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.     

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.     

Geophone-seabed coupling effect and its correction

By summing geophone and hydrophone data with opposite polarity responses to water layer reverberation, the ocean bottom cable dual-sensor acquisition technique can effectively eliminate reverberation, broaden the frequency bandwidth, and improve both the resolution and fidelity of the seismic data. It is thus widely used in industry. However, it is difficult to ensure good coupling of the geophones with the seabed because of the impact of ocean flow, seafloor topography, and field operations; therefore, geophone data are seriously affected by the transfer function of the geophone-seabed coupling system. As a result, geophone data frequently have low signal-to-noise ratios (S/N), which causes large differences in amplitude, frequency, and phases between geophone and hydrophone data that severely affect dual-sensor summation. In contrast, the hydrophone detects changes in brine pressure and has no coupling issues with the seabed; thus, hydrophone data always have good S/N. First, in this paper, the mathematical expression of the transfer function between geophone and seabed is presented. Second, the transfer function of the geophone-seabed is estimated using hydrophone data as reference traces, and finally, the coupling correction based on the estimated transfer function is implemented. Using this processing, the amplitude and phase differences between geophone and hydrophone data are removed, and the S/N of the geophone data are improved. Synthetic and real data examples then show that our method is feasible and practical.     

大容量气枪震源子波时频特性及其影响因素

通过分析福建街面水库气枪实验的近场水听器记录,研究气枪子波时频特性及其受沉放深度和工作压力的影响,并结合气泡模型解释气泡振荡过程。数据分析表明：①气枪子波由主脉冲和气泡脉冲组成。主脉冲振幅大,持时短,频带宽,通常应用于浅部探测;气泡脉冲能量集中在低频段,垂直穿透深,水平传播远,通常应用于深部探测。②随沉放深度的增加,主脉冲振幅变化很小,气泡脉冲振幅增加,初泡比减小,气泡周期减小,低频段主频增加。沉放深度为10m时,主脉冲振幅和初泡比最大,可应用于浅部探测;沉放深度为25m时,气泡脉冲振幅很大,初泡比最小,可应用于深部探测。③工作压力增加时,主脉冲振幅、气泡脉冲振幅、初泡比、气泡周期等随之增大,低频段主频则减小。     

Research Note: Low-frequency pneumatic seismic sources

Pneumatic seismic sources, commonly known as airguns, have been serving us well for decades, but there is an increasing need for sources with improved low-frequency signal and reduced environmental impact. In this paper, we present a new pneumatic source that is designed to achieve these goals by operating with lower pressures and larger volumes. The new source will release more air creating larger bubbles with longer bubble periods than airguns. The release of the air will be tuned so that the rise time will be longer and the sound pressure level and its slope will be lower. Certain engineering features will eliminate cavitation. Larger bubbles increase low-frequency content of the signal, longer rise times decrease mid-frequency content and the elimination of cavitation reduces high-frequency content. We have not yet built a full-scale version of the new source. However, we have manufactured a small-scale low-pressure source incorporating most of the engineering features, and tested it in a lake. Here, we present the lake data that, as expected, show a significant reduction in the sound pressure level, increase in rise time, decrease in slope and decrease in high-frequency content while maintaining the same low-frequency content when the source prototype is operated at low pressure compared with high pressure. Synthetic data produced by numerical modelling of the full-scale proposed pneumatic source suggest that the new source will improve the low-frequency content and can produce geophysically useful signal down to 1 Hz.     

Spectral variations of underwater river sounds

Passive acoustic monitoring of the self‐generated noise of particle impacts has been shown to be correlated to bedload flux and bedload size. However, few studies have concentrated on the role of acoustic wave propagation in a river. For the first time, the river environment is modeled as a Pekeris waveguide, where a wave number integration technique is used to predict the transformation of sounds through their propagation paths. Focusing on the distance of a hydrophone from the channel bed and cutting off the low frequencies produced by impacts between gravel particles, we demonstrate that acoustic propagation modifies the spectral content of bedload‐generated sound. Acoustic signals analyzed with the proposed model are interpreted by comparison to Helley–Smith bedload data obtained during flood conditions on the large gravel‐bedded Arc‐en‐Maurienne River, France. This study shows that careful attention to acoustic propagation effects is required when estimating bedload grain size distribution with hydrophones in rivers, especially for rivers with slopes higher than 1%. Bedload monitoring with a hydrophone is particularly appropriate for large gravel‐bed rivers – especially so during large floods, when in situ sampling is difficult or impractical and the impact of acoustic propagation is weaker relative to the self‐generated noise of bedload impacts. Copyright © 2017 John Wiley & Sons, Ltd.     

Sea surface shape derivation above the seismic streamer

The rough sea surface causes perturbations in the seismic data that can be significant for time‐lapse studies. The perturbations arise because the reflection response of the non‐flat sea perturbs the seismic wavelet. In order to remove these perturbations from the received seismic data, special deconvolution methods can be used, but these methods require, as input, the time varying wave elevation above each hydrophone in the streamer. In addition, the vertical displacement of the streamer itself must also be known at the position of each hydrophone and at all times. This information is not available in conventional seismic acquisition. However, it can be obtained from the hydrophone measurements provided that the hydrophones are recorded individually (not grouped), that the recording bandwidth is extended down to 0.05 Hz and that data are recorded without gaps between the shot records. The sea surface elevation, and also the wave‐induced vertical displacement of the streamer, can be determined from the time‐varying pressure that the sea waves cause in the hydrophone measurements. When this was done experimentally, using a single sensor seismic streamer without a conventional low cut filter, the wave induced pressure variations were easily detected. The inversion of these experimental data gives results for the sea surface elevation that are consistent with the weather and sea state at the time of acquisition. A high tension approximation allows a simplified solution of the equations that does not demand a knowledge of the streamer tension. However, best results at the tail end of the streamer are obtained using the general equation.     

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.     

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.     

LOW-FREQUENCY STANDARD HYDROPHONE * FOR CALIBRATING LINE HYDROPHONE ARRAYS (SEISMIC STREAMERS) AND MARINE SEISMIC SOURCES **

The design of a standard hydrophone with a maximally flat (Butterworth) response in the frequency range 8.0 Hz-1.0 kHz is described. The standard hydrophone has been developed primarily for calibrating line hydrophone arrays (seismic streamers) and marine seismic sources. The standard hydrophone has been used successfully during the past eight years for monitoring the output of a single air gun. It can be used for the calibration of a marine seismic streamer.     

Measurement of bedload gravel transport: The calibration of a self-generated noise system

Self-generated noise (SGN) is a passive acoustic technique for the measurement of bedload transport through the acoustic detection of intergranular collisions using a submerged hydrophone system. SGN is of interest because of the possibilities it offers to the measurement of transport parameters at the frequencies of turbulent or oscillating current scales. This paper describes the background of the technique, and reports the results from laboratory experiments on the relationship between SGN and the absolute transport rate.     

Seismic on floating ice: data acquisition versus flexural wave noise

Geophysical surveying of the Arctic will become increasingly important in future prospecting and monitoring of the terrestrial and adjacent areas in this hemisphere. Seismic data acquired on floating ice are hampered with extensive noise due to ice vibrations related to highly dispersive ice flexural waves generated by the seismic source. Several experiments have been conducted on floating ice in van Mijenfjorden in Svalbard in the Norwegian Arctic to specifically analyse the extent of flexural waves recorded with various seismic receivers and sources deployed both on top of ice and in the water below. The data show that flexural waves are severely damped at 5 m or deeper below the ice and hydrophone data suffer less from these vibrations compared with data recorded on the ice. Aliasing of single receiver hydrophone data can to some extent be suppressed using an in-line line source of detonating cord. Experiments on ice on shallow water show prominent guided wave modes often referred to as Scholte waves propagating along the seabed. In this case, both flexural and Scholte waves interfere and make a complicated pattern of coherent noise. On shallow water, the positioning and type of the seismic source must be evaluated with respect to the coherent noise generated by these waves. Geophone strings of 25 m effectively suppress both flexural and Scholte waves due to their relative short wavelengths. An airgun generates relative more low-frequency energy than a surface source of detonating cord. Accordingly, seismic mapping of deep seismic horizons seem to be best achieved using geophone strings of such length and an airgun source. For shallow targets, the use of hydrophones in combination with detonating cord is an appropriate solution. Seismic surveying in the Arctic always have to follow environmental restrictions of not disturbing or harming wildlife and not causing permanent footprints into the vulnerable tundra, which implies that the choice of seismic acquisition strategy might occur as a trade-off between optimum data quality and environmental constraints.     

WAVELET ESTIMATION USING THE MULTITAPER METHOD1

An accurate estimate of the seismic wavelet on a seismic section is extremely important for interpretation of fine details on the section and for estimation of acoustic impedance. In the absence of well-control, the recognized best approach to wavelet estimation is to use the technique of multiple coherence analysis to estimate the coherent signal and its amplitude spectrum, and thence construct the seismic wavelet under the minimum-phase assumption. The construction of the minimum-phase wavelet is critically dependent on the decay of the spectrum at the low-frequency end. Traditional methods of cross-spectral estimation, such as frequency smoothing using a Papoulis window, suffer from substantial side-lobe leakage in the areas of the spectrum where there is a large change of power over a relatively small frequency range. The low-frequency end of the seismic spectrum (less than 4 Hz) decays rapidly to zero. Side-lobe leakage causes poor estimates of the low-frequency decay, resulting in degraded wavelet estimates. Thomson's multitaper method of cross-spectral estimation which suffers little from side-lobe leakage is applied here, and compared with the result of using frequency smoothing with the Papoulis window. The multitaper method seems much less prone to estimating spuriously high coherences at very low frequencies. The wavelet estimated by the multitaper approach from the data used here is equivalent to imposing a low-frequency roll-off of some 48 dB/oct (below 3.91 Hz) on the amplitude spectrum. Using Papoulis smoothing the equivalent roll-off is only about 36 dB/oct. Thus the multitaper method gives a low-frequency decay rate of the amplitude spectrum which is some 4 times greater than for Papoulis smoothing. It also gives more consistent results across the section. Furthermore, the wavelet obtained using the multi-taper method and seismic data only (with no reference to well data) has more attractive physical characteristics when compared with a wavelet extracted using well data, than does an estimate using traditional smoothing.     

基于EEMD分解的气压改正方法研究

利用超导重力仪观测数据精确测定低于1 mHz的地球自由振荡简正模式的分裂频率,是在不与任何弹性系数发生联系的情况下改善一维密度模型的有效方法.但在该频段台站局部气压变化对重力观测数据的影响成为主要干扰来源,且具有频率依赖特性,因此精细地开展气压改正成为利用超导重力数据检测低频自由振荡信号的必要手段.本文基于EEMD方法,提出了一种具有频率依赖特性的气压改正方法.该方法将重力观测和气压变化分解成处于不同频段的本征模态函数,并在相应频段上分别进行重力-气压变化的回归分析,计算得到具有频率依赖特性的气压导纳值,精细地消除气压变化对重力观测的影响,并以此对微弱低频地球自由振荡信号开展高分辨率分析.基于本文提出的气压改正方法,利用大地震后的超导重力数据检测了频率小于1.5 mHz的低频地球自由振荡及其频谱分裂现象.研究结果表明：利用该方法进行气压改正后检测得到的各简正模具有更高的信噪比,估计的本征频率误差水平明显降低,获得的基频球型振荡₀S₂和₀S₃以及一阶球型振荡₁S₂的分裂谱峰的估计精度更高,同时还检测到了部分环型振荡在重力观测中的耦合现象.对低频地球振荡的高分辨率检测结果验证了基于EEMD分解提出的气压改正方法的有效性,同时再次证明了超导重力仪观测数据在低频地球自由振荡检测中的优势.     

IMPROVED LOW-FREQUENCY DECAY ESTIMATION USING THE MULTITAPER SPECTRAL ANALYSIS METHOD1

Spectral analysis is one of the most ubiquitous signal processing tools used in exploration geophysics. Among many applications, it is used simply to look at the frequency content of seismic traces, to find notches, to estimate wavelets under the minimum-phase assumption, and to match broadband synthetic seismograms to seismic data. Seismic spectra exhibit very large dynamic ranges, particularly at low frequencies. Estimation of low-frequency decay is very important for accurate modelling. However, when using traditional spectral estimates incorporating smoothing windows, too much sidelobe energy leaks from high power into low power areas, spoiling our ability to estimate low-frequency spectral decay. The multitaper method of spectral analysis due to D. Thomson does not employ just a single window, but rather a set of orthogonal data tapers. It is possible to have much less sidelobe contamination, while maintaining a stable estimate. The trace is tapered by each of a subset of the orthogonal tapers, and a raw spectral estimate produced in each case. These are combined to produce a final spectral estimate. The technique can be made adaptive by applying different weights to the different raw spectra at different frequencies. A comparison of seismic spectral estimation using this multitaper technique with a traditional approach having the same analysis bandwidth and stability demonstrates the very different estimates of spectral decay in the areas of high dynamic range. The multitaper approach provides estimates with much reduced sidelobe leakage, and hence is a very appealing method for reflection seismology.     

Estimating the permeability of the Nojima Fault Zone by a hydrophone vertical seismic profiling experiment

Abstract A multi-offset hydrophone vertical seismic profiling (VSP) experiment was done in a 747 m deep borehole at Nojima Hirabayashi, Hyogo prefecture, Japan. The borehole was drilled to penetrate the Nojima Fault, which was active in the 1995 Hyogo-ken Nanbu earthquake. The purpose of the hydrophone VSP is to detect subsurface permeable fractures and permeable zones and, in the present case, to estimate the permeability of the Nojima Fault. The analysis was based on a model by which tube waves are generated when incident P-waves compress the permeable fractures (or permeable zones) intersecting the borehole and a fluid in the fracture is injected into the borehole. Permeable fractures (or permeable zones) are detected at the depths of tube wave generation, and fracture permeability is calculated from the amplitude ratio of tube wave to incident P-wave. Several generations of tube waves were detected from the VSP sections. Distinct tube waves were generated at depths of the fault zone that are characterized by altered and deformed granodiorite with a fault gouge, suggesting that permeable fractures and permeable zones exist in the fault zone. Tube wave analysis shows that the permeability of the fault gouge from 624 m to 625 m is estimated to be approximately 2 × 10⁻¹² m².     

震源和水听器沉放深度对单道地震勘探的影响

为研究震源和水听器电缆沉放深度对单道地震勘探的影响,建立鬼波影响模型,采用雷克子波模拟震源子波,分析因震源及水听器电缆沉放深度产生的鬼波对一次有效反射地震波的影响,并采用不同震源和水听器电缆沉放深度的数据对本次研究结论进行验证。研究表明：地震剖面分辨率和震源及水听器电缆沉放的总深度呈反比;随着震源或水听器电缆沉放深度的增加,地震剖面的分辨率递减,接收地震波能量从低值递增到最大值,再从最大值缓慢降低到稳定值;当水听器电缆沉放深度小于168.8/f(m)时,接收地震波能量随震源沉放深度变化的波动相对平缓,水听器电缆沉放深度浅有利于地震波的一致性,但沉放深度过浅时接收地震波能量将被极大削弱,会明显降低地震波的有效穿透深度;当震源沉放深度约为292.3/f(m)时,一次有效反射波的峰值和震源鬼波的峰值重叠,接收地震波能量最大;当震源沉放深度大于584.6/f(m)时,不考虑地震波能量在水中衰减的情况下,震源沉放深度继续增加对接收地震波能量的增加无效。     

坚硬顶板诱发煤体冲击破坏的微震效应

利用TDS-6微震采集系统测试了忻州窑煤矿组合煤岩试样变形破裂直至冲击破坏过程中的微震信号,特别是冲击破坏前后微震频谱的变化规律.采用SOS微震监测系统,对忻州窑煤矿8929工作面冲击矿压的微震活动规律进行了监测,结果表明：（1）冲击前兆信号呈现低频特征,随着与主震间隔的时间越短,主频越低.主震信号频谱较宽,但低频（0～50 Hz）成份增加,余震信号呈现高频特性.（2）微震信号的频次、累积能量与主频之间呈负相关关系.尤其当工作面顶板来压以及诱发冲击矿压时,微震信号的主频达到最低值.（3）现场监测表明,冲击前兆微震信号的主频为0～50 Hz,且振幅较低.冲击主震信号频谱较宽,但较低频（0～20 Hz）成份明显增加,同时振幅达到极值.余震信号主频为0～200 Hz,呈现高频、低振幅特征.     

Moment magnitude (Mw) from hydrophone records of low energy volcanic quakes

Earthquake magnitude calibration using hydrophone records has been carried out at Campi Flegrei caldera, an active area close to the highly populated area of Naples city, partly undersea. Definite integrals of the hydrophone records amplitude spectra, between the limits of 1 and 20 Hz, were calculated on a set of small volcano-tectonic earthquakes with moment magnitudes ranging from 1 to 3.3. The coefficients of a linear relationship between the logarithm of these integrals and the magnitude were obtained by linear optimization, thus defining a useful equation to calculate the moment magnitude from the hydrophone record spectra. This method could be easily exported to other volcanic areas, where submerged volcanoes are monitored by networks of hydrophones and seismic sensors on land. The proposed approach allows indeed magnitude measurements of small magnitude earthquakes occurring at sea, thus adding useful information to the seismicity of these volcanoes.

     

卵石土场地地震反应特征振动台试验研究

为研究卵石土场地地震反应特征,基于四川成都典型卵石土场地,通过振动台模型试验研究卵石土场地在不同地震波、不同地震强度激励下的加速度峰值放大系数、加速度频谱反应及动土压力反应,并且对其场地地震反应非线性效应及土体动剪应力-动剪应变关系进行分析。结果表明：卵石土场地表层土层对地震波具有明显的放大效应,加速度峰值放大系数介于1～1.4之间,下部土层放大效应较小,加速度峰值放大系数介于0.9～1.2之间。卵石土场地对地震波具有低频放大,高频滤波的作用,滤波频率上、下限随激励强度的增大逐渐向低频方向移动。激励强度较小时,土体尚未破坏,动土压力在地震过程中逐渐增大;随着激励强度的增大,动土压力反应明显增大,表现出骤减后逐渐增大的现象。在激励强度较小时(SN1),中部土体最先进入非线性反应阶段,地震波在中部土层能量损耗最大;激励强度较大时(EL3),土体均发生了较大变形,土体最大动剪应变达到1.7%,此时卵石土场地对地震波的放大作用明显减弱。