Detection of Unobserved Ground Targets by Use of Seismic Location Stations

: The detection method of moving unobserved ground military heavy trucks is considered in paper. The 3D coordinates detection set and the seismic location method are used in our study. The computational algorithm of determination of the bearing angle and distance to unobserved ground target are presented, the computational uncertainties are estimated. The triangular form of 3D detectors in one seismic location cell is offered. It allows minimizing distance and angle uncertainties in target direction. To improve the detection effectiveness, the optimal geometry disposition of 3D detectors in seismic location cells is determined. The offered method is effective in case of terrain relief where seismic-acoustic signal is propagated by approximately straight line. Therefore, this method can be applied only to homogenous medium, for example, to mountainous, solid rocky soil.


Introduction
For successful planning of a military operation the battle field commanders have to know full information about battle space [1]. But there could be such a critical situation during military operations that it might not be possible to observe terrestial targets, particularly unobserved enemy ground military heavy trucks located on mountainous terrain. In this situation, there is indefinity in information, thus the risk of wrong decision making under uncertainty on destroying of the targets is rised, the probability of unexpected attack is increased. Therefore, the detection of faraway moving unobserved military heavy trucks is very important.
To solve this problem, the passive (hidden) means is offered to reveal the heavy trucks movement. The advantages of passive means are noiselessness and low energy consumption (< 0.01 W). This power is provided by autonomous energy sources (battery, accumulator, solar cells), which is very important during war operations at the field conditions.
Firstly, this problem of the secretive revealing of unobserved military heavy machineries (tank or other armoured trucks) in mountainous or forestry regions has been considered in [2,3]. The offered method cannot directly be applied in complex conditions of landscape (serially located mountains, hills and valleys). This method is effective in case of terrain relief when seismic-acoustic signal is propagated by approximately straight line. Therefore, this algorithm can be applied only to homogenous medium, that is, to mountainous, solid rocky soil.
The location parameters of unobserved target can be determined by seismic and acoustic methods, and by their combination [4][5][6][7]. Each of them has advantages and lacks. The application of the combination of these methods raises the reliability of detection because noise factors for various physical effects phenomena are negligibly correlated [8]. The application of detection seismic-acoustic means is caused by neighbourhood of physical processes, which occur during longitudinal seismic and acoustic wave propagation along of soil-air interface with various densities.
We can see in frequency range of surface seismic waves  Hz) that there is a sufficiently high and stable acoustic electric sensitivity of piezoelectric detectors [3]. At that, the range of a low frequency (1-10 Hz) is of a special interest, and subsurface seismic vibrations are weakly depended on climate factors.
Permanent seismic-acoustic waves arise in the surface soil owing to not only the movement of heavy armoured trucks, but also trains, aircraft, helicopters, rain, wind and processes in Earth interior. By application of space-separated 3D detectors and seismic location cells of triangular form [9] we can determine a bearing angle (azimuth) of the source of seismic waves and a distance. Then, by use of the computer base of standard spectral seismic-acoustic data, we can determine a type of the armoured trucks.
The detection method of location of the moving unobserved ground military heavy trucks is considered in paper. The 3D coordinates detection set and the seismic location method are used in our study. The computational algorithm for determination of bearing angle and distance to unobserved ground target are presented, the computational uncertainties are estimated. The triangular form of 3D detectors in one seismic location cell is offered. It allows minimizing distance and angle uncertainties in target direction. In order to improve detection effectiveness, the optimal geometry disposition of 3D detectors in seismic location cells is determined.
The goal of this work is the development of the determination algorithm of angle parameters of unobserved target direction (bearing angle measurement) based on seismic-acoustic data, distance to target and optimal geometry of disposition of 3D detectors in seismic location cell.

Determination of Space Parameters
Let us consider the principle of seismic location station (SLS) operation. The chart of geometry of SLS components is shown in Fig. 1 [3,10]. There are three 3D detectors in seismic location cells (SLCi) for detection of seismic-acoustic waves (Fig. 2): here, d is the distance between detectors; i,(i = 1,…, n) is the serial number of cell, n is the number of seismic location cells; SW is the input signal from seismic-acoustic wave. Let us suppose cells are brought into a line and the distance between them is equal to l. Then, the distance between the first cell and the last one is L = (n − 1) · l. 120° is the angle between vertical surfaces of 3D detectors 3D-detectors are made based on piezoelectric sensors. Polymer matrix PVDF + Pb(ZrTiO3) + (SiO2)6 and PVDF + Pb(ZrTiO3) + BaTiO3 hybrid nanocompozites are used in these sensors as highly sensitive piezoelectric materials [11]. When we compare output power on various sides of 3D-detectors, we can determine the direction of incident wave. That is, this kind of 3D sets allows determining exactly the space position of the target. The physical principles of 3D detectors operation and of sensible piezoelectric sensors making are given in [9,10].
The design of 3D-detectors is shown in Fig. 3 in [3, p. 187]. We can determine the direction of seismic sound source by comparing the fixed signal power from vertical sides of piezoelectric detectors (sensors). Then, we calculate an average value of the obtained data from all 3D detectors of considered SLS and determine the direction of target for this station. Now, for determination of the target location we can apply the triangle method by separately considering each pair of SLS. We calculate the average value of the obtained average data and determine the coordinates of target.
Below, we present the calculation formulas of direction vectors, the distance to target R and the evaluation of calculations uncertainty in dependence on the distance l and the number n of SLS.
Let us consider angle parameters in the direction of target, the distance between the target and SLS, and the optimal disposition geometry of 3D detectors in seismic location cell.
Let us take the coordinate system shown in Fig. 3. Let SLC1 is situated in (0, 0) point, and SLC2 is situated in (l, 0) point. If the direction of the target with (x0, y0) coordinates in SLC1 sets itself by α angle, and in SLC2 sets itself by β angle, then The distance to the target is The quantity R is identical with the coordinate y0 (it is clear from Fig. 3), that is y0 = R.
It is clear that the larger |sin(β − α)|, the lower the calculation uncertainty.
To reduce the uncertainty dR, three D1, D2 and D3 3D detectors in one SLC are placed angularly under 120° to one another (see Fig. 2) [10].
So, detecting signals by piezoelectric sensors on vertical sides of 3D detectors we can determine the direction (bearing angle), the distance to the source of seismic waves (the target) and the evaluation of measurement uncertainties.
Generated on horizontal sides of 3D detectors the signals are filtered (sorted out) because they are formed by acoustic waves from flying targets (aircraft, helicopter etc.).
From Eq. (3) we obtain the uncertainty of determination of the target space coordinates From Eq. (3) we obtain the uncertainty of determination of the target space coordinates dR ≈ 2 m.
From Eq. (1) we obtain the distance of the unobserved target R = 1600 m. Various spectrums of seismic-acoustic waves (etalon spectrums) generated by moving armoured machineries (T-55, T-72. T-80, T-90 tanks: BMP-2, BMP-1, BMD, BRDM-2, BRDM, BTR-80, BTR-60 and heavy trucks) are stored in the memory of electronic block AB. Therefore, taking into account that the power of propagated seismic-acoustic waves is inversely proportional to the square of the distance, we can determine the type of target by comparison of seismic-acoustic spectrums detected by 3D detectors with etalon ones.

Conclusion
The paper deals with the determination of the hidden movement of enemy heavy armoured trucks (targets) based on seismic acoustic location. The algorithms of determination of the angle parameters and the distance to the unobserved ground target are developed based on seismic acoustic data.
The triangular form of 3D detectors in one seismic location cell is offered. It allows minimizing the distance and angle uncertainties in target direction. The optimal dislocation geometry of 3D detectors in seismic location cell for decreasing measurement uncertainty is determined.
If the width of seismic location station is L = 2000 m, α = 50 o and β = 110° then the uncertainty of determination of the target space coordinates is dR ≈ 2 m in the distance of R ≈ 1600 m. This method can be applied only to homogenous medium, for example, to mountainous, solid rocky soil.
In heterogeneous geological subsoil environment, there are lots of interference and diffraction processes of seismic-acoustic waves. So, we must carry out field experiments and have reference (etalon) data generated by moving of known armoured machineries.