VLF propagation loss to a buried seismometer

The propagation loss from a shallow underwater explosion to a buried vertically polarized seismometer over the frequency band between 3 and 15 Hz was measured during the 1983 Ngendei Experiment. The data was recorded in 5.5-km-deep water in the southwest Pacific with a triaxial borehole seismograph emplaced 50 m within the basaltic basement. It is found that the average power decays as r^-3 (r is slant range) beyond 30 km and that the propagation loss is minimal between 6-9 Hz. At shorter ranges, the propagation loss is more complicated and exhibits a stronger frequency dependence. Power in the first water wave is estimated for both buried and ocean-bottom sensors. The ocean-bottom power is approximately 7 dB higher than that recorded by the buried sensor, and both exhibit similar decay rates     

Ocean-bottom seismometer observations of seismic activity at Loihi Seamount,Hawaii

This study reports the result of deep ocean-bottom seismometer recording of an undersea volcanic event in progress. An array of five three-component, isolated sensor ocean-bottom seismometers (ISOBS) was deployed for 28 days on the summit and flanks of Loihi Seamount, Hawaii, to monitor seismicity. The deployment was prompted by reports from the Hawaiian Volcano Observatory (HVO) of a swarm of small-magnitude events located beneath the active submarine volcano in late September, 1986. Monitoring of this earthquake swarm by the University of Hawaii commenced 1 October 1986. Although seismicity tapered off rapidly after 11 October, more than 200 events were located. Systematic changes in spatial clustering during the initial swarm activity suggest changing patterns of stress within this crustal volume, possibly due to induced stress resulting from magma movement in the underlying crust or deep portions of the edifice.Most of the very shallow (< 10 km) events were located beneath the summit and southwest flank of the elongate edifice. No shallow tremor was observed despite a search through the data, although such tremor may have ceased prior to deployment of the ocean-bottom seismometers (OBS). Constraints on the association between seismicity and observed topographic and tectonic elements of Loihi are also of primary importance. Many of the earthquakes located near the steep flanks generated rock falls that were recorded on the OBSs. This is consistent with the results of dredge and bottom photography data indicating that the flanks are covered with fragments of shattered lava pillows and flows. Dike intrusion and mass wasting are major influences on the morphology of Loihi. Intact flows have been observed near the deep portion of the south rift zone; however, few events were located in that region during this swarm.     

An ocean bottom,microprocessor based seismometer

We describe the design and construction of an ocean bottom seismometer configured as a computer, based on an Intersil IM6100 microprocessor plus appropriate peripheral devices. The sensors consist of triaxial 1 Hz seismometers and a hydrophone, each sensor channel being filtered prior to digitizing so that typical noise spectra are whitened. Digital data are recorded serially on magnetic tape. The instrument is placed on the ocean bottom by allowing it to fall freely from just below the surface. An acoustic system allows precise determination of instrument position, acoustic recall, and transmission of operational information to the surface. Release from an expendable anchor is accomplished by redundant pyrotechnic bolts which can be fired by acoustic command or by precision timers.The operational flexibility provided by the micro-computer, which executes the DEC PDP8/E instruction set, enables optimum use of the 6-hr recording capacity (at 128 samples/second/channel) in the context of the particular experiment being performed.

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Deep structure of the transition zone from the Baltic shield to the Barents Sea basin based on seismic data

Results of the analysis and interpretation of the records of 17 ocean bottom seismometers designed at the Shirshov Institute of Oceanology, Russian Academy of Sciences (a three-component geophone and a hydrophone), installed with an interval of 10–20 km along a profile in the transition zone from the Baltic shield to the Barents Sea basin are presented. The studies were carried out in 1995 from R/V Professor Kurentsov. An air gun with a chamber volume of 80 1 was used as the source of seismic waves with a shooting interval of 250 m. The longest range of records of deep refracted and wide-angle reflected waves (up to 300 km) was reached with the hydrophones. Two-dimensional seismic modeling allowed us to refine the earlier versions of the seismic cross section of the earth’s crust and uppermost mantle in the study region. New data confirmed that, in the central area of the Barents Sea, the “granitic-metamorphic” layer of the crust with a seismic velocity of 6.2 km/s typical of the Baltic Shield is absent. In this region, a thin consolidated crust with a seismic velocity of 6.8 km/s is covered with a thick (more than 25 km) sedimentary layer. In this layer, a local low-velocity zone probably exists, which causes a strong attenuation of the “crustal” waves.     

南海北部潮汕坳陷海区海底地震仪调查实验

潮汕坳陷被认为是一个具有良好油气前景的中生代残留沉积坳陷,其中生代地层也被新近的钻井证实。为研究其盆地深部构造,“十五”863课题跨越该区进行了深地质调查。调查采用5台国产海底地震仪记录深部地震资料。处理结果显示本次调查清楚地记录到了来自地壳内部和Moho面的震相,这是国产海底地震仪在南海地区的首次成功实践。海底地震仪记录揭示沿测线的地壳在南海形成过程中减薄程度较低,中生代地层速度较高,代表致密的岩石,这些因素可能不利于油气的储集,需要在勘探中避开。     

Beam patterns of an underwater acoustic vector hydrophone located away from any reflecting boundary

A vector hydrophone is composed of two or three spatially collocated but orthogonally oriented velocity hydrophones plus an optional collocated pressure hydrophone. A vector hydrophone may form azimuth-elevation beams that are invariant with respect to the sources' frequencies, bandwidths and radial location (in near field as opposed to the far field). This paper characterizes the spatial matched filter beam patterns (a.k.a. fixed or conventional or maximum signal-to-noise ratio beam patterns) and the minimum variance distortionless response (MVDR) beam patterns associated with a single underwater acoustic vector hydrophone distant from any reflecting boundary.     

A three-component ocean bottom seismograph for controlled source and earthquake seismology

A three-component pop-up ocean-bottom seismograph was built at the Institute of Oceanographic Sciences in 1978. It is constructed around a buoyant 71 cm diam aluminium alloy forged sphere which contains three 4.5 Hz orthogonal geophones and an external hydrophone. The instrument will record continuously in analogue mode for over eight days using a modified reel-to-reel tape-recorder running at 1.5 mm s^-1. The geophones have a bandwidth of 2–25 Hz and the hydrophone bandwidth is 5–40 Hz. Ballast release is by pre-set clock or by acoustic command.Fifty-four deployments have been carried out in five cruises for the loss of only one instrument. Good recordings of dropped weights, airguns, explosions and earthquakes have been obtained.     

Twelve years of tracking 52-Hz whale calls from a unique source in the North Pacific

A unique whale call with 50–52 Hz emphasis from a single source has been tracked over 12 years in the central and eastern North Pacific. These calls, referred to as 52-Hz calls, were monitored and analyzed from acoustic data recorded by hydrophones of the US Navy Sound Surveillance System (SOSUS) and other arrays. The calls were noticed first in 1989, and have been detected and tracked since 1992. No other calls with similar characteristics have been identified in the acoustic data from any hydrophone system in the North Pacific basin. Only one series of these 52-Hz calls has been recorded at a time, with no call overlap, suggesting that a single whale produced the calls. The calls were recorded from August to February with most in December and January. The species producing these calls is unknown. The tracks of the 52-Hz whale were different each year, and varied in length from 708 to 11,062 km with travel speeds ranging from 0.7 to 3.8 km/h. Tracks included (A) meandering over short ranges, (B) predominantly west-to-east movement, and (C) mostly north-to-south travel. These tracks consistently appeared to be unrelated to the presence or movement of other whale species (blue, fin and humpback) monitored year-round with the same hydrophones.     

台湾海峡西南部的海陆联合深地震探测资料特点与处理对策*

为调查长乐—南澳断裂带和滨海断裂带的展布、形态和深部构造背景, 福建省地震局使用大容量气枪震源和海底地震仪将深地震探测拓展到台湾海峡西南部, 配合陆上的水库气枪震源、吨级爆破点和流动地震仪实现了海陆双向激发接收。文章详细介绍了2013—2015年间在台湾海峡西南部采集的6条二维广角地震剖面的观测系统、采集参数和数据预处理方法。对资料的整理分析表明: 震源激发参数和仪器接收点位选择合理, 大部分共接收点道集记录能清晰地识别出Pg、PmP、Pn、Sg、SmS等震相; 通过气枪固定点多次激发进行叠加的方法, 获得了信噪比相当于吨级爆破的共炮点道集记录。陆上台站数据品质较佳, 而海底地震仪数据信噪比较低, 可能是由于海底吸收衰减较为严重。数据处理中针对台湾海峡西南部沉积层速度较低且基底面起伏剧烈的问题, 将共接收点域拾取的走时分选到炮域, 避免了表层改正残差造成深部构造假象的问题。     

Measurements of ambient noise spectra in the south Norwegian Sea

Measurements have been made of ambient sea noise north of the Shetland Islands in the southern part of the Norwegian Sea. Shore-terminated, omnidirectional hydrophones, bottomed in 240 and 350 fathoms, were used. Twenty minute samples of single hydrophone outputs were recorded during daylight hours about every 3 days over a 4-year period from the summer of 1957 to 1961. Amplitude levels were measured at selected frequencies in the band 30 cps to 1 kc. Matching observations of wind and weather were coordinated with the noise data. The noise spectrum was found to be relatively flat compared with the spectrum measured in deep ocean areas of the western North Atlantic. Also, the pressure levels at frequencies above 100 cps are considerably higher. Seasonal changes were observed in the spectrum below 400 cps. The annual excursion in level from winter high to midsummer low was about 5 db at 50 cps and 3.5 db at 100 cps. Although no seasonal changes occurred in the upper spectrum, day-to-day variations in level at the higher frequencies generally followed patterns of change in local weather. There was good correlation between wind velocity and ambient noise at 800 and at 1000 cps. The character of sea-noise in the area seems to be shaped to two factors. First, a combination of thermal structure and a very gradual bottom slope result in high propagation losses. This in turn restricts the acoustic access at a bottomed hydrophone to a localized field. Second, except for a few summer calms, weather in the islands is severe, and strong winds and high sea states are sustained throughout the year.     

Closed-form underwater acoustic direction-finding with arbitrarilyspaced vector hydrophones at unknown locations

This paper introduces a novel ESPRIT-based closed form source localization algorithm applicable to arbitrarily spaced three-dimensional arrays of vector hydrophones, whose locations need not be known. Each vector hydrophone consists of two or three identical but orthogonally oriented velocity hydrophones plus one pressure hydrophone, all spatially co-located in a point-like geometry. A velocity hydrophone measures one Cartesian component of the incident sonar wavefield's velocity-vector, whereas a pressure hydrophone measures the acoustic wavefield's pressure. Velocity-hydrophone technology is well established in underwater acoustics and a great variety of commercial models have long been available. ESPRIT is realized herein by exploiting the nonspatial inter-relations among each vector hydrophone's constituent hydrophones, such that ESPRIT's eigenvalues become independent of array geometry. Simulation results verify the efficacy and versatility of this innovative scheme     

Closed-form underwater acoustic direction-finding with arbitrarilyspaced vector hydrophones at unknown locations

This paper introduces a novel ESPRIT-based closed-form source localization algorithm applicable to arbitrarily spaced three-dimensional arrays of vector hydrophones, whose locations need not be known. Each vector hydrophone consists of two or three identical but orthogonally oriented velocity hydrophones plus one pressure hydrophone, all spatially co-located in a point-like geometry. A velocity hydrophone measures one Cartesian component of the incident sonar wavefield's velocity vector, whereas a pressure hydrophone measures the acoustic wavefield's pressure. Velocity-hydrophone technology is well established in underwater acoustics and a great variety of commercial models have long been available. ESPRIT is realized herein by exploiting the nonspatial interrelations among each vector hydrophone's constituent hydrophones, such that ESPRIT's eigenvalues become independent of array geometry. Simulation results verify the efficacy and versatility of this innovative scheme     

Real-time geophysical measurements on the deep seafloor usingsubmarine cable in the southern Kurile subduction zone

A permanent real-time geophysical observatory using a submarine cable was developed and deployed to monitor seismicity, tsunamis, and other geophysical phenomena in the southern Kurile subduction zone. The geophysical observatory comprises six bottom sensor units, two branching units, a main electro-optical cable with a length of 240 km and two land stations. The bottom sensor units are: 1) three ocean bottom broadband seismometers with hydrophone; 2) two pressure gauges (PGs); 3) a cable end station with environmental measurement sensors. Real-time data from all the undersea sensors are transmitted through the main electro-optical cable to the land station. The geophysical observatory was installed on the continental slope of the southern Kurile trench, southeast Hokkaido, Japan in July 1999. Examples of observed data are presented. Sensor noises and resolution are mentioned for the ocean bottom broadband seismometers and the PGs, respectively. An adaptable observation system including very broadband seismometers is scheduled to be connected to the branching unit in late 2001. The real-time geophysical observatory is expected to greatly advance the understanding of geophysical phenomena in the southern Kurile subduction zone     

Ocean bottom seismograph development at Hawaii Institute of Geophysics

Three distinct ocean bottom seismograph (OBS) systems have been developed at the Hawaii Institute of Geophysics to satisfy the different requirements for short-range refraction and anisotropy experiments, long-range refraction experiments, and short-term and semi-permanent monitoring for earthquakes. One system, originally designed for semi-permanent use in conjunction with a monster buoy of the IDOE North Pacific Experiment has been modified for emplacement off Oahu. It contains 3-component 1 Hz seismometers and a hydrophone and obtains power and transmits data via tow conductor cable. Two additional systems were designed for short-term use: a 2 Hz telemetering system (TOBS); and 4.5 Hz free-fall pop-up system (POBS). The TOBS contains 3-component seismometers and a hydrophone and transmits data to the ship via light-weight single-conductor electromechanical cable and an HF-VHF radio link from a surface buoy. The bottom package also includes a backup tape recorder. This system exhibits the advantages of real-time data acquisition (e.g. precise timing, rapid appraisal of data quality, optimum use of explosives, and common recording with other data) and the complexities and difficulties associated with a deep-sea mooring. However, use of cable with near neutral bouyancy permits the design of a deep-water system with low weights and stress levels. The POBS is a self-contained package containing a vertical and single horizontal seismometer, hydrophone, cassette tape recorder, and pre-set timed release. This system is relatively simple and inexpensive. Total weight of 150 kg in air (before launch) permits emplacement and retrieval from a ship with no special equipment by two (strong) persons. Experience to data suggests that the optimum deployment scheme for many studies is a combination of TOBS's and POBS's.Hawaii Institute of Geophysics Contribution 835.     

仿生水听器设计、制造与测试

根据仿生鱼类侧线细胞纤毛原理,设计、制造并测试了一种新型双纤毛压阻水听器,用于检测水下声信号.与传统的压电水听器相比,这种水听器具有体积小和矢量性的优点.利用聚氨酯仿生细胞壁作为透声帽,利用硅油模拟细胞液和压阻微梁模拟感觉纤毛三层拾振结构,从而提高纤毛式水听器的可靠性.封装以后的纤毛式水听器采用振动台和矢量水听器校准系...     

Large aperture seismic imaging at a convergent margin: Techniques and results from the Costa Rica seismogenic zone

In March and April 1995 a cooperative German, Costa Rican, and United States research team recorded onshore-offshore seismic data sets along the Pacific margin of Costa Rica using the R/V Ewing. Off the Nicoya Peninsula we used a linear array of ocean bottom seismometers and hydrophones (OBS/H) with onshore seismometers extending across much of the isthmus. In the central area we deployed an OBS/H areal array consisting of 30 instruments over a 9 km by 35-km area and had land stations on the Nicoya Peninsula adjacent to this marine array and also extending northeast on the main Costa Rican landmass. Our goal in these experiments was to determine the crustal velocity structure along different portions of this convergent margin and to use the dense instrument deployments to create migrated reflection images of the plate boundary zone and the subducting Cocos Plate. Our specific goal in the central area was to determine whether a subducted seamount is present at the location of the 1990, M 7 earthquake off the Nicoya Peninsula and can thus be linked to its nucleation. Subsequently we have processed the data to improve reflection signals, used the data to calculate crustal velocity models, and developed several wide-aperture migration techniques, based on a Kirchhoff algorithm, to produce reflection images. Along the northern transect we used the ocean bottom data to construct a detailed crustal velocity model, but reflections from the plate boundary and top and bottom of the subducting Cocos plate are difficult to identify and have so far produced poor images. In contrast, the land stations along this same transect recorded clear reflections from the top of the subducting plate or plate boundary, within the seismogenic zone, and we have constructed a clear image from this reflector beneath the Nicoya shelf. Data from the 3-D seismic experiment suffer from high-amplitude, coherent noise (arrivals other than reflections), and we have tried many techniques to enhance the signal to noise ratio of reflected arrivals. Due to the noise, an apparent lack of strong reflections from the plate boundary zone, and probable structural complexity, the resulting 3-D images only poorly resolve the top of the subducting Cocos Plate. The images are not able to provide compelling evidence of whether there is a subducting seamount at the 1990 earthquake hypocenter. Our results do show that OBS surveys are capable of creating images of the plate boundary zone and the subducting plate well into the seismogenic zone if coherent reflections are recorded at 1.8 km instrument spacing (2-D) and 5 km inline by 1 km crossline spacing for 3-D acquisition. However, due to typical high amplitude coherent noise, imaging results may be poorer than expected, especially in unfavorable geologic settings such as our 3-D survey area. More effective noise reduction in acquisition, possibly with the use of vertical hydrophone arrays, and in processing, with advanced multiple removal and possibly depth filtering, is required to achieve the desired detailed images of the seismogenic plate boundary zone.     

A simple deep-towed vertical array for high-resolution reflection seismic profiling

A simple, low cost, deep-towed system for high-resolution reflection seismic profiling is described. It consists of a vertical array with two hydrophones having a separation of 2.2 m and rigidly mounted onto streamlined tow bodies. Improvement of the signal-to-noise ratio is attained by simple stacking of the hydrophone outputs after signal conditioning and travel time corrections. The suppression of side echoes and surface reflections is achieved by an analog procedure which in effect improves the directional characteristics of the array. A circuit for automatic gain control is included to enhance weak signals as well as to suppress ringing.Results in Kiel Bay and over the crest of the Jan Mayen Ridge (northern Atlantic) suggest that this simple vertical array may supplement air gun systems better than conventional, surface pinger-type equipment.Institute of Geophysics     

High-frequency ambient noise and its impact on underwater tracking ranges

A theoretical model for the vertical directionality and depth dependence of high frequency (8 to 50 kHz) ambient noise in the deep ocean is developed. The anisotropic noise field at a variety of depths and frequencies is evaluated and displayed. It was found that at high frequencies and deep depths, a bottom-mounted hydrophone receives the maximum noise energy from overhead rather than from the horizontal. This leads to the consideration of an oblate hydrophone receiving response pattern for underwater tracking ranges that would provide a constant signal-to-noise ratio (SNR) for an acoustic source located anywhere in a circular area centered above the hydrophone. Two of the desirable characteristics of this type of pattern are the increase in receiving range of a bottom-mounted sensor and the decrease of the dynamic range of signals that a signal processor must handle.     

Detection of small earthquakes along the Pacific-Antarctic Ridge from T-waves recorded by abyssal ocean-bottom observatories

A number of seismoacoustic T-wave events were observed between January 2003 and January 2004 by broadband ocean-bottom seismometers installed on the French Polynesia seafloor at depths of 4,000?C5,000?m, well below the conjugate depth of the SOFAR channel. Using T-wave arrival times, we located 89 T-wave events along the Pacific-Antarctic Ridge. Among these, 63 events were not detected by land-based seismic observations of the United States Geological Survey (USGS), which was nearly twice the number of earthquakes reported by the USGS in the area during the observation period. We used a simple method to estimate earthquake magnitude from T-wave energy. The magnitudes of the events unidentified by the USGS ranged from 2.3 to 5.5, whereas magnitudes of the earthquakes reported by the USGS ranged from 4.1 to 6.2. Our study suggests that T-wave observations with abyssal ocean-bottom seismometers can improve the detection of small earthquakes and help our understanding of the seismotectonics of remote oceanic areas.     

Extended-aperture underwater acoustic multisource azimuth/elevationdirection-finding using uniformly but sparsely spaced vector hydrophones

Aperture extension is achieved in this novel ESPRIT-based two-dimensional angle estimation scheme using a uniform rectangular array of vector hydrophones spaced much farther apart than a half-wavelength. A vector hydrophone comprises two or three spatially co-located, orthogonally oriented identical velocity hydrophones (each of which measures one Cartesian component of the underwater acoustical particle velocity vector-field) plus an optional pressure hydrophone. Each incident source's directions-of-arrival are determined from the source's acoustical particle velocity components, which are extracted by decoupling the data covariance matrix's signal-subspace eigenvectors using the lower dimensional eigenvectors obtainable by ESPRIT. These direction-cosine estimates are unambiguous but have high variance; they are used as coarse references to disambiguate the cyclic phase ambiguities in ESPRIT's eigenvalues when the intervector-hydrophone spacing exceeds a half-wavelength. In one simulation scenario, the estimation standard deviation decreases with increasing intervector-hydrophone spacing up to 12 wavelengths, effecting a 97% reduction in the estimation standard deviation relative to the half-wavelength case. This proposed scheme and the attendant vector-hydrophone array outperform a uniform half-wavelength spaced pressure-hydrophone array with the same aperture and slightly greater number of component hydrophones by an order of magnitude in estimation standard deviation. Other simulations demonstrate how this proposed method improves underwater acoustic communications link performance. The virtual array interpolation technique would allow this proposed algorithm to be used with irregular array geometries