SITAR Seafloor Imaging and Toxicity: Assessment of Risk caused by buried waste

Transcription

1 SITAR Seafloor Imaging and Toxicity: Assessment of Risk caused by buried waste Contract EVK3-CT RTD project under the 5 th Framework Programme of the European Union Specific Programme: Energy, Environment and Sustainable Development Sub-programme: Environment and Sustainable Development WP-1 Deliverable 10 Multiple Aspect scattering measurement requirements Produced by NTNU Jens M. Hovem, hovem@tele.ntnu.no Magne A. Larsen, malarsen@tele.ntnu.no 13/11/2003 1

2 EXECUTIVE SUMMARY The acoustic scattering signature and the characteristics of a buried object depend on the incidence and scattering angles. This dependence will be used to estimate important information about the target, like size and shape. In the project the principle of Multiple Aspect will be applied by using a source mounted on a ROV to ensonify a portion of the seabed and to record the scattered field with a receiver array. This gives the possibility of adapting the geometry of the experiment and thereby to collect 2-D images of the scattered field from multiple positions and incidence angles. The 2-D images are slices of the whole volumetric image. The sampling requirements will be optimised to enable image reconstruction of a buried container with the minimum required dimension. This will in particular include the design of the transmitted acoustic waveform, the position accuracy of the source and receivers, and the coverage and sampling of the scattered field. This report discusses the experimental procedure and sampling requirements for the main sea trial in the SITAR project. The main conclusion is that measurements with a variety of different incidence and scattering angles must be performed. Different pulse shapes and frequencies should also be tried out in order to exploit the advantage of each type. 2

3 INDEX 1. Introduction page 4 2. Equipment page LBL page Parametric source TOPAS page ROV page Hydrophone chain page Recording system page CTD page 7 3. Experimental procedure page 8 4. Transmitter waveforms and frequencies page Waveform page Frequency page 9 5. Sampling of the scattered field page Conclusion page References page 13 3

4 1 Introduction The main objective of WP-1 is the development of acoustic methods and instrumentation for imaging of waste barrels/containers of small dimension buried in unconsolidated sea sediments. One of the research lines within this work package will focus on the study and experimental verification of Multiple Aspect (MA) acoustic sub-bottom scattering measurement techniques. This refers to measurement techniques where the receivers are separated in space from the acoustic source. Another name for MA is bistatic, as opposed to monostatic, where source and receivers are at the same place. The test site for the main sea trial is Möja Söderfjärd bay (figure 1) located in the Stockholm archipelago. The bay has since early 20 th century until 1965 been used by the Swedish Armed Forces as a dumpsite for ammunition as well as ordinary waste. According to available information the amount of munitions can be as much as several tons. All kinds of different types and sizes of munitions has been dumped, piece by piece or packed in boxes. The target we will focus on in the MA scattering measurements has been identified with video camera to be a semi-buried box. We do not know what its content is. The bottom depth is around 70 m. The Swedish Geological Survey has reported that the upper sediment in the area is muddy with a high content of gas. Figure 1: Test site 4

5 2 Equipments A ship from the Swedish Navy (figure 2) will be used to operate the equipment from. It is 30 meter long and its weight is 700 tons. The crew consists of officers and conscripts. A shed containing instruments will be positioned at the stern of the ship. The ship will carry instruments as a conventional ROV with video camera, echo sounder and DGPS. The parametric source will be mounted on the FOI ROV. Figure 2: Operating vessel 2.1 LBL A commercial LBL system from Sonardyne will be used in the measurements. An LBL (Long-Base-Line) system has two segments. The first segment comprises a number of acoustic transponders moored in fixed locations on the seabed. The positions of the transponders are described in a coordinate frame fixed to the seabed. The distances between them form the "baselines" used by the system. The second segment comprises an acoustic transceiver which is installed on the vessel. The distance from the transducer to a transponder can be measured by causing the transducer to transmit a short acoustic signal which the transponder detects and causes it to transmit an acoustic signal in response. The time from the transmission of the first signal to the reception of the second is measured, and as sound travels through the water at a known speed, the distance between the transducer and the transponder can be estimated. The process is repeated for the remaining transponders and the position of the vessel relative to the array of transponders is then calculated or estimated. A ROV-track system consists of 4 transponders, an acoustic transceiver on the ROV, an umbilical cable and software. The four transponders are deployed the on the seabed in a rough square (~ 500*500m). Ranges are measured to the transponders while the boat is being tracked using its DGPS system. This enables the geographical seabed locations of the transponders to be established automatically. The ROV may be tracked in X-Y coordinates with a repeatability of a fraction of a metre, while conducting the instrumented survey. 5

6 2.2 TOPAS As transmitter in the the experimental system we use a parametric source, the TOPAS (TOpographic PArametric Sonar) 120 (figure 3) manufactured by Kongsberg Maritime. With a parametric sonar the nonlinearity of high-amplitude sound propagation is exploited to generate low frequency sound. The sonar transmits two high-frequency (primary frequency) signals that interact in the water to give the desired difference frequency (secondary frequency). A characteristic of this type of sonar is that it has a very narrow main lobe (~ 6 degrees) even at low frequencies that varies little with frequency and it has low side lobes. Figure 3: TOPAS image 2.3 ROV The TOPAS is mounted on a remotely operated vehicle (ROV) called PLUMS (PLatform for Underwater Measurement Systems) developed at FOI. Its main characteristics is its good attitude stability. It has three axes rate gyro aided by a magnetic compass and two pendulums. Its roll, pitch and yaw angles are controlled to an accuracy of less than one degree. 2.4 Hydrophone chain A FOI homemade hydrophone chain or a commercial one from Vibrometric will be used. It consists of eight hydrophones with 2 m separation between each. The hydrophones should be calibrated in order to know the relative output signal levels. 6

7 SITAR Multiple Aspect scattering measurement requirements 2.4 Recording system The recorded pings for each run will be sampled at 200 khz and stored in files on disc. ROV navigation data; X-Y coordinates, depth, pitch, roll and course will be stored in files containing UT (Universal Time) stamps. The time stamps for the GPS clock triggered pulses should be saved for each ping. It would then be possible to know exactly what portion of the seabed was ensonified for an arbitrary ping. 2.6 CTD The sound velocity profiler from Falmouth Scientific, Inc. (FSI) (figure 4) uses Conductivity, Temperature and Depth sensors to determine sound velocity based on the UNESCO 44 standard [1]. Stable CTD measured parameters allows more precise measurement of sound velocity then direct acoustic time-of-flight measurement. Figure 4: CTD 7

8 3 Experimental procedure The experimental procedure (see figure 5) is described briefly as follows, The boat is anchored at the test site. Four transponders will be deployed at the bottom to track the position of the ROV. ROV runs with parametric source (TOPAS) in vertical monostatic mode. The purpose is to find the exact position of the target. Deployment of the receiving hydrophone chain. Bistatic scattering measurements. The ROV will be in several different positions around the target and the TOPAS in different angles towards the target. The hydrophone chain is moved to a position transversely to the first one, and the bistatic scattering measurements are repeated. Recovery of the hydrophone chain and the transponders. Velocity profiles should be recorded on a regular basis. Careful quality checks of the data should be done onboard, and backup of data made. Measurements from the bottom without the target should also be carried out in order to do inversion for geoacoustic parameters. Figure 5: Experimental procedure 8

9 4 Transmitter waveforms and frequencies The bottom sediments are soft with a high content of gas. The target is a squared box. This information could give a conception of suitable transmitter waveforms and frequencies for the measurements. 4.1 Waveform The TOPAS has CW, Single pulse (Ricker) and chirp capability (figure 6). Figure 6: Waveforms The capability of the sonar to detect a target in presence of reverberation is dependent of the energy of the waveform, and not on its shape. For a single-frequency sine pulse (CW) the only way to increase the signal energy under limitation of peak power is to increase the length of the pulse. With an effective pulse length t, the resolution in range (i.e. the capability to discriminate between to targets) r is given by r = c 2 t As we see from the above formula, increasing the pulse length has a negative impact on the resolution in range. A Ricker wavelet is a zero-phase wavelet (symmetrical about zero time) and is widely used in seismic modelling because it provides sharper definition and less distortion between features in the subsurface. The disadvantage of using an impulse like this is that the pulse energy is low due to the very short pulse length. Consequently, the peak power has to be very high if a reasonable system performance is to be obtained. When a chirp (linear frequency modulation) is used, the effective pulse length after compression with matched filtering is t = 1/ b, and hence the resolution in range is 9

10 r We can therefore use a long pulse length and still maintain good resolution. c 2B 4.2 Frequency A wave undergoes attenuation when it propagates in material media. The energy per unit area in the direction of propagation of a spherical wave front falls off inversely as the square of the distance from the source. This is called geometric spreading. A further cause of energy loss along a ray path arises because the ground is imperfectly elastic in its response to the passage of waves. Elastic energy is gradually absorbed into the medium by internal frictional losses, leading eventually to the total disappearance of the acoustic disturbance. Since the sediment is attenuating, the propagation velocity varies with frequency (frequency dispersion). If the amount of absorption per wavelength is constant, it follows that higher frequency waves attenuate more rapidly than lower frequency waves as a function of time or distance. The shape of a pulse with a broad frequency content therefore changes continuously during propagation due to the progressive loss of higher frequencies. Thus if the target is buried in the sediments, we could fail to detect it if we are using too high frequencies. The presence of gas in sediments reduces the elastic moduli and density and, hence, the compressional sound wave velocity. The increased attenuation due to the gas will put a limit on the upper transmitter frequency.. At an interface between two layers there is generally a change of propagation velocity resulting from the difference in physical properties of the two layers. At such an interface the energy within an incident pulse is partitioned into transmitted and reflected pulses. The relative amplitudes of the transmitted and reflected pulses are functions of angle of incidence and the velocities and densities of the two layers. A discussion on the impact of frequency on resolution is given in [2]. The simple model consists of a thin layer bounded at its top and bottom by layers of identical physical properties (but with properties different from the thin layer). A simple waveform is reflected at the top and the bottom interface. It is shown that the character of the reflections is indistinguishable for layers whose thickness is less than about λ l /8. Here λ l is the wavelength computed using the velocity v l of the thin layer, and is related to frequency f by vl λ l = f Hence a higher frequency improves the resolving power. For layers as thin as this the amplitude of the reflection is found to be approximately proportional to the thickness and inversely proportional to the wavelength. 10

11 5 Sampling of the scattered field Imagine the direct ray path from a source, via a target to a receiver. The scattering angle in elevation in plane with incidence angle is now referred to as the scattering angle. The scattering angle in azimuth is referred to as the bistatic angle. For our experimental procedure, the incidence and bistatic angle are determined by the position of the source, while the scattering angle is fixed by the position of the hydrophone chain. The directional characteristics of the scattered field from a target can reveal its orientation or shape. A strong response at the receiver exists for specular reflection. This happens when the bistatic angle is zero and the scattering angle equals the incidence angle. Mixed layer 0 0 Angles=-20 : 5 : 70 Sd. = 50 m Depth - m Sound speed - m/s Range - m Figure 7: Ray tracing 11

12 Sound velocity profiles can be used to model the scenario with ray tracing. In figure 7 a hypothetical velocity profile consisting of a constant velocity layer overlaying another layer with negative gradient is shown to the left. To the right we see the propagation of sound represented as rays, for which propagation is completely governed by Snell s law of refraction. Three types of arrivals at the receivers can be identified. The first arrival corresponds to the direct ray path from source to receiver. The amplitude level of this arrival is rather small because of the narrow beam width and side lobe level of the source. The second arrival is the bottom reflected rays, and is the one we want to extract from the traces in order to say something about the target or bottom. Some of the energy is transmitted through the water-bottom interface and diffracted at subsurface interfaces. The third arrival is the bottom-surface reflected rays. The water-air interface is pressure-release, which means no amplitude attenuation but inversion of phase. The signals are furthermore attenuated by absorption in water and bottom and geometric spreading. The signal amplitudes for each path add linearly and may therefore interfere at the receivers. For more information on ray tracing and sound propagation in water in generally, see [3] and [4]. An analytic solution for scattering from a cylinder has been presented in [5]. The model does not apply for square boxes, since the fundamental basis of the model is the cylinder shape. However, it is instructive to see how the computed 3-D scattering function (figure 8) can aid in choosing the incidence, scattering and bistatic angles that give a strong response at the receivers (figure 9). For most practical target shapes, no analytic solutions exist and hence exhaustive numeric modelling must be used. Figure 8: Scattering function of a cylinder 12

13 Figure 9: Dependence of scattering on incidence and bistatic angles 6 Conclusion The sampling requirements should be optimised to enable image reconstruction of a buried container with minimum dimensions. Measurements with different types of transmitted waveforms (chirp, Ricker) and different frequency ranges should be carried out. This in order to exploit the advantages (resolution, bottom penetration, energy level) of each type. The positioning accuracy of the ROV should be good in order to have a reasonable estimate of the angle of incidence, alternatively what part of the bottom is ensonified. Furthermore, the narrow beam width of the parametric source increases the demand for good ROV position accuracy. Sound velocity profiles should be produced in order to calculate the exact travel path of sound from source to receivers. Sufficient coverage and sampling of the scattered field is critical for 3-D image reconstruction of the target. The ROV will fly m over the bottom. The hydrophone chain will be positioned about 15 m from the known target. With 8 hydrophones in the chain and 2 m separation between each, a distance of about 5 m of lowermost hydrophone above bottom should be chosen. Results from scaled tank experiments at the University of Bath (SubTask 1.2.5) suggest that this configuration will cover a wide range of scattering angles of interest. Measurements with the TOPAS oriented so as to produce 13

14 lots of different combinations of incidence angle and bistatic angle should then be performed. Incidence angles producing specular reflection arrivals at the hydrophones should be prioritised. Results from the scaled tank experiments suggest repeated measurements from the source held at the same position in order to increase SNR by averaging the received signals. The bottom should also be ensonified without target to make geo-acoustic inversion possible. 7. References [1] C. Chen and F. Millero, (1977), JASA 62:1129. [2] M.B. Widess, Geophysics, vol. 38, no. 6 (December 1973) p [3] H. Medwin and C.S. Clay, Fundamentals of Acoustical Oceanography, Academic Press, [4] J.M. Hovem, Kompendium i Marin Akustikk, NTNU, 2000 [5] Z. Ye, (1997), JASA 102:877 14