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1.
This paper applies a full-field technique to invert bottom sound profile and bottom reflectivity from simulated acoustic data in a shallow water environment. Bottom sound-speed profile and bottom reflectivity have been traditionally estimated using seismic reflection/refraction techniques when acoustic ray paths and travel time can be identified and measured from the data. However, in shallow water, the many multipaths due to bottom reflection/refraction make such identification and measurement rather difficult. A full-field inversion technique is presented here that uses a broad-band source and a vertical array for bottom sound-speed and reflectivity inversion. The technique is a modified matched field inversion technique referred to as matched beam processing. Matched beam processing uses conventional beamforming processing to transform the field data into the beam domain and correlate that with the replica field also in the beam domain. This allows the analysis to track the acoustic field as a function of incident/reflected angle and minimize contamination or mismatch due to sidelobe leakage 相似文献
2.
Siderius M. Snellen M. Simons D.G. Onken R. 《Oceanic Engineering, IEEE Journal of》2000,25(3):364-386
In October 1997, the EnVerse 97 shallow-water acoustic experiments were jointly conducted by SACLANT Centre, TNO-FEL, and DERA off the coast of Sicily, Italy. The primary goal of the experiments was to determine the sea-bed properties through inversion of acoustic data. Using a towed source, the inversion method is tested at different source/receiver separations in an area with a range-dependent bottom. The sources transmitted over a broadband of frequencies (90-600 Hz) and the signals were measured on a vertical array of hydrophones. The acoustic data were continuously collected as the range between the source and receiving array varied from 0.5 to 6 km. An extensive seismic survey was conducted along the track providing supporting information about the layered structure of the bottom as well as layer compressional sound speeds. The oceanic conditions were assessed using current meters, satellite remote sensing, wave height measurements, and casts for determining conductivity and temperature as a function of water depth. Geoacoustic inversion results taken at different source/receiver ranges show sea-bed properties consistent with the range-dependent features observed in the seismic survey data. These results indicate that shallow-water bottom properties may be estimated over large areas using a towed source fixed receiver configuration 相似文献
3.
Acoustic propagation in shallow water is examined. Multipath propagation and extensive boundary interactions, which along with a host of other phenomena produce a highly variable and often unpredictable acoustic field, are discussed. The responsible mechanisms, and hence the acoustic effects, cover a wide range of temporal and spatial scales and are classified as either deterministic or random, although the two types often act in concert. Because of extensive interactions with the sound field, the bottom can severely degrade waterborne propagation, although the sea bottom (and subbottom) can provide a seismic path that not only is relatively stable, but exists even under environmental conditions that preclude an effective waterborne path. Propagation in the bottom is particularly significant at very low frequencies. These various aspects of shallow-water acoustics are illustrated using the results of experiments conducted in diverse geographic areas 相似文献
4.
Dziak R. P. Fox C. G. Matsumoto H. Schreiner A. E. 《Marine Geophysical Researches》1997,19(2):137-162
The oceanic T-waves of earthquakes associated with the 1992 Cape Mendocino earthquake sequence were recorded and analyzed using fixed hydrophone arrays located throughout the north-east Pacific Ocean. The T-waves of these events were well recorded with high S/N ratios and strong acoustic energy present over a 0–64 Hz bandwidth. The smallest event recorded by the hydrophone arrays from the sequence had a local magnitude of 2.4. The hydrophone records of the three largest shocks in the sequence (ML 6.9, 6.2, 6.5) exhibited both T-waves and lithospheric phases from these events. Low-pass filtering (2 Hz) of the lithospheric phases yielded a clear P-wave arrival for epicentral distances of <10°, but no apparent S-wave. A seafloor cable-break was detected immediately after the second M>6 aftershock, possibly the result of a submarine slide. The direct P-wave hydrophone records from the second large aftershock showed a relatively high-amplitude, high-frequency arrival, consistent with seismic analyses which used this information to infer rupture direction. The rupture direction was toward the location of the cable break, thus rupture directivity possibly played a role in initiating the slide event. Modelling of the T-wave propagation path, using the Parabolic Equation model, produced estimates of the acoustic transmission loss from epicenter to receiver. The transmission loss to the most distant phones is typically 10-20 dB , and can be as large as 50–70 dB for acoustic propagation paths that cross the continental margin. The amount of acoustic energy each earthquake released into the ocean at the seafloor–water interface was estimated applying the transmission loss and instrument response to the recorded T-wave signals. This acoustic source power level was calculated for 41 events with magnitudes over a recorded range of 2.4ML6.9, with 17 of these events having their seismic moment estimates available through the NEIC. Ground displacement spectra were estimated from the acoustic power spectra and showed no indication of a corner frequency. Thus empirical analyses relating source level to magnitude and seismic moment were necessary to quantitatively derive an earthquake's size from hydrophone records. The results of indicator variable regression analyses suggest that T-wave source level increases linearly with the event's local magnitude and seismic moment. Furthermore, the source power level versus magnitude relationships for oceanic and continental earthquakes are significantly different, probably illustrating differences in the seismic and acoustic propagation paths from hypocenter to the hydrophone receivers. The results indicate that acoustic measurements provide a reasonable estimate of magnitude and seismic moment of an oceanic earthquake that was not detected by land-based seismic networks. 相似文献
5.
In this paper, we address the problem of detecting an inhomogeneity in shallow water by observing changes in the acoustic field as the inhomogeneity passes between an acoustic source and vertical line array of receivers. A signal processing scheme is developed to detect the perturbed field in the presence of the much stronger primary source signal, and to estimate such parameters as the time when the inhomogeneity crosses the source-receiver path, its velocity, and its size. The effectiveness of incoherent, coherent, and partially coherent spatial processing of the array signals is evaluated using models and data obtained from experiments in a lake. The effect of different bottom types is also considered, and it is shown that partially coherent processing can have a significant advantage depending on the bottom type. Estimates of the minimum input signal-to-noise ratios (SNRs) for which the diffracted signal can be observed are presented. 相似文献
6.
A computational case study of coupled-mode 400-Hz acoustic propagation over the distance 27 km on the continental shelf is presented. The mode coupling reported here is caused by lateral gradients of sound-speed within packets of nonlinear internal waves, often referred to as solitary wave packets. In a waveguide having unequal attenuation of modes, directional exchange of energy between low- and high-loss modes, via mode coupling, can become time dependent by the movement of waves and can cause temporally variable loss of acoustic energy into the bottom. Here, that bottom interaction effect is shown to be sensitive to stratification conditions, which determine waveguide properties and, in turn, determine modal attenuation coefficients. In particular, time-dependent energy loss due to the presence of moving internal wave packets is compared for waveguides with and without a frontal feature similar to that found at the shelfbreak south of New England. The mean and variability of acoustic energy level 27 km distant from a source are shown to be altered in a first order way by the presence of the frontal feature. The effects of the front are also shown to be functions of source depth. 相似文献
7.
Acoustic echoes obtained during high-resolution shallow marine seismic surveys contain information about the statistical nature of the sedimentary bottom and its spatial variability. Use of a broad-band seismic source and an appropriately chosen data acquisition window makes the acoustic responses particularly amenable to quantitive analysis. The work reported utilizes experimental frequency-domain spatial coherence functions of along-track acoustic echoes as empirical metrics of bottom character, and by virtue of their correlation with known sediment types, as objective bases for remote sediment classification. Theoretical relationships between parameters describing sediment surface topographies and echo coherence are derived for the case of dominant water-sediment interface acoustic scattering. The diverse experimental data base was acquired from the Grand Banks of Newfoundland using a 1- to 10-kHz Huntec DTS system. Bottom photographs, cores, and grab samples combined with expert geological synthesis provide qualitative and quantitative control. 相似文献
8.
San Simón Bay in the innermost part of the Ría de Vigo is characterized by an abundance of very shallow gas accumulations
and methane seeps. During the expeditions of April–June–September 2004 within the Spanish-funded Gs2G project, detailed very
high-resolution seismic and field investigations were carried out to study the shallow gas and the seeps. Direct gas fluxes
also were measured from bubble streams. For the first time, the surface area and gas front depth of a shallow gas field has
been mapped and quantified in the inner bay of Ría de Vigo. This field overlaps spatially with the distribution of Holocene
mud within the bay. Seismic data show 3.6 km2 affected by acoustic turbidity but this surface can be extended up to 9.5 km2 of San Simón’s muddy subtidal area. Mounded turbidity superimposed on the main gas field has been mapped and characterized
as anthropogenically (mussel rafts) mediated gas accumulations. Different acoustic anomalies have been identified and interpreted
as being due to gas escapes from the present seabed sediment. The very high resolution of the seismic data makes it possible
to identify a new type of seep, here named ‘acoustic smoke.’ A direct relationship can be observed between the gas front of
accumulations and escape features, both acoustic seeps and pockmarks. The methane flux has been estimated from the subtidal
environment in San Simón based on detected acoustic targets and direct measurements of current bubble flow. The total estimated
methane flux from the seabed into the water column ranges from 10.1 to 48.8 t/year, and into the atmosphere from 7.0 to 34.2 t/year.
The intertidal San Simón environment is also actively venting methane, as indicated by the presence of bubbling during high
tide and white patches of Beggiatoa sp. 相似文献
9.
Gas in sediments has become an important subject of research for various reasons. It affects large areas of the sea floor
where it is mainly produced. Gas and gas migration have a strong impact on the environmental situation as well as on sea floor
stability. Furthermore, large research programs on gas hydrates have been initiated during the last 10 years in order to investigate
their potential for future energy production and their climatic impact. These activities require the improvement of geophysical
methods for reservoir investigations especially with respect to their physical properties and internal structures. Basic relationships
between the physical properties and seismic parameters can be investigated in shallow marine areas as they are more easily
accessible than hydrocarbon reservoirs. High-resolution seismic profiles from the Arkona Basin (SW Baltic Sea) show distinct
‘acoustic turbidity’ zones which indicate the presence of free gas in the near surface sediments. Total gas concentrations
were determined from cores taken in the study area with mean concentrations of 46.5 ml/l wet sediment in non-acoustic turbidity
zones and up to 106.1 ml/l in the basin centre with acoustic turbidity. The expression of gas bubbles on reflection seismic
profiles has been investigated in two distinct frequency ranges using a boomer (600–2600 Hz) and an echosounder (38 kHz).
A comparison of data from both seismic sources showed strong differences in displaying reflectors. Different compressional
wave velocities were observed in acoustic turbidity zones between boomer and echosounder profiles. Furthermore, acoustic turbidity
zones were differently characterised with respect to scattering and attenuation of seismic waves. This leads to the conclusion
that seismic parameters become strongly frequency dependent due to the dynamic properties of gas bubbles. 相似文献
10.
It is extremely difficult to determine shallow ocean bottom properties (such as sediment layer thicknesses, densities, and sound speeds). However, when acoustic propagation is affected by such environmental parameters, it becomes possible to use acoustic energy as a probe to estimate them. Matched-field processing (MFP) which relies on both field amplitude and phase can be used as a basis for the inversion of experimental data to estimate bottom properties. Recent inversion efforts applied to a data set collected in October 1993 in the Mediterranean Sea north of Elba produce major improvements in MFP power, i.e., in matching the measured field by means of a model using environmental parameters as inputs, even using the high-resolution minimum variance (MV) processor that is notoriously sensitive and usually results in very low values. The inversion method applied to this data set estimates water depth, sediment thickness, density, and a linear sound-speed profile for the first layer, density and a linear sound-speed profile for a second layer, constant sound speed for the underlying half space, array depth, and source range and depth. When the inversion technique allows for the array deformations in range as additional parameters (to be estimated within fractions of a wavelength, e.g., 0.1 m), the MFP MV peak value for the Med data at 100 Hz can increase from 0.48 (using improved estimates of environmental parameters and assuming a vertical line array) to 0.68 (using improved estimates of environmental parameters PLUS improved phone coordinates). The ideal maximum value would be 1.00 (which is achieved for the less sensitive Linear processor). However, many questions remain concerning the reliability of these inversion results and of inversion methods in general 相似文献
11.
Peter G. Simpkin 《Marine Geophysical Researches》2005,26(2-4):171-181
Technical specifications for small seismic sound sources used in seismic profilers may provide details of the pressure signatures
as measured on the primary acoustic axis and in the far field but will rarely provide information regarding off-axis emission
or of the form of the signatures in the near field. Since shallow water geophysical mapping is now attracting more attention
in research establishments and in the commercial sector, off-axis information could become an important issue. The use of
a wide angle seismic configuration on a small scale will demand that directional source information be included in data processing
activities if the full potential of the data is to be realised. Direct measurement of the full near and far field characteristics
of a source are often not practical, however modelling of the acoustic field may offer a tractable alternative and a means
of rapidly generating source functions appropriate for advanced data processing. One such modelling approach reviewed here
uses a time domain method to synthesise off-axis signatures using a far field, on-axis reference signature. 相似文献
12.
The shallow refracted path through sea floor sediments plays a significant role in the transmission of acoustic energy at low frequencies. For bottom grazing angles of 90/spl deg/ to 25/spl deg/, low-frequency acoustic energy was observed to come from reflected paths. For bottom grazing angles of 25/spl deg/ to 10/spl deg/ the dominant source of low-frequency acoustic energy is from shallow refracted paths through the sediments. At angles less than 10/spl deg/, low-frequency acoustic energy is received from both the refracted and the reflected paths. The refracted path is possible because of the positive gradient within the sediment. The sudden emergence of the refracted arrival is related to the overall sound path length in the sediment and sediment absorption of sound. Since sediment absorption is directly proportional to frequency, only low-frequency energy is transmitted via this path. The refracted path may well exist where unconsolidated sediments of at least a few hundred feet are present. 相似文献
13.
Naresh Kumar Thakur Pasupuleti Prasada Rao N. Vishwanath Sanjeev Rajput Bhaskarabhatla Ashalatha 《Marine Geophysical Researches》2007,28(4):373-378
Seismic tomography is an effective means of estimating velocity and structure from multichannel seismic (MCS) reflection data.
In this study we have followed a 2D approach to arrive at the probable velocity field configuration from multichannel seismic
data and infer the presence of gas hydrates/free-gas in the offshore Kerala-Konkan region, along the eastern part of a seismic
line on which a bottom simulating reflector (BSR) has previously been identified. Tomographic modeling consists of the identification
of reflection phases and picking of respective travel times for various source-receiver positions. These picks were then utilized
to arrive at a 2D velocity field following a forward and inversion approach using a ray tracing technique. The modeling for
the first time brought out the finer scale velocity structure under the region of investigation. Modeling through the 2D approach
shows lateral variation in velocity field along the studied segment of the seismic line. The results indicate a thin (∼50–60 m)
sedimentary cover with velocity ranging from 1,770 to 1,850 m/s. A sedimentary layer with high P-wave velocity 1,980–2,100 m/s
below the sea floor was interpreted as the hydrate layer. The thickness of this layer varies between 110 and 140 m. The hydrate
layer is underlain by a low-velocity layer having velocities in the range 1,660–1,720 m/s. This low velocity may represent
a free gas layer, whose thickness varies between 50 and 100 m located below the hydrated layer. The investigation suggests
the occurrence of gas hydrate underlain by free gas in some parts of the Kerala-Konkan offshore region. 相似文献
14.
V. A. Lazarev A. I. Malekhanov L. R. Merklin V. I. Romanova V. I. Talanov A. I. Khil’ko 《Oceanology》2013,53(6):755-761
Experimental results of the seismic profiling with bottom penetration up to 1000 m based on broadband signals and conducted in the Caspian Sea sites are presented. Use has been made of synchronized sequences of probing pulses with linear frequency modulation at a frequency deviation of 50 to100 Hz. The pulses were emitted by a towed sound source of an original design (acoustic power up to 300 W, frequency ranged from 100 to 1000 Hz) and received by a standard digital seismic streamer. The processing of the signals involved the matched filtering of the individual pulses and the trajectory accumulation of a long sequence of pulses lengthwise the horizontal-homogeneous reflecting layers of the bottom structure. The adaptive stacking procedure taking into account the linear inclinations of the individual layers allowed us to enlarge the stacking interval by up to 100 pulses and to increase the effective depth and the spatial resolution of the seismic profiling, which gave us a total increase of more than 30 dB in the S/N ratio. In our view, the seismic profiling using low-power (about 100 W) and broadband (up to several hundred Hz) coherent sound sources represents a promising technology for decreasing the hazardous impact on aquatic ecosystems. The approach developed is an alternative to the conventional technology of marine seismic prospecting based on powerful pulse sources of the shock type (air guns, sparkers) in the low frequency range (less than ～200 Hz). 相似文献
15.
通过分析天然气水合物在海洋中的6种主要赋存状态类型,总结了每种赋存状态之间的相互转化关系及其物性参数计算方法,并应用到地震波场的正演模拟中。对比研究了声波模型、弹性波模型和双相介质模型对各种水合物赋存地层的响应特征,结果表明:1)当地层中存在孔隙充填型水合物且下伏地层不含游离气时,双相介质模拟的含水合物层底界表现负极性特征;当充填结核型水合物时,弹性介质和双相介质模拟的水合物底界反射呈负极性;2)当地层充填颗粒包裹型水合物且下伏地层含游离气时,无论是低频(25 Hz)条件还是提高子波主频(40 Hz),3种介质模拟水合物的地震响应特征都很明显,但水合物层底界反射振幅随偏移距变化的关系存在差异;3)当沉积薄层中充填颗粒间胶结型水合物且下伏地层含游离气时,弹性介质和双相介质模拟水合物薄层底界的反射振幅随偏移距的增大而减小;将水合物类型改为颗粒支撑型并提高子波主频,声波介质和弹性介质模拟水合物层底界的反射振幅随偏移距的增大而减小。 相似文献
16.
17.
在南黄海某一典型的砂质海底区域，采用全向性声源和全向性接收水听器开展了频率范围为6-24 kHz的海底反向声散射测量。测量结果表明，在避免海面散射干扰并满足远场条件的情况下，本次实验获得了掠射角范围为18~80°的海底反向声散射强度，其数值为-41.1~24.4 dB。在有效掠射角范围内，声散射强度总体上随掠射角的增大呈现出增大趋势，但对于不同的频率，其变化趋势有所不同，反映出不同的散射机理。在20°、40°和60°掠射角处，在6-24 kHz的频率范围内反向声散射强度总体上呈现出正相关的频率依赖性，其线性相关斜率分别为0.2229 dB/kHz、0.5130 dB/kHz、0.1746 dB/kHz。在最大掠射角80°处，反向声散射强度未呈现出明显的频率相关性。 相似文献
18.
张训义 《海洋学报(英文版)》1985,4(4):641-647
The high-efficiency tandem piston air gun is a new pneumatic seismic source with simple structure, convenient operation and low cost. It can create high-energy acoustic pulse signals, and effectively attenuate bubble responses. This paper briefly introduces the air gun with 0.8 liter at a pressure of 140 kg/cm2. The measurement shows that the gun has reached a maximum peak-peak sound pressure up to about 11.7 bar-meters. The ratio (P1-P2) is about 2.79. The energy of the acoustic pulse is concentrated within 200 Hz and the main frequency corresponding to the spectrum value is about 30 Hz. Its lowest frequency is below 10 Hz. 相似文献
19.
The self-starter is improved using the operator of the split-step Pade solution. In addition to providing greater stability and being applicable closer to the source, the improved self-starter is an efficient forward model for geoacoustic inversion. It is necessary to solve only O(10) tridiagonal systems of equations to obtain the acoustic field on a vertical array located O(10) wavelengths from a source. This experimental configuration is effective for geoacoustic inverse problems involving unknown parameters deep in the ocean bottom. For problems involving depth-dependent acoustic parameters, the improved self-starter can be used to solve nonlinear inverse problems involving O(10) unknown sediment parameters in less than a minute on the current generation of workstations 相似文献
20.
Wilmut M.J. Chapman N.R. Heard G.J. Ebbeson G.R. 《Oceanic Engineering, IEEE Journal of》2007,32(4):940-947
This paper describes a simple approach for inferring the depth and track of a sound source at short ranges by inversion of acoustic field data at a set of sea bottom hydrophones. At short ranges, the acoustic field consists of a dominant Lloyd mirror (LM) signal from the direct and surface-reflected ray paths and a series of bottom-reflected paths that modulate the LM signal. A computationally efficient propagation model based on the method of images is developed to calculate replica fields for the inversion. The matched field inversion method for inferring the source depth and track is demonstrated using data from an experiment carried out in shallow water off the east coast of Canada. The estimated values were in very good agreement with independent measurements taken during the experiment. 相似文献