MODELLING OF GI GUN SIGNATURES1

In 1989 a new type of marine seismic source was introduced. This new air-gun, which consists of two air chambers instead of one, is called the GI gun. The main feature of this gun is that the bubble created by the gun is stabilized by an injection of extra air from the second chamber at a later time. This injection mechanism reduces the amplitude of the bubble oscillations, which also means that the acoustic signal from a GI gun shot is characterized by a very clean primary pulse followed by very small bubble oscillations. A method for calculating the acoustic signal generated by a GI gun is presented. Based on the solution of a damped Kirkwood–Bethe equation, the far-field pressure of single GI guns and of arrays of GI guns is calculated. It is shown that the optimal values for injection start time and injection period vary with injector volume and gun depth. It is also shown that the precision in the firing time for the injector should be of the order of 4 ms, while the precision of the injection period should be of the order of 8 ms. Modelled and measured far-field signatures have been compared, and the relative error energy is found to be less than 3.5% for all examples.     

A PULSED PLASMA JET ACOUSTIC SOURCE FOR PROFILING THE OCEAN FLOOR1

A plasma gun source has been successfully used to obtain sub-bottom profiles. The profiles show better penetration than with a 3-5 kHz source and more resolution than with an air gun. The plasma gun source is compact, self-contained, and requires no complex auxiliary equipment. The device was deployed from large research vessels and a small boat. The plasma gun produces sufficient acoustic energy and with its characteristically short pulse length and broad bandwidth it is an attractive sub-bottom profiler, especially in shallow fresh or salt water.     

鄂霍次克微板块东部俯冲带区域地震b值及应力场特征

古登堡-里克特震级-频度关系式中的b值与剪切应力(或偏应力)大小被认为存在着负相关的关系,因此b值常被用作估算区域应力大小的指标.本文利用1970-2018年鄂霍次克微板块东部俯冲带区域的地震目录,使用最大似然法对该区域的b值进行空间扫描,得到了该区域沿海沟走向不同区域及不同深度的b值分布,进而调查与分析其应力状态及地球动力学特征.结果显示不同俯冲区域的b值分布具有4个共同特征:1)地壳范围内的高b值特征,表明其剪切应力较低;2)俯冲板片与上覆板块耦合强烈的区域b值较低,表明该位置剪切应力较高;3)弧前区域b值较高,表明其剪切应力较低;4)海沟东侧的太平洋板块与软流层接触的区域b值较高,表明该位置剪切应力较低.上述这些b值分布结果及其剪切应力分布,是能够与俯冲带的地球动力学结构与特征相关联的.我们也发现在水平方向上,在浅部区域(0~40km深度范围),勘察加地区的剪切应力相对于北日本地区和千岛地区更低;在更深的区域(40~80km深度范围),千岛地区弧前区域的剪切应力较低.由本文俯冲带区域力平衡估算得到的俯冲接触面上的剪应力大小能够解释鄂霍次克微板块东部俯冲带不同接触界面上的b值大小及其分布差异.本文得到的日本海沟附近区域平均b值在大地震前后的时间变化揭示了该区域震间、震前和震后的应力演化过程.     

Sublethal effects of cupric ion activity on the grazing behaviour of three calanoid copepods

This research (1) characterized the effects of sublethal cupric ion activities on the grazing behavior of two estuarine copepods (Acartia tonsa, Acartia hudsonica) and one nearshore, neritic copepod (Temora longicornis) and (2) compared the sensitivity of short-term sublethal behavioral assays with that of longer-term acute toxicity tests. A nitrilotriacetate-trace-metal-ion buffer system at 27‰. S was used to quantify and control the free cupric ion activity. Acute toxicity tests were used to determine the mortality of A. tonsa and T. longicornis over 72 h within the approximate cupric ion activity range of 10⁻¹³ to 10^−9.5 M. 24 h survival was not affected within the approximate cupric ion activity range of 10⁻¹³ to 10^−9.7 M, the range used for subsequent grazing activity experiments after 24 h exposure to Cu. Grazing activity was significantly diminished at cupric ion activities of ≈ 10⁻¹⁰ M for A. tonsa and T. longicornis, and at ≈ 10⁻¹¹ M for A. hudsonica. A hormetic pattern of response in feeding activity was observed with A. tonsa and T. longicornis. Grazing activity was found to be a sensitive measure of sublethal Cu stress compared with the acute toxicity tests. Grazing activity was affected at environmentally relevant cupric ion activities.     

TBT toxicity on the marine microalga Nannochloropsis oculata

Commercial antifouling formulations containing TBT are the major source of organotin contamination in coastal waters. In view of the persisting TBT residues (13 ng Sn l⁻¹) in the coastal waters of South Korea, an attempt has been made to evaluate the growth response and biochemical composition of laboratory-cultured Nannochloropsis oculata to TBT toxicity. It is evident that the persisting concentration level of TBT is high enough to cause adverse effect on the microalgal species. The EC₅₀ (24 h) was found to be at 0.89 nM level of TBT for this marine eustigmatophyte N. oculata. Photosynthetic pigment content was significantly affected. At elevated TBT concentrations of 1.0 nM, especially pronounced changes in biochemical composition was found. TBT tolerance of N. oculata and its growth as well as biochemical responses are discussed.     

气枪震源波形走时变化分析的反褶积处理

利用云南省宾川主动源基地周边10km范围内7个流动台站2017年2月气枪震源集中激发时段记录的气枪信号,使用干涉法计算了直接使用波形和将波形与参考台反褶积后的格林函数两种处理方法下气枪信号的走时变化。使用了气枪震源附近不同方位角的两个参考台的数据,以分析不同参考台对走时变化结果的影响。结果表明:反褶积能较为有效的压制气枪震源的时间、空间和频谱轻微不重复性导致的走时变化;反褶积能降低一些来源不明的伪走时变化;使用CKT2的数据反褶积的结果略优于CKT0,其可能原因是气枪阵列沿方位角的非均匀辐射。     

Impact of air gun noise on the behaviour of marine fish and squid

In this study various species of captive marine fish and one species of squid were exposed to the noise from a single air gun. Six trials were conducted off the coast of Western Australia with each trial using a different noise exposure regime. Noise levels received by the animals ranged between 120 and 184 dB re 1 μPa(2).s (SEL). Behavioural observations of the fish and squid were made before, during and after air gun noise exposure. Results indicate that as air gun noise levels increase, fish respond by moving to the bottom of the water column and swimming faster in more tightly cohesive groups. Significant increases in alarm responses were observed in fish and squid to air gun noise exceeding 147-151 dB re 1 μPa SEL. An increase in the occurrence of alarm responses was also observed as noise level increased.     

利用功率谱检测气枪信号

使用北京台网记录的数据,利用功率谱方法对中海油渤海勘探部在渤海湾进行的气枪试验进行了分析。通过北京台网部分台站噪声功率谱与含有气枪信号信息的噪声功率谱进行的比较,发现频率0．2Hz左右含气枪信号信息的噪声功率谱明显高于不含气枪信号信息噪声功率谱。     

非均匀黏弹性介质中声波模拟的非规则网格方法

本文研究了满足衡Q模型的黏弹性介质中声波模拟的时间域数值解法.通过采用有理式基函数描述频率相关的体积模量,使模型满足地震波黏性吸收的衡Q要求.结合弹性波模拟的非规则、非结构网格方法——格子法,本文提出了一种非均匀黏弹性介质中声波模拟的非规则网格方法.非规则、非结构网格的使用,可以精细地刻画地下介质中复杂速度、Q值的变化,及界面的形状.通过和解析解的比较及复杂模型算例分析,验证了该方法的精度及有效性.     

PERFORMANCE OF 2000 AND 6000 PSI AIR GUNS: THEORY AND EXPERIMENT*

Air guns have been used in various applications for a number of years. They were first used in coal-mining operations and were operated at up to 16000 psi charge pressures. Later, single air guns, operated at 2000 psi, found application as an oceanographic survey tool. Air gun arrays were first used in offshore seismic exploration in the mid-1960's. These early arrays were several hundred cubic inches in total volume and were operated at 2000 psi; they were either tuned arrays or several large guns of the same size with wave-shape kits. Today's arrays have total volumes greater than 5000 cu in. and are typically operated at 2000 psi. Recently, higher-pressure, lower-volume arrays operated at 4000–5000 psi have been introduced; guns used in these arrays are descendants of the coal-mining gun. On first thought one would equate increased gun pressure linearly with the amplitude of the initial pulse. This is approximately true for the signature radiated by a “free-bubble” (no confining vessel) and recorded broadband. The exact relation depends on the depth at which the gun is operated; from solution of the free-bubble oscillation equation, the relation is If P_c,1= 6014.7 psia, P_c,2= 2014.7 psia and P_{O, 1}=P_{O, 2}= 25.8 psia (corresponding to absolute pressure at 25 ft water depth), then Experiments were conducted offshore California in deep water to determine the performance of several models of air guns at pressures ranging from 2000 to 6000 psi and gun volumes ranging from 5 to 300 cu in. At a given gun pressure, the initial acoustic pulse P_a correlated with gun volume V_c according to the classical relation For 1 ms sampled data the ratio varied between 4.5 and 5.5 dB depending on gun model. Pulse width of the 2000 psi signatures indicated they are compatible with 2 ms sample-rate recording while pulse width of the 6000 psi signatures was greater, indicating they are less compatible with 2 ms sample-rate recording. Conclusions reached were that 2000 psi air guns are more efficient than higher pressure guns and are more compatible with 2 ms sample-rate requirements.     

The explosive activity of Karymskii Volcano, Kamchatka: Acoustic and seismic observations

Karymskii Volcano typically shows explosive activity with great variations in the frequency and energy of explosions. This is demonstrated here for three time segments of the volcano’s activity (1970–1973, 1976–1980, and 1996–2000). We examine various types of seismic and acoustic emission as controlled by crater morphology and the character of activity. The explosion funnels migrated over the crater area, and the 1976 effusive-explosive eruption occurred at two centers of lava flow effusion; this is here explained by the fact that magma as it was moving along the conduit was stratified to form a set of vertical filaments. The shape of shock waves in air recorded in August 2011 favors the hypothesis that the leading explosive mechanism during that period was a fragmentation wave that was produced in a gas-charged, viscous, porous magma during decompression. One notices that the shape of some shock waves in air recorded in 2011 indicates the occurrence of air blasts above the crater. The air blasts may have been caused by combustible volcanic gases such as carbon monoxide and hydrogen (CO and H₂), which entered the atmosphere in sufficient amounts.     

移动式气枪震源系统概述

气枪震源在海洋地震勘探方面获得了很大成功,近年来在陆域水体的应用越加广泛。目前,大容量气枪用于陆地的研究主要固定在某一处水库或者水域进行气枪震源试验,因此,发展易安装、拆卸、运输的可移动式气枪震源系统是有必要的。通过福建省地震局近年来的陆海联测工作,详细地介绍了移动式气枪震源系统的整体架构和工作流程。移动式气枪震源系统是一种绿色环保、经济实用、自动化程度高、震源激发能量精度高、重复性好、探测距离大的震源激发系统,该系统的使用填补了国内相关领域技术装备的空白。     

Seismoelectric and electroseismic responses to a point source in a marine stratified model

We investigate the seismoelectric/electroseismic wavefields excited by a point source in an air/seawater/three-layered porous medium configuration containing a hydrocarbon layer. The results show that if an explosive source for excitation is used, receivers at seafloor can record the coseismic electromagnetic fields accompanying the P, S, fluid acoustic waves and the interface responses converted from the acoustic waves at seafloor interface and from the seismic waves at the interfaces beneath the seafloor. Employing a vertical electric dipole source shows that, with the exception of the interface responses converted from electromagnetic waves at seafloor, the interface responses converted from transmitted electromagnetic waves at the interfaces beneath the seafloor can also be identified. Given that the strength of the explosive source is within excitation capability of industry air guns, the generated interface responses from the hydrocarbon layer can be detected by current electromagnetic sensors considering the low ambient noise at the seafloor. Our results demonstrate the feasibility of the seismoelectric method applied to marine hydrocarbon exploration. Electroseismic modelling results suggest that it is not practical to employ this method to prospect marine hydrocarbon layer due to the weak interface response signal, unless a much larger current is injected into seafloor.     

Forced imbibition into a limestone: measuring P‐wave velocity and water saturation dependence on injection rate

Quantitative interpretation of time‐lapse seismic data requires knowledge of the relationship between elastic wave velocities and fluid saturation. This relationship is not unique but depends on the spatial distribution of the fluid in the pore‐space of the rock. In turn, the fluid distribution depends on the injection rate. To study this dependency, forced imbibition experiments with variable injection rates have been performed on an air‐dry limestone sample. Water was injected into a cylindrical sample and was monitored by X‐Ray Computed Tomography and ultrasonic time‐of‐flight measurements across the sample. The measurements show that the P‐wave velocity decreases well before the saturation front approaches the ultrasonic raypath. This decrease is followed by an increase as the saturation front crosses the raypath. The observed patterns of the acoustic response and water saturation as functions of the injection rate are consistent with previous observations on sandstone. The results confirm that the injection rate has significant influence on fluid distribution and the corresponding acoustic response. The complexity of the acoustic response —‐ that is not monotonic with changes in saturation, and which at the same saturation varies between hydrostatic conditions and states of dynamic fluid flow – may have implications for the interpretation of time‐lapse seismic responses.     

DEVELOPMENT OF MORE EFFICIENT AIRGUN ARRAYS: THEORY AND EXPERIMENT*

Source strength of an airgun array may be increased by:

• — utilizing higher pressure,
• — increasing total array volume,
• — employing more guns,
• — improving gun efficiency.

One measure of gun efficiency is “specific source strength”, P_a*, defined as source strength per unit quantity of air used. Typical units are MPa m/l. Most developments are directed toward increasing gun pressure and/or gun volume to increase source strength of the array. These efforts require that more air compressors be installed onboard the ship. Consequently, a larger ship may be needed for the additional compressors, guns, and auxiliary equipment. A development program was initiated in 1976 to increase source strength of the array without using a larger ship. New guns were designed and built—one for 41.4 MPa and 7.37 liter (6000 p.s.i./450 in³) operation and another with 13.8 MPa and 4.92 liter (2000 p.s.i./300 in³) capability. Experiments were conducted with these new guns (and existing guns) over a range of pressures from 13.8 to 41.4 MPa (2000 to 6000 p.s.i.). Design of the new guns was aided by a mathematical model. The model relates physical dimensions of the airgun to acoustic pressure in the water. It consists of four nonlinear differential equations relating

• — shuttle motion,
• — bubble pressure,
• — chamber pressure,
• — bubble radius.

The last equation is the “free-bubble-oscillation equation” and represents the ideal case of a pressurized bubble released instantaneously in water. The three other equations modify this ideal case; the four equations together model an airgun of the type manufactured by Bolt Associates, Inc.     

台网噪声评估及其对气枪震源激发效果影响的研究

为了研究背景噪声对长江地学气枪主动源实验激发效果的影响,收集了240个台站激发前72小时的背景噪声数据及20个定点激发的叠加结果进行实验分析.先剔除异常台站,按照国标将台网噪声水平划分为高、中、低3个等级进行评估,从而研究气枪波形信噪比与台站背景噪声的关系.结果表明:① 台站接收能力与背景噪声水平密切相关;② 气枪信号的信噪比在一定范围内受背景噪声的影响大于距离衰减的影响;③ 长江马鞍山—安庆流域段不同定点激发的场地条件基本一致;④ 随气枪信号叠加次数的增加,低噪台站的信噪比增长速度快于中、高噪台站,若想获得同样的叠加效果,高噪台站需要的叠加次数远大于低噪台站.      

2011年日本9.0级地震前后应力场变化

通过对2011年日本9.0级地震前后不同时段震源机制解进行分析,利用FMSI震源机制反演法求解,得到此次地震前后内陆地区的应力场。研究发现,此次地震发生前,应力场比较均衡稳定,挤压应力接近水平向,而拉张应力比较复杂,方向不确定,表明该地区的地震以逆断层和走滑为主。地震发生后,日本东北地区东南部应力场变得不太稳定,该地区地震以正断层为主,最小挤压应力由垂向转变为水平向。此研究结果可以用来分析该地区地震地质背景和断层形成条件,对地球动力环境研究具有一定参考意义。     

甘肃祁连山主动源不同枪组激发效果对比

大容量气枪震源激发探测因具绿色环保、传播距离远和高度重复性等特点成为地下介质探测的新方法,利用其进行地下介质物性变化高精度探测,则对不同激发条件下气枪传播信号的一致性提出要求。本文选取了祁连山主动源不同枪组（4支和3支气枪）在激发环境一致情况下分别激发时各远台波形记录,通过信号叠加、互相关及频谱分析等方法,对比分析了各台站记录的不同枪组激发信号,及其与震中距的关系,并讨论了不同枪组激发对地下介质走时变化观测的影响。结果表明：（1）不同枪组的激发信号波形相关度随震中距增大而降低,信号震相随激发能量有微弱的变化,震中距超过50 km后其变化几乎分辨不出;（2）远台接收信号的能量大小随主动源激发能量变化,各台站能量变化主要体现在优势频率范围内;（3）接收台站台基越坚固不同激发能量均方根振幅比越小;（4）叠加一定数量的波形可以有效提高远台记录信号的信噪比,增加气枪激发能量则相对更容易提高信号信噪比。     

金属矿床采掘过程围岩失稳状态的声发射监测实践

本文以声发射技术在岩土工程中的应用为基础,结合国内外的研究成果,对小铁山矿脉内采准工程中采准和回采巷道的稳定性进行了实时监测,确定了不同岩性的稳固性、开挖后的松动范围和巷道的合理支护方式及支护时间. 从岩石应力和声发射次数与能量的关系出发,通过实测一定时期内巷道的声发射特征,对比相应的破坏形式,确定了巷道开挖后破坏形式和声发射的关系,为脉内采准工程合理性和地压控制措施提供了理论指导, 同时也证明了声发射监测技术在岩矿地下开挖工程地压活动监测中的有效性.     

Research note: Laboratory study of the influence of changing the injection rate on the geometry of the fluid front and on P‐wave ultrasonic velocities in sandstone

Forced imbibition was performed in reservoir sandstone by injecting water into a dry sample. The injection was monitored with X‐ray computed tomography and acoustic acquisition to simultaneously visualize the displacement of the fluid and quantify its presence by calculating saturation and P‐wave velocities. We observed a strong influence when changing the injection rates on the acoustic response. Upon decreasing the injection rate from 5 mL/h to 0.1 mL/h, P‐wave velocities decreased sharply: 100 m/s in 1 h. This behaviour is related to the partially saturated conditions of the sample (76% of saturation) before decreasing the injection rate. The air that is still trapped is free to move due to a decrease of pore pressure that is no longer forced by the higher injection rate. After 1 hour, P‐wave velocities started increasing with small changes in saturation. Stopping injection for 16 hrs decreased saturation by 8% and P‐wave velocities by 100 m/s. Restarting injection at 5 mL/h increased saturation to 76% while P‐wave velocities fluctuated considerably for 2 hrs until they stabilized at 2253 m/s. Through the computed tomography scans we observed a water front advancing through the sample and how its shape changed from a plane to a curve after decreasing the injection rate.