立体气枪阵列延迟激发震源特性及在浅海区OBS探测中的应用

利用中、小容量气枪组成的立体气枪阵列延迟激发震源和海底地震仪(OBS)在我国北部浅海海域开展了人工地震深部地球物理探测试验.基于水深条件和压制水体虚反射、提升低频能量的需要,使气枪震源有足够的输出能量和高品质子波特性,研究了立体气枪阵列延迟激发震源工作机理,经远场子波理论模拟优选了组合参数并进行了海上试验工作.结果表明,中、小容量气枪组成的立体气枪阵列延迟激发震源,适应了浅水海域的激发环境,降低了由虚反射造成的局部陷波和干扰作用,有效地改善了OBS信号的品质,获得了Ps,Pg,PmP,Pn等多种震相.创新了由中、小容量气枪组成的立体气枪阵列延迟激发震源在浅海区OBS探测中的应用,也填补了南黄海海域深地震探测数据的空白,为南黄海、渤海深部地壳结构研究及含油气盆地形成演化研究提供了重要的基础资料.     

大容量气枪震源子波激发特性分析

大容量气枪水库激发作为陆地震源的可行性与有效性已经得到成功验证.为进一步提高气枪震源激发效果,本文通过水库气枪激发试验对单枪容量为2000 in³的气枪震源激发子波特征及规律进行了研究.依据近场水听器和远场短周期地震仪记录数据,分析气枪震源沉放深度、工作压力等不同激发条件对压力脉冲和气泡脉冲的影响.有助于人们根据不同尺度地下结构探测对震源激发信号的要求,调整气枪激发参数和激发环境,获得最佳激发效果.试验结果表明：（1）沉放深度对压力脉冲波形影响较小,其优势频率不随沉放深度而改变;（2）随着沉放深度从5 m增加到11 m,气泡脉冲的优势频率由5 Hz增加至7 Hz,其最大振幅亦近线性递增;（3）工作压力越大,激发压力脉冲能量越强,而对气泡脉冲的影响主要体现在主频降低.适合远距离深穿透地下结构探测的低频信号主要来自大容量气枪所激发气泡的反复振荡,由于气枪振荡过程非常复杂,本文通过较为简洁的数学和物理模型进行了解释.     

EFFICIENT DESIGN OF AIR-GUN ARRAYS*

A new technique is developed for shaping the pressure bubble pulse radiated by an array of air-guns. It involves the proper spacing of identical units. It is shown that considerable shortening of the pressure bubble pulse can be achieved provided there is sufficient mutual coupling between all the air-guns. The existing air-gun array technique for reducing the bubble pulse involves the redistribution of the energy of the bubble pulses which are produced by an array of variable sized air-guns such that no energy of the bubble pulses is radiated along the perpendicular to the array axis and only the sum of the initial pulses produced by the air-guns forming the array is radiated along that direction. However, the new air-guns array technique involves the damping of the bubble pulses which are produced by an array of identical air-guns by means of mutual interaction and the effective pressure pulse radiated by the array is given by the sum of the damped bubble pulses produced by the mutually coupled identical air-guns. Preliminary field trials gave results consistent with the theoretical predictions. A comparison between the waveforms of the pressure bubble pulses radiated by a single air-gun and by an array of four identical air-guns shows that, due to the presence of mutual coupling between the four air-guns, effective damping of the bubble pulse radiated by the array is about 50% greater than that of the bubble pulse radiated by the single air-gun.     

Fired Models of Air-gun Source and Its Application

Air-gun is an important active seismic source. With the development of the theory about air- gun array, the technique for air-gun array design becomes mature and is widely used in petroleum exploration and geophysics. In order to adapt it to different research domains, different combination and fired models are needed. At the present time, there are two fired models of air-gun source, namely, reinforced initial pulse and reinforced first bubble pulse. The fired time, space between single guns, frequency and resolution of the two models are different. This comparison can supply the basis for its extensive application.     

THE RADIATION OF ACOUSTIC WAVES FROM AN AIR-GUN*

A theoretical model is developed for predicting three important parameters of the pressure pulse radiated by an air-gun, namely the rise time, the amplitude of the initial pulse, and the period of the bubble pulse. A knowledge of these three parameters is essential for the efficient design of air-guns arrays. The prediction of the amplitude of the initial pulse is based on the assumption that the initial pulse is radiated by a spherical source with surface area equal to that of the air-gun ports and not by a spherical source with initial volume equal to that of the air-gun chamber, as has been assumed previously. A simple equation is obtained for predicting the period of the bubble pulsation, taking into account the effect of the air-gun body, boundaries such as the sea-surface and seabed and the presence of a number of identical air-guns placed at the same depth and fired simultaneously.     

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.     

沉放深度对气枪震源激发信号影响的试验研究

以2014年11月福建街面水库气枪震源激发试验观测数据为研究对象,利用基于互相关的时延估计法对不同沉放深度及不同水深的气枪激发展开研究,分析沉放深度及激发环境水深对气枪震源激发信号到时的影响。结果显示:激发位置不变,气枪沉放深度在8—30 m范围内变化时,震中距为13 km的测震台站记录到的气枪信号的首脉冲到时差异很小,而气泡脉冲到时则有明显差异,且随着沉放深度的增加,气泡脉冲信号到时提前,8 m沉放深度与30 m沉放深度气泡脉冲的到时差近80 ms,这种变化与布设在库底的海底地震仪记录到的信号变化相一致,分析认为这种变化与气枪沉放深度增加引起的气枪激发信号的周期减小有关;在不同水位开展的相同沉放深度的激发信号差异不大。因此,应用气枪震源监测地壳介质变化时需注意气枪震源沉放深度变化所带来的影响。      

TEST RESULTS OF A NEW TYPE OF EFFICIENT SMALL AIRGUN ARRAY*

Recently the author developed and demonstrated (Safar 1980) an efficient method for operating the airgun. The method involves the generation of a short seismic pulse from the pressure bubble pulses radiated by an airgun when fired several times at the same optimum depth but with different chamber pressures. The purpose of this paper is to present and discuss the test results obtained when implementing the same method using a two-dimensional airgun array. The array consists of seven 0.65 liter airguns fired simultaneously at the same depth but with different chamber pressures. It is shown that the far-field pressure pulse radiated by the seven 0.65 liter airgun array is similar to that radiated by the Flexichoc seismic source. It is concluded that the proposed airgun array can be used as a subarray to form an extremely powerful super-long array suitable for deep seismic exploration. The author would like to thank the Chairman and Board of Directors of the British Petroleum Co. Ltd for permission to publish this paper. Thanks are also due to Mike Symes and Lovell Cox for carrying out the field tests and Seismograph Service (England) Ltd for providing the airguns.     

AN EFFICIENT METHOD OF OPERATING THE AIR-GUN*

A new technique is developed for generating a short seismic pulse from the bubble pulses which are radiated by an air-gun. The new technique, which is useful in well velocity surveys and vertical seismic profiling, can be implemented by firing a single air-gun several times at the same depth but with different chamber pressures. A record obtained by this procedure from a well-geophone clamped at a depth of 2450 m gave a maximum peak-to-peak amplitude within the first 100 ms of the effective seismic pulse at least ten times any later peak-to-peak amplitude.     

CALIBRATION OF MARINE SEISMIC SOURCES USING A HYDROPHONE OF UNKNOWN SENSITIVITY*

The absolute amplitude of the pressure pulse radiated by a marine seismic source is one of the prime criteria used in evaluating its performance. A technique for this measurement is proposed which is applicable to all sources which radiate a bubble pulse. The technique is described with reference to an air-gun array in which the pulse radiated from a single gun is compared with that radiated by the full array. The advantage of the method is that absolute values of pressure are obtained without any need for a calibrated hydrophone. In theory this may seem a trivial advantage but in practice sensitivity factors for the hydrophone channel cannot always be relied upon. The proposed technique is illustrated by an example.     

海洋地震资料采集过程中近场检波器的应用

海洋地震资料采集中空气枪震源近场检波器不仅可以较好地进行震源质量实时监控,更重要的是可以较好地预测远场子波.经国外大量的远场子波实测试验与预测结果对比,利用近场检波器预测远场子波,与实测远场子波吻合度很高,因此,利用近场检波器预测远场子波是一项非常关键的技术,然而国内在该技术领域还不是很成熟,目前只用来做一些震源的质量监控和近场子波记录.较为详细地描述了国外的部分试验结果,并根据国外的试验结果和国内的具体使用情况,提出了近场检波器放置位置、记录长度和采样率合理化选取的建议.并建议国内相关领域的专家尽快研发拥有自主知识产权的相关软件,利用近场子波来有效地预测远场子波.     

THE SCALING OF AIRGUN ARRAYS,INCLUDING DEPTH DEPENDENCE AND INTERACTIONS*

The article provides a theoretical basis for the extension of the method of scaling law deconvolution to three dimensions using airgun arrays as a sound source. Earlier papers by the author required the dimensions of the scaled sources to be different while the depths and firing pressures were maintained the same in order to preserve the same dynamics of the scaled sources at scaled time. However, this forces the source ghost to be considered as part of the impulse response of the earth rather than as part of the downgoing source wave. And, in fact, the dynamics of the scaled sources are not the same at the same depth because the ghost reflection modulates the behaviour of the oscillating bubbles generated by the airguns, and this modulation does not scale. To force the sources to scale properly, including the ghost interaction, the larger source must be put at greater depth, where hydrostatic pressure is greater, and the initial firing pressure must be adjusted accordingly. Thus, the depth, initial firing pressure and gun volume are all variables. The interaction among guns in scaled airgun arrays also scales exactly if the geometry of an array and the depth of its deployment are scaled by the same factor.     

COMPUTATION OF SIGNATURES OF LINEAR AIRGUN ARRAYS*

Far-field signatures from an airgun array are usually obtained by carrying out extensive field measurements. In order to decrease the need for such measurements, we have developed a method for computing signatures from linear airgun arrays where the distances between the airguns are such that the non-linear interaction among the airguns is negligible. The signature from a single airgun of a given type is computed from the following airgun parameters: airgun chamber volume, chamber pressure, airgun depth and position of the waveshape plate within the chamber. For calibration purposes, a recorded signature for one set of airgun parameters has to be provided for each type of airgun. The signatures are computed by using empirical relations between signature properties and the airgun parameters, and by treating the primary and bubble pulses separately. The far-field signature from a linear airgun array can now be computed by summation of the delayed signatures from the airguns in the array. Practical results are shown for an array with different PAR (Bolt) 1500 C airguns.     

Influencing factors of seismic signals generated by un-tuned large volume airgun array in a land reservoir

Un-tuned large volume airgun array in a water reservoir is recently proposed as a new way to generate seismic waves on land. It can be used to explore the earth velocity structure and its temporal variations as well. However, the characteristics of seismic signals (especially far-field signals) from an airgun array in a reservoir and its affecting factors (firing pressure, airgun towing depth, water level of the reservoir, etc.) has not been adequately studied. We analyzed the seismic data collected from field experiments at Binchuan Transmitting Seismic Station in 2011 and 2013 and found that (1) The similarity of seismic signals decrease with distance, which is most likely induced by the decay of signal amplitude and signal to noise ratio (SNR); (2) The amplitudes of far-field airgun signals are almost linearly proportional to the firing pressure; (3) The towing depth of airgun has less effects on the far-field signals; (4) The amplitudes of far-field airgun signals are proportional to the water level of the reservoir.     

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.     

气枪震源激发模式及应用

气枪震源是一种重要的人工地震震源。气枪阵列理论的提出,使得气枪阵列设计技术日趋成熟,并能够在石油勘探和地球物理探测中得到更加广泛的运用。气枪震源在不同领域中应用时,需要不同的组合和激发模式,以适应不同的探测要求。加强主脉冲和加强气泡脉冲,是目前两种主要的气枪激发模式。通过比较研究两种气枪激发模式,讨论各种激发模式在激发时间、气枪间距、频率、分辨率等方面的差异,为气枪震源的广泛运用提供依据。     

基于气枪震源信号和背景噪声的2016年呼图壁6.2级地震前后波速变化特征初步研究

本文选取呼图壁气枪发射台周边流动台记录的2016年6月至2017年12月气枪震源信号和背景噪声信号,分别采用互相关时延检测法和移动窗互谱法研究2016年12月呼图壁6.2级地震前后波速变化特征,得到以下初步结果:呼图壁6.2级地震前后,距6.2级地震60 km的流动台记录的气枪震源信号未发现明显的波速变化,距6.2级地...     

ON THE CALIBRATION OF THE WATER GUN PRESSURE SIGNATURE*

A simple field method was proposed by the author in 1976 for measuring the absolute amplitude of the pressure pulse radiated by marine seismic sources which radiate a bubble pulse. The proposed method involves the recording of the near-field pressure signature radiated by the water gun using a wide-band hydrophone. The key feature of the proposed method is that a knowledge of the hydrophone sensitivity and its distance from the water gun are not required. It is shown that the absolute amplitudes of the pressure pulses radiated by the S80 and P400 water guns obtained using the proposed method are in agreement with those obtained using a Ref-Tek hydrophone.     

Optimum strong-motion array geometry for source inversion

Optimum strong-motion array geometry for source inversion is quantitatively determined for each of three types of earthquake faults: strike—slip, dip-slip and offshore subduction thrust. The estimation is done by using the source inversion method^1,2 previously developed and based on Wolberg's prediction analysis.³ Only the far-field S waves are taken into account in source inversion; we test only the ability to estimate seismic moments of subfault elements. A comparison of the optimum array geometries obtained in this study with the ones proposed on the basis of an empirical judgment at the 1978 International Workshop on Strong-Motion Earthquake Instrument Arrays⁴ shows that the Workshop geometries are not the best ones as far as source studies are concerned. The most preferable array involves two different kinds of stations regardless of the target fault type: stations close to the fault and aligned parallel to its strike, and stations surrounding the fault area with good azimuthal coverage. The former stations resolve the later stage of the rupturing process while the latter stations resolve the earlier stage.     

Marine seismic sources: QC of wavefield computation from near-field pressure measurements

A commercial marine seismic survey has been completed with the wavefield from the n-element (single guns and clusters) airgun array measured for every shot using an array of n + 2 near-field hydrophones, n of which were required to determine the source wavefield, the remaining two providing a check on the computation. The source wavefield is critical to the determination of the seismic wavelet for the extraction of reflection coefficients from seismic reflection data and for tying the data to wells. The wavefield generated by the full array of interacting airguns can be considered to be the superposition of n spherical pressure waves, or notional source signatures, the n hydrophone measurements providing a set of n simultaneous equations for each shot. The solution of the equations for the notional source signatures requires three ingredients: the geometry of the gun ports and near-field hydrophones; the sensitivity of each hydrophone recording channel; and the relative motion between the near-field hydrophones and the bubbles emitted by the guns. The geometry was measured on the back deck using a tape measure. A calibration data set was obtained at the approach to each line, in which each gun was fired on its own and the resulting wavefield was measured with the near-field hydrophones and recorded. The channel sensitivities, or conversion from pressure at the hydrophones to numbers on the tape, were found for each near-field hydrophone channel using the single gun calibration data, the measured geometry, and the peak pressure from each gun, known from the manufacturer’s calibration. The relative motion between the guns and hydrophones was obtained from the same calibration data set by minimizing the energy in the computed notional source signatures at the guns which did not fire. The full array data were then solved for the notional source signatures, and the pressure was computed at the two spare hydrophones and compared with the actual recordings. The rms errors were 5.3% and 2.8% and would have been smaller if the hydrophone channel sensitivities had been properly calibrated beforehand and if the movement of the guns with respect to the hydrophones had been more restricted. This comparison of the predicted and measured signatures at spare hydrophones can, in principle, be done on every shot and we recommend that this be implemented as a standard quality control procedure whenever it is desired to measure the wavefield of a marine seismic source.