Rapid geoacoustic characterization using a surface ship of opportunity

The problem of rapid classification of the sea-floor sediment is addressed using horizontal line array (HLA) acoustic data from a passing surface ship. The data are beamformed to improve signal-to-noise ratio. The rapid geoacoustic characterization (RGC) algorithm involves extracting acoustic observables from the data (normalized striation slope, time spread, and transmission-loss slope). A simple single homogenous sediment layer over an acoustic half-space model is used to compute forward estimates of the acoustic observables. An exhaustive search over the two-parameter model is performed. The two parameters searched over are the sediment compressional speed (Cp), which is a polynomial function of the mean grain size (/spl phi/), and sediment thickness (H). This approach provides a real-time technique for classifying the sediment in a way that successfully reproduces the basic physics of propagation.     

Rapid geoacoustic characterization: applied to range-dependent environments

The problem of rapid environmental assessment in a range-dependent environment is addressed. For rapid assessment, the exact geoacoustic parameters are not required, nor is it a requirement that the exact structure of the acoustic field (location of peaks and s) be matched by an acoustic prediction model. The parameters that are relevant are the overall transmission loss (incoherent TL), the time spread (/spl tau/), and the slopes of the range/frequency interference patterns (/spl beta/, the waveguide invariant). The rapid geoacoustic characterization algorithm uses a homogeneous single-sediment layer overlying a hard acoustic basement model to optimally match the predicted acoustic observables with those estimated from data. The approach is presented here and is applied to the range-dependent benchmark cases TC1 and TC2 from the Inversion Techniques Workshop held in Gulfport, MS, in May 2001. The technique successfully reproduces the acoustic observables and estimates the sediment sound-speed, density, and attenuation profiles, as well as the sediment thickness.     

Estimating geoacoustic parameters using matched-field inversion methods

In this paper, we use matched-field inversion methods to estimate the geoacoustic parameters for three synthetic test cases from the Geoacoustic Inversion Techniques Workshop held in May 2001 in Gulfport, MS. The objective of this work is to use a sparse acoustic data set to obtain estimates of the parameters as well as an indication of their uncertainties. The unknown parameters include the geoacoustic properties of the sea bed (i.e., number of layers, layer thickness, density, compressional speed, and attenuation) and the bathymetry for simplified range-dependent acoustic environments. The acoustic data used to solve the problems are restricted to five frequencies for a single vertical line array of receivers located at one range from the source. Matched-field inversion using simplex simulated annealing optimization is initially used to find a maximum-likelihood (ML) estimate. However, the ML estimate provides no information on the uncertainties or covariance associated with the model parameters. To estimate uncertainties, a Bayesian formulation of matched-field inversion is used to generate posterior probability density distributions for the parameters. The mean, covariance, and marginal distributions are determined using a Gibbs importance sampler based on the cascaded Metropolis algorithm. In most cases, excellent results were obtained for relatively sensitive parameters such as wave speed, layer thickness, and water depth. The variance of the estimates increase for relatively insensitive parameters such as density and wave attenuation, especially when noise is added to the data.     

双船双方位角地震探测技术方法及应用效果

南海东沙海域的陆坡台地区, 浅表断裂及生物礁发育众多, 浅层新生界多为碳酸盐岩层, 中深层的中生界内部构造复杂, 断裂较多, 常规单船窄方位地震剖面显示中深层反射品质较差。利用双船可以设计灵活的观测系统, 形成双方位角地震探测方式, 对于地下同一反射点的照明, 可以获得比窄方位角探测更好的效果, 也可以改善高速屏蔽层下方、高陡倾角斜层等区域的成像质量。文章通过重点技术的攻关研发, 进行了单源激发、双缆接收的双方位角采集试验, 成功实现该方法在南海海域中深层地震勘探中的应用。     

Benchmarking geoacoustic inversion methods using range-dependent field data

Over the past decade, inversion methods have been developed and applied to acoustic field data to provide information about unknown ocean-bottom environments. An effective inversion must provide both an estimate of the bottom parameters and a measure of the uncertainty of the estimated values. This paper summarizes results from the Office of Naval Research (ONR)/Space and Naval Warfare Systems Command (SPAWAR) Geoacoustic Inversion Techniques Workshop, test cases 4 and 5. The workshop was held to benchmark present-day inversion methods for estimating geoacoustic profiles in shallow water. The format of the workshop was a blind test to estimate unknown geoacoustic profiles by inversion of measured acoustic transmission loss data in octave bands and reverberation envelopes. The data sets for test cases 4 and 5 were taken at two locations in shallow water, one in the East China Sea and the other along the southwest coast of Florida. The limitations of the data and the limits to the knowledge of the sites are discussed. In both cases, impulsive sources were used in conjunction with air-deployed sonobuoys. Since the measured data was incoherent, only methods consistent with total energy matching were applicable. Comparisons between the different inversion techniques presented at the workshop are discussed. For test cases 4 and 5, a precise metric was unavailable for comparison.     

Comparison of high-resolution seismic profiles and the geoacoustic properties of Eckernförde Bay sediments

High-resolution seismic profiles of Eckernförde Bay and the adjacent Baltic Sea were collected, and the geoacoustic properties of sediments there were measured. Bulk densities averaged ~ 1.35 g cm^–3 and ranged from ~ 1.2 to ~ 1.7 g cm^–3. Compressional wave velocities in gas-free sediments averaged ~ 1460 m s^–1 and ranged from ~ 1425 to ~ 1555 m s^–1. In nongassy sediments, bulk density variations typically controlled changes of acoustic impedance. Impedance changes were usually too small and closely spaced to be resolved seismically, although, at certain sites, significant impedance changes are far apart enough that they correlate one-to-one with seismic reflectors. Where free gas is present, velocity decreases and wave energy is scattered, causing a prominent seismic reflector.     

Matched-field geoacoustic inversion of low-frequency source tow data from the ASIAEX East China Sea experiment

Geoacoustic inversion results based on data obtained during the Asian Seas International Acoustics Experiment (ASIAEX) 2001 East China Sea experiment are reported. The inversion process uses a genetic-algorithm-based matched-field-processing approach to optimize the search procedure for the unknown parameters. Inversion results include both geometric and geoacoustic variables. To gauge the quality of the inversion, two different analyses are employed. First, the inversion results based upon discrete source-receiver ranges are confirmed by continuous source localization over an interval of time. Second, separate inversions at many different ranges are carried out and the uncertainties of the parameter estimation are analyzed. The analysis shows that both methods yield consistent results, ensuring the reliability of inversion in this study.     

Perturbative inversion of geoacoustic parameters in a shallow waterenvironment

In many strategic shallow water areas, the geoacoustic properties of the sub-bottom are largely unknown. This paper demonstrates that inverse theory and measured data from a single hydrophone can be used to accurately deduce the geoacoustic properties of the sub-bottom, even when the initial background geoacoustic model is a highly inaccurate estimate. Since propagation in shallow water is very sensitive to the geoacoustic properties of the sub-bottom the inverse technique is a vitally important, practical, and inexpensive means to improve sonar performance prediction in a potentially hostile environment. To provide ground truth for the inverse technique, measured data collected during Project GEMINI were compared to the inverse solutions. Detailed, site-specific geoacoustic models were developed for two array locations and the finite-element parabolic equation (FEPE) model was used to estimate transmission loss (TL). The model estimates from FEPE compared well with the measured data and the detailed geoacoustic models were considered as “ground truth.” To test the efficacy of the technique, initial background geoacoustic models were constructed assuming no a priori information of the bottom. The resultant inverse solution was used to predict the geoacoustic properties at each of the sites. The final results were in excellent agreement with the measured data and the resulting TL estimates derived from the inverse technique were as good or better than the TL estimates obtained from the detailed, site-specific geoacoustic models     

海啸地震震源机制解统计

为探索海啸地震发震时的规律,统计分析了1976-2010年的美国全球CMT地震矩心矩张量数据,并结合美国海洋与大气局的海啸灾害数据库,从海啸事件和较大海啸事件两个方面对震源机制解展开了对比统计分析,给出了海啸地震在震源方面的统计特征。     

天然气水合物准三维地震调查面元参数优化研究

在我国某海域对天然气水合物研究过程中,使用了单源单缆准三维地震勘探方法,获得了天然气水合物矿体较好的三维成像效果.影响天然气水合物矿体三维成像的因素很多,其中,面元参数的选取是最直接的影响因素之一.本文分析了大量试验数据,结果表明,在确定了电缆的排列长度为2400m(192道)的前提下,面元横向最佳优化尺寸应为50m,处理面元为原始采集面元大小的1/2(即面元大小为12.5mx25m)时,可以兼顾地层的真实性和连续性.以此为基础可获得较高质量的准三维地震处理资料.     

Marine 3D seismic volumes from 2D seismic survey with large streamer feathering

Marine Geophysical Research - Strong ocean current influences a marine seismic survey and forces the streamer off-course from the survey line. The sideway drift of the streamer results in that the...     

福建沿海港口建设中的活动断裂与地震安全

本文研究了福建沿海港湾的活动断裂与地震安全问题,结果表明,福建沿海地区NE、NW向断裂发育,晚更新世以来都具有明显的张性活动,严格地控制了沿海湾的形成和发育,福建沿海地区历史上地震活动频繁,发生过多次破坏性地震。因此,在港口建设中应认真研究活动断裂与地震安全问题。     

A seismic reconnaissance survey of the northern Congo Fan

To study the time-varying influence of the Congo River and the Benguela Current on the deposition at the Angola Continental Margin, a high-resolution reflection seismic survey was carried out on the northern Congo Fan. Four seismostratigraphic units have been defined for the upper 800 m (1000 ms TWT) of the data. The units record different depositional environments, ranging from pre-establishment of the Congo River drainage system to the influence of the Benguela Current. An indication of a general change in the turbidite system is provided by a shift in channel distribution and a relocation of the depocentre of coarse material. The ascent of salt is recorded up to the Pliocene. Gas that has migrated out of Lower Cretaceous shales and that was produced from large quantities of organic matter in the younger sediments can be found on the flanks and on top of the salt domes. In a few places, this gas even ascends to the ocean floor along structural pathways through the topmost unit.     

Time-domain geoacoustic inversion of high-frequency chirp signal from a simple towed system

An inversion method using a towed system consisting of a source and two receivers is presented. High-frequency chirp signals that have been emitted from the source are received after multiple penetrations and reflections from the shallow water sub-bottom structure and are processed for geoacoustical parameter estimation. The data are processed such that a good resolution and robustness is achieved via matched filtering, which requires information about the source signal. The inversion is formulated as an optimization problem, which maximizes the cost function defined as a normalized correlation between the measured and modeled signals directly in the time domain. The very fast simulated reannealing optimization method is applied to the global search problem. The modeled time signal is obtained using a ray approach. An experiment was carried out in the Mediterranean Sea using a towed source and receiver system. The inversion method is applied to the experimental data and results are found to be consistent with previous frequency-domain analyses using measurements from a towed horizontal array of receivers and measurements on a vertical array.     

Quantitative seafloor characterization using a bathymetric sidescansonar

Bathymetry and backscatter measurements from a 120-kHz phase-difference sonar are analysed in terms of statistical and spectral characteristics. Data from a multisensor, multiscale survey of the Juan de Fuca Ridge are compared across three distinct geological provinces: sediment pond, ridge flank, and axial valley. The detrended bathymetry follows a Gaussian distribution; the power spectral density can be approximately described by a power law. The composite multiscale power spectrum demonstrates a similar slope spanning a spatial frequency range from about 0.005 to 50 cycles/m, corresponding to a range of geological features from a few hundred meters down to several centimeters. The backscattering strength and grazing-angle dependencies agree with previous empirical studies; data from a sediment-pond region are shown to match theoretical predictions of the composite-roughness model. Histograms of the echo amplitude are characterized by a multimodal Rayleigh probability density function. For all analyses, the data show distinct differences among the three provinces     

A high-resolution seismic profiling system using a deep-towed horizontal hydrophone streamer

A seismic reflection profiling system utilising a surface air gun source and a deep-towed horizontal hydrophone streamer has been developed for high resolution studies in the deep ocean. The instrument is deployed on a conventional armoured single conductor cable at depths of up to 6 km. Seismic data from the 30 m long streamer is wide-band frequency modulated up the towing cable to the ship together with a high frequency monitor from a 3.5 kHz echo-sounder mounted on the instrument package. The geometry of the system allows an order of magnitude improvement in spatial resolution compared with that obtained from standard surface source/receiver configurations. The summed hydrophones of the streamer attenuate cable-generated mechanical noise, and the 3.5 kHz sea-surface and bottom reflected returns provide receiver positioning information. The system has been successfully deployed at depths of 5 km in the Vema Fracture Zone in the North Atlantic, and although initially difficulties were experienced in balancing the streamer, subsequent profiles across the transform fault show details of sub-bottom structure which on conventional surface records are generally masked by diffraction hyperbolae.     

An implosive seismoacoustic source for seismic experiments on the ocean floor

Bottom shots have been used for a number of years in seismic studies on the ocean floor. Most experiments utilized explosives as the energy source, though researchers have recognized the usefulness of collapsing water voids to produce seismoacoustic signals. Implosive sources, however, suffered generally from a lack of control of source depth. We present a new experimental tool, called SEEBOSEIS, to carry out seismic experiments on the seafloor utilizing hollow glass spheres as controlled implosive sources. The source is a 10-inch BENTHOS float with penetrator. Inside the sphere we place a small explosive charge (two detonators) to destabilize the glass wall. The time of detonation is controlled by an external shooting device. Test measurements on the Ninetyeast Ridge, Indian Ocean, show that the implosive sources can be used in seismic refraction experiments to image the subbottom P-wave velocity structure in detail beyond that possible with traditional marine seismic techniques. Additionally, the implosions permit the efficient generation of dispersed Scholte waves, revealing upper crustal S-wave velocities. The frequency band of seismic energy ranges from less than 1 Hz for Scholte modes up to 1000 Hz for diving P-waves. Therefore, broadband recording units with sampling rates >2000 Hz are recommended to sample the entire wave field radiated by implosive sources.     

Localization of a broadband source using a matched-mode procedurein the time-frequency domain

In this paper, we present a nonconventional matched-mode procedure for localizing a broadband source in the time-frequency domain. This hybrid coherent and incoherent approach exploits both the temporal and spatial characteristics of the multimode arrival structure at a receiving sensor array. In the previous work, a time-domain technique was developed to deal with narrowband signals coherently. It consists of the following three steps. The first step employs a receiving sensor array to separate the modes by the conventional modal filtering approach. The second step is to estimate the energy and relative arrival times of the various modes which arrive at the receiver. The last step uses the differences of modal travel times to estimate the source range, and uses the ratios of modal energies to estimate the source depth. Here, we employ bandpass filters to divide the received broadband signal into several subfrequency bands, and apply the first and second steps of the previously developed coherent narrowband technique to the subfrequency bands in the time domain. The results obtained from subfrequency bands are then combined incoherently in the frequency domain to produce an estimate of the source position. Numerical simulation of an experiment with explosive sources at the shallow water site of the Yellow Sea is presented