q)|＝|B _ReMFT(f _j,α _p)|·|B _TrMFT(f _j,β _q)|

Wherein, f _jwith β _qmeet Vertical Launch beam direction index q, divide covering of the fan sequence number k _sand the relation between signal frequency index j three.

Multi-frequency launching beam formation method is divided and the sequential firing of multifrequency signal by covering of the fan, and the parallel processing to echoed signal, effectively the emitting times of cross array be reduced to from vertical beam direction number Q and divide covering of the fan number K, the time needed by structure one hardwood three-dimensional acoustics image is from (2R _max× Q/c) be reduced to (2R _max× K/c), significantly shorten the time of cross array scanning observation scene, make it can carry out real-time three-dimensional acoustics imaging within the specific limits.

In addition, in multi-frequency launching beam formation method, because when launching, the signal of different frequency is launched in order successively, therefore there is when propagating the time delay that length is pulse width τ in the sonar wave beams signal of side frequency.For eliminating the time delay of Firing Sonar beam signal, usually needing to compensate it in Wave beam forming calculates, but performance cost and the storage space of system can be increased like this.Therefore, the present invention proposes a kind of post processing mode, carries out delay compensation to Firing Sonar beam signal.

For building three-dimensional acoustics image, need the three dimensional space coordinate (x, y, z) knowing that beam intensity result is corresponding.Wherein, (x, y) and beam direction angle (α _p, β _q) there is corresponding relation, z represents range information, and the relation of itself and the sonar signal travel-time t in water is as follows:

$z=\frac{ct}{2}$

The time delay of signal be reacted to spatially then representation signal at the range difference of water transmission.Therefore, time delay can convert range information to, compensates in the z value of correspondence.If transmit as reference with first frequency in each covering of the fan, then the time delay value Γ of each frequency signal _jcan be expressed as:

Γ _j＝(j-1)τ

According to the coordinate y of beam intensity result, corresponding beam direction call number q can be obtained, by Vertical Launch beam direction index q, divide covering of the fan sequence number k _swith relation between signal frequency index j three is known, through the new distance value z ' of over-compensation is:

$z^{\′}=z-\frac{{\mathrm{c\Γ}}_j}{2}=z-\frac{c\mathrm{\τ}\[q-(k_S-1)J-1\]}{2}$

As the above analysis, when master control PC receives the beam intensity result (x of underwater signal processing system-computed, y, z, i) afterwards (wherein i is beam intensity value), the distance value z ' that can convert new according to coordinate information wherein, transmitting time delay is compensated, subsequently according to new three dimensional space coordinate (x, y, z ') rebuild, correct real-time three-dimensional acoustic picture can be obtained.Adopt above-mentioned post processing mode to carry out delay compensation at master control PC end, not only can obtain target three-dimensional coordinate information accurately, also can not increase performance cost and the storage space of system.If do not carry out delay compensation, in final three-dimensional acoustics image, straight target will be shown as the tilting body being divided into K section (dividing covering of the fan number).

The present invention is directed to underwater 3 D acoustic imaging system hardware spending and calculated amount huge, and the not enough difficult problem of the real-time of cross array, to reduce the emitting times of cross array for starting point, the emitting times of cross array is reduced to the covering of the fan number of division from vertical beam direction number, significantly shorten the time needed for structure one frame three-dimensional acoustics image, improve the real-time of cross array.Employing array element quantity is the cross array of (M+N), obtaining with array element quantity is the wave beam performance index (main lobe width, side lobe peak) that the two dimensional surface receiving array of (M × N) is close, significantly reduce the hardware complexity of three-dimensional acoustics imaging system, and the computational requirements of beamforming algorithm is reduced 3 orders of magnitude, solve two difficult problems of restriction real-time three-dimensional acoustic imaging system development.In addition, the invention allows for a kind of three-dimensional acoustics image rebuilding method based on delay compensation, avoid the performance cost of system and the increase of storage space.

Claims (6)

1. a multi-frequency launching beam formation method, is characterized in that, comprise step:

(1) beam direction of emission array in cross array is divided into multiple covering of the fan, in each covering of the fan, launch the fan-shaped sonar wave beams signal of a series of different frequency successively, the fan-shaped sonar wave beams signal of each frequency points to a beam direction in corresponding covering of the fan;

(2) after in each covering of the fan, the fan-shaped sonar wave beams signal transmitting of all frequencies terminates, the receiving array in cross array is utilized to receive sonar echo signal, the frequency information that in each covering of the fan, all fan-shaped sonar wave beams signals are corresponding is extracted by discrete Fourier transformation, and Wave beam forming calculating is carried out in the frequency domain that frequency information is corresponding, obtain beam intensity result.

2. multi-frequency launching beam formation method as claimed in claim 1, it is characterized in that, in each covering of the fan, be the fan-shaped sonar wave beams signal of τ successively to different beam direction fire pulse width, the time-domain expression launching fan-shaped sonar wave beams signal is:

$\begin{array}{c}{s}_{nMFT}\left(t\right)=s_n\left(t,f_1,{\mathrm{\β}}_{q\left(k_S,1\right)}\right)+s_n\left(\left(t-\mathrm{\τ}\right),f_2,{\mathrm{\β}}_{q\left(k_S,2\right)}\right)\\ +...+s_n\left(\left(t-J\mathrm{\τ}\right),f_J,{\mathrm{\β}}_{q\left(k_S,J\right)}\right)\end{array}$

Wherein,

$s_n(t,f_j,{\mathrm{\β}}_q)=\left\{\begin{array}{cc}Asin(2{\mathrm{\πf}}_{j}t+{\mathrm{\θ}}_n(f_j,{\mathrm{\β}}_q\left)\right);& 0\≤t\≤\mathrm{\τ}\\ 0;& otherwise\end{array}\right.$

θ _n(f _j,β _q)＝2πf _j·(n-1)dt·sinβ _q/c

In formula: s _nMFTt () is time-domain function, A is signal amplitude, θ _n(f _j, β _q) be phase offset parameter, dt is for launching array element distance, f ₁, f ₂... f _jfor the frequency of each fan-shaped sonar wave beams signal, β _qfor vertical beam deflection, vertical beam call number q is about division covering of the fan number k _swith the function of frequency indices j, k _s=1,2 ..., K, j=1,2 ..., J, n are for launching array element call number, and J is the number of beam direction, and c is sonar velocity of propagation under water.

3. multi-frequency launching beam formation method as claimed in claim 2, it is characterized in that, number frequency of each fan-shaped sonar wave beams signal increases progressively according to the step frequency of Δ f, i.e. f _j+1-f _j=Δ f, should meet 1/ Δ f≤τ between step frequency Δ f and pulse width τ.

4. multi-frequency launching beam formation method as claimed in claim 3, it is characterized in that, the beam pattern of emission array is expressed as:

$\left|B_{TrMFT}(f_j,{\mathrm{\β}}_q)\right|=|\underset{n=1}{\overset{N}{\Σ}}{S}_n\left(k_j\right)\·\exp (-{\mathrm{j\θ}}_n(f_j,{\mathrm{\β}}_q\left)\right)|$

Wherein:

$S_n\left(k_j\right)=\underset{l=1}{\overset{L}{\Σ}}{s}_n(l,f_j,{\mathrm{\β}}_q)\·\exp (-j\frac{2\mathrm{\π}}{L}l\·k_j)$

$k_j=\frac{f_j}{f_s}L$

In formula: N is the element number of array of emission array, L is that DFT computing is counted, and l is its call number, f _sfor sample frequency, k _jfor frequency f _jline spectrum number.

5. multi-frequency launching beam formation method as claimed in claim 4, it is characterized in that, the beam pattern expression formula of receiving array is:

$\left|B_{\mathrm{Re}MFT}(f_j,{\mathrm{\α}}_p)\right|=|\underset{m=1}{\overset{M}{\Σ}}{S}_m\left(k_j\right)\·\exp (-{\mathrm{j\θ}}_m(f_j,{\mathrm{\α}}_p\left)\right)|$

Wherein:

$S_m\left(k_j\right)=\underset{l=1}{\overset{L}{\Σ}}{s}_m\left(l\right)\·\exp (-j\frac{2\mathrm{\π}}{L}l\·k_j)$

${\mathrm{\θ}}_m(f_j,{\mathrm{\α}}_p)=2{\mathrm{\πf}}_j\·(m-\frac{M+1}{2})dr\·{\mathrm{sin\α}}_p/c$

In formula: M is the element number of array of receiving array, k _jfor frequency f _jline spectrum number, S _m(k _j) be echo signal sample data s _mleaf transformation in the L point discrete Fourier of (l), θ _m(f _j, α _p) be the phase offset parameter of receiving array, dr is for receiving array element distance, α _pfor horizontal beam deflection.

6. the method for reconstructing of a three-dimensional acoustics image, it is characterized in that, the multi-frequency launching beam formation method described in any one of claim 1 ~ 5 is utilized to obtain beam intensity result (x, y, z, i), wherein i is beam intensity value, (x, y, z) be three dimensional space coordinate that beam intensity result is corresponding, adopt distance value z ' to compensate transmitting time delay, and adopt three dimensional space coordinate (x, y after compensating, z ') rebuild, obtain correct real-time three-dimensional acoustic picture;

The distance value z ' of described compensation is:

$z^{\′}=z-\frac{{\mathrm{c\Γ}}_j}{2}=z-\frac{c\mathrm{\τ}\[q-(k_S-1)J-1\]}{2}$

In formula, Γ _jfor the time delay value of each frequency signal, τ is pulse width, and q is beam direction call number, k _sfor dividing covering of the fan sequence number, j is signal frequency index.