Burying receivers for improved time‐lapse seismic repeatability: CO2CRC Otway field experiment

4D seismic is widely used to remotely monitor fluid movement in subsurface reservoirs. This technique is especially effective offshore where high survey repeatability can be achieved. It comes as no surprise that the first 4D seismic that successfully monitored the CO₂ sequestration process was recorded offshore in the Sleipner field, North Sea. In the case of land projects, poor repeatability of the land seismic data due to low S/N ratio often obscures the time‐lapse seismic signal. Hence for a successful on shore monitoring program improving seismic repeatability is essential. Stage 2 of the CO2CRC Otway project involves an injection of a small amount (around 15,000 tonnes) of CO₂/CH₄ gas mixture into a saline aquifer at a depth of approximately 1.5 km. Previous studies at this site showed that seismic repeatability is relatively low due to variations in weather conditions, near surface geology and farming activities. In order to improve time‐lapse seismic monitoring capabilities, a permanent receiver array can be utilised to improve signal to noise ratio and hence repeatability. A small‐scale trial of such an array was conducted at the Otway site in June 2012. A set of 25 geophones was installed in 3 m deep boreholes in parallel to the same number of surface geophones. In addition, four geophones were placed into boreholes of 1–12 m depth. In order to assess the gain in the signal‐to‐noise ratio and repeatability, both active and passive seismic surveys were carried out. The surveys were conducted in relatively poor weather conditions, with rain, strong wind and thunderstorms. With such an amplified background noise level, we found that the noise level for buried geophones is on average 20 dB lower compared to the surface geophones. The levels of repeatability for borehole geophones estimated around direct wave, reflected wave and ground roll are twice as high as for the surface geophones. Both borehole and surface geophones produce the best repeatability in the 30–90 Hz frequency range. The influence of burying depth on S/N ratio and repeatability shows that significant improvement in repeatability can be reached at a depth of 3 m. The level of repeatability remains relatively constant between 3 and 12 m depths.     

江苏东海深井观测地震波形及其信噪比研究

为了探索在高噪声干扰地区获取高信噪比地震信息的有效途径,本文分析了我国第一个超深井地震观测站江苏东海地壳活动国家野外科学观测研究站设置在地面与井下三个不同深度处的地震仪所记录的波形及其信噪比特征。结果显示：由于观测研究站周围强烈的噪声扰动,地面地震仪记录中无法识别M_L0.8地方震波形,而深井地震仪可清晰地记录到该小震波形,且深井地震仪可观测到较地面地震仪更多的零级或负震级地震;井下三组不同深度地震仪所记录波形的信噪比均远高于地面地震仪,且不同深度地震波形信噪比的平均值随仪器深度的增大而增加。井下1 559.5 m处的地震仪的波形平均信噪比为69.20 dB,2 545.5 m处的信噪比达到74.15 dB,均达到高保真波形的信噪比值,这说明1 500 m深处地震仪所观测到的波形可以有效地避免地面干扰,因此深井地震观测能够提供高信噪比的波形资料,为研究震源过程和场地效应等提供真实可靠的基础资料,这也预示着深井观测将促进深井地震学的研究与发展。     

Numerical study on screening of surface waves using a pair of softer backfilled trenches

Vibration screening effectiveness of a pair of in-filled trenches is numerically studied under conditions of plane strain using PLAXIS 2D in a linear elastic, isotropic, and homogeneous half-space acted upon by a steady-state vertical excitation. Barrier efficiency is analyzed in terms of reduction of vertical and horizontal vibration components with respect to the variations in barrier geometry and in-fill material parameters. The geometric features are normalized with respect to Rayleigh wavelength of vibration in soil. The in-fill trench material is considered to be softer and accordingly assigned lower shear wave velocities than the half-space. Non-dimensional design charts are developed and the key parameters participating in wave screening process are identified. Some recommendations are made regarding the optimal selection of these parameters. Comparison with a few published results shows close agreement. Such an isolation scheme may be advantageous in circumstances where a single trench requires unrealistic depth.     

波场延拓深度滤波方法

消除面波是地震数据处理中的一个重要内容．本文提出了基于15°波动方程的深度滤波方法．由于面波与有效反射波具有不同的传播深度,可利用波场向下延拓方法将二者进行波场分离．把向下延拓后的波场中集中在地表附近的面波能量切除后,再将波场重新延拓回原始的观测面,达到去除干扰的目的．实际资料处理显示：方法计算稳定,消除面波能力强,能更好地保持波场的有效成份和幅值,符合波场的实际传播状态,表明该方法正确可行．     

Scattered ground‐roll attenuation using model‐driven interferometry

Scattered ground roll is a type of noise observed in land seismic data that can be particularly difficult to suppress. Typically, this type of noise cannot be removed using conventional velocity‐based filters. In this paper, we discuss a model‐driven form of seismic interferometry that allows suppression of scattered ground‐roll noise in land seismic data. The conventional cross‐correlate and stack interferometry approach results in scattered noise estimates between two receiver locations (i.e. as if one of the receivers had been replaced by a source). For noise suppression, this requires that each source we wish to attenuate the noise from is co‐located with a receiver. The model‐driven form differs, as the use of a simple model in place of one of the inputs for interferometry allows the scattered noise estimate to be made between a source and a receiver. This allows the method to be more flexible, as co‐location of sources and receivers is not required, and the method can be applied to data sets with a variety of different acquisition geometries. A simple plane‐wave model is used, allowing the method to remain relatively data driven, with weighting factors for the plane waves determined using a least‐squares solution. Using a number of both synthetic and real two‐dimensional (2D) and three‐dimensional (3D) land seismic data sets, we show that this model‐driven approach provides effective results, allowing suppression of scattered ground‐roll noise without having an adverse effect on the underlying signal.     

Ground-roll separation of seismic data based on morphological component analysis in two-dimensional domain

Ground roll is an interference wave that severely degrades the signal-to-noise ratio of seismic data and affects its subsequent processing and interpretation. In this study, according to differences in morphological characteristics between ground roll and reflected waves, we use morphological component analysis based on two-dimensional dictionaries to separate ground roll and reflected waves. Because ground roll is characterized by low-frequency, low-velocity, and dispersion, we select two-dimensional undecimated discrete wavelet transform as a sparse representation dictionary of ground roll. Because of a strong local correlation of the reflected wave, we select two-dimensional local discrete cosine transform as the sparse representation dictionary of reflected waves. A sparse representation model of seismic data is constructed based on a two-dimensional joint dictionary then a block coordinate relaxation algorithm is used to solve the model and decompose seismic record into reflected wave part and ground roll part.The good effects for the synthetic seismic data and application of real seismic data indicate that when using the model, strong-energy ground roll is considerably suppressed and the waveform of the reflected wave is effectively protected.     

基于接收阵列的时域地震波束形成方法

地震勘探中,针对目的层地震数据信噪比较低情况,为改善信噪比,本文提出一种基于接收阵列的时域地震波束形成方法(TSBBRA).该方法的理论依据是波束形成原理,文中通过对相控震源激发定向地震波场的分析,提出在反射地震勘探中,在信号接收端同样能生成定向地震波的思路.具体地,论文给出在地震数据集上先对单炮记录分组,再对组内记录时域内延时、叠加,生成对应定向地震波场的地震记录的方法.TSBBRA方法适于杂波干扰强,且杂波与目的层反射波方向不一致的地震条件.本文先通过波场模拟,展示了TSBBRA方法具有形成定向地震波场的能力;而后通过对比相控震源与TSBBBRA时间剖面,说明了TSBBRA方法的有效性;最后在吉林某油页岩矿区,用TSBBRA方法,分别针对浅层目标与深层目标,生成定向地震波数据.与原始数据相比,目标层信噪比提高了3~35 dB,在时间剖面上观测到的深部目标信号从200 ms扩展到1000 ms,TSBBRA剖面资料与测区附近区域的地质与地震资料一致.由此得出结论,TSBBRA方法有助于改善目的层地震资料信噪比,当关注深部目的层时,由于TSBBRA方法能有效压制噪声,因此可用于能够实现更大的勘探深度.实验中,我们给出了浅层勘探结果,由于本方法对震源及观测系统没有限制,实际应用中TSBBRA方法可作为深部资源勘探领域提高深部地震资料信噪比的有效方法.     

Vibration isolation by trenches in continuously nonhomogeneous soil by the BEM

Vibration isolation of structures from ground-transmitted waves by open trenches in isotropic, linearly elastic or viscoelastic soil with a shear modulus varying continuously with depth is numerically studied. Both an exponential and a linear shear modulus variation with depth are used in this work. Waves produced by the harmonic motion of a rigid surface machine foundation are considered. The problem is solved by the frequency domain boundary element method employing the Green's function of Kausel-Peek-Hull for a thin layered half-space. Thus only the trench perimeter and the soil-foundation interface need essentially to be discretized. The proposed methodology is first tested for accuracy by solving two Rayleigh wave propagation problems in nonhomogeneous soil with known analytical solutions and/or for which experimental results are available. Then the method is applied to vibration isolation problems and the effect of the inhomogeneity on the wave screening effectiveness of trenches is studied.     

Study of effects of focal depth on the characteristics of Rayleigh waves using finite difference method

This paper presents a relationship between the focal depth in terms of Rayleigh-wave wavelength and the dominant frequency of Rayleigh waves generated in a homogeneous half-space. Rayleigh waves were simulated using a (2, 4) staggered grid P-SV wave finite difference algorithm with VGR-stress imaging technique as a free surface boundary condition. VGR is an acronym for vertical grid-size reduction. The simulated seismic responses using P-wave and SV-wave sources at different focal depths revealed Rayleigh-wave generation up to certain focal depth only for the considered frequency bandwidth. A shift of normalized spectral shape of Rayleigh wave towards lower frequency with increasing focal depth was inferred. Largest spectral amplitude was obtained in the wavelength for which the ratio of focal depth to the wavelength of Rayleigh wave was around 0.17 in the case of P-wave source and 0.9 in the case of SV-wave source. An exponential decrease of spectral amplitude of Rayleigh wave with the departure of the ratio of focal depth to Rayleigh wave wavelength from the above mentioned values was obtained.     

利用多种地震数据联合反演剪切波速度结构的可靠性检测

本文模拟使用青藏高原东南缘区域台网及国家台网的170个宽频台站基于背景噪声、天然地震面波、P波接收函数反演时的实际数据,对青藏高原东南缘假定的初始模型进行恢复,通过计算初始模型台站下方纯路径频散、提取各台站对间的瑞雷波频散曲线、计算理论接收函数以及反演剪切波速度结构来测试使用不同单项数据与联合使用多种数据反演对初始模型的恢复程度。结果表明,同时使用接收函数、基于噪声经验格林函数的群速度、相速度频散以及基于天然地震面波的相速度频散联合反演的剪切波速度结构,充分利用了几种数据的分辨率优势,清晰地分辨出中下地壳及上地幔顶部的低速层。此外,本文也分析了实际数据处理中出现的计算误差、随机噪声干扰对计算结果稳定性的影响。结果显示:对于面波频散,加入1%的误差后,联合反演的结果仍可很好地反映低速层的形态,但是当误差提升至5%后,对最终结果则产生了一定程度的影响;而在接收函数中加入4%的随机噪声时,虽然地幔低速层的上界面和下界面会略微受到随机噪声的影响,但是低速层的深度范围和速度值均得到了较好的恢复。     

An experimental study of the isolation performance of trenches on ground-borne vibration from underground tunnels

This paper presents a study of the isolation performance of trenches on train-induced ground-bore vibration from underground tunnels. A series of model tests was conducted to perform the study. Both train loads and sinusoidal sweep loads were applied to the model tunnel invert by a shaker. The response of the soil at the surface of the ground was measured by accelerometers. Three types of isolation trenches, i.e., an open trench, an expanded Polystyrene(EPS)-filled trench, and a Duxseal-filled trench, were modelled in the experiment. The results showed that the open trench and the EPS-filled trench increased the dynamic response of the surface soil in front of the trench, but the Duxseal-filled trench had little effect on the dynamic response of the soil on the surface in this area. Although all three types of isolation trenches reduced the response of the soil behind the trench, the open trench and the Duxseal-filled trench were clearly more effective. The test results also showed that, as the excitation frequency increased, the vibration isolation trench was more effective in the near field. The effects of the width, depth, and position of an open trench on isolation performance also were examined in the tests.     

The effectiveness of trenches in reducing seismic motion

The effect of placing barriers in the travel path of P, SV and SH seismic waves has been studied using time-domain solutions of plane-strain finite element programs for two-dimensional crustal models. The wavefields considered propagate parallel to the free surface of an elastic medium consisting of a single layer over a halfspace. Barriers take the form of open-air trenches. Effects of damping are assessed by considering representative viscoelastic conditions. Computations are presented as the ratio of spectral energy observed at a point with the barrier system in place in the model to the spectral energy observed at the same point without the barrier system in the model. These spectral ratios are dependent upon the direction of wave propagation. The calculations brought to light the marked role of surficial layering and attenuation properties of the surface rocks or soils on the effectiveness of seismic trench barriers. Barrier models without these features cannot in general reliably predict seismic wavefields at the surface. In the range of cases studied, trench depth d rather than width is the most sensitive parameter. When the ratio d/λ, the ratio of trench depth to the wavelength of shear waves, is greater than about 0·6, power spectral ratios of 0·06 and less are found for SH waves and the vertical component of SV motion for frequencies of 4–6 Hz. By contrast, for frequencies less than 3–4 Hz, power spectral ratios from unity to about two and greater are observed, indicating amplification for the horizontal component of wave motion. Spectral ratios calculated at some locations in front of the barrier system show over two-fold amplification. Spectral ratios also change significantly with the relative location of the free surface observation point.     

Analysis and design of open trench barriers in screening steady-state surface vibrations

The problem of vibration isolation by rectangular open trenches in a plane strain context is numerically studied using a finite element code, PLAXIS. The soil media is assumed to be linear elastic, isotropic, and homogeneous subjected to a vertical harmonic load producing steady-state vibration. The present model is validated by comparing it with previously published works. The key geometrical features of a trench, i.e., its depth, width, and distance from the source of excitation, are normalized with respect to the Rayleigh wavelength. The attenuation of vertical and horizontal components of vibration is studied for various trench dimensions against trench locations varied from an active to a passive case. Results are depicted in non-dimensional forms and conclusions are drawn regarding the effects of geometrical parameters in attenuating vertical and horizontal vibration components. The screening efficiency is primarily governed by the normalized depth of the barrier. The effect of width has little significance except in some specific cases. Simplified regression models are developed to estimate average amplitude reduction factors. The models applicable to vertical vibration cases are found to be in excellent agreement with previously published results.     

饱和地基中空沟近场隔振的现场试验与二维半解析边界元分析

动力机器运行和车辆行驶等会产生振动污染,危及邻近建筑物安全和干扰精密仪器设备正常运行等。这些振动污染可通过在地基中设置空沟的方式来降低或消除。针对饱和地基上明置动力机器基础的环境振动影响及空沟近场隔振问题,进行了饱和地基上空沟近场隔振的现场试验,并对试验结果进行了无量纲化分析;基于饱和土半解析边界元法,分别推导了动力机器基础环境振动影响和空沟近场隔振的边界元方程;在此基础上,详细研究了空沟对动力机器基础振动影响的隔振效果,分析了空沟深度、宽度和距振源距离对其隔振效果的影响。结果表明:空沟能够有效的降低动力机器基础的环境振动影响;空沟宽度对其隔振效果影响相对较小,而空沟深度对其隔振效果影响较大,为获得较好的隔振效果,空沟深度建议取1倍Rayleigh波波长;空沟距振源距离对其隔振效果也有较大影响,距离越远则隔振效果也越好,当被保护建筑距振源较远时,建议空沟在被保护建筑附近设置。此外,在某些特殊情况下,空沟隔振系统会由于共振现象而出现隔振效果劣化的现象,在工程设计中应予以注意。     

可控震源定向照明方法的仿真研究

当野外噪声很强,即使使用组合震源地震也无法获得满意信噪比的地震数据时,本文提出了一种基于可控震源阵列的定向照明控制方法,采用该方法可形成定向地震波.通过仿真研究合成了8激震器可控震源阵列分别采用简单组合及定向照明技术得到的单炮地震记录,可以看出采用合适的延时参数,定向照明单炮地震记录的反射波信噪比高于组合地震情况.定量的计算结果表明,实验条件下采用0.89 ms延时参数,各反射波信噪比分别提高了10.19 dB,3.23 dB和1.02 dB.由此可见,可控震源定向照明地震技术是一种提高原始地震资料信噪比的有效方法.     

Effects of source and cavity depths on wave fields in homogeneous half spaces

In homogeneous half spaces excited by small buried, spherically symmetric P-wave sources, Rayleigh waves (R-waves) could be generated. The effects of the source depth on the induced wave pattern and propagation behavior of R-waves are analyzed using the thin layer method. When a cavity is present in a homogeneous half space, R-waves could be formed in the scattered wave field. It is found that the energy of R-waves in the incident (direct) surface wave-field is related to the ratio of the source depth to the wavelength of R-waves; R-waves have relatively strong energy when the ratio is less than 1; the buried source induced R-waves approximately travel at the velocity of the planar R-waves in the range of the offset beyond about one wavelength; the energy of R-waves in the back-scattered surface wave-field depends on the ratio of the depth of cavity to the wavelength of R-waves; and for the case of the cavity presented at depths less than one wavelength, R-waves can be clearly observed in the back-scattered wave field. The results are helpful for selecting the source depth and the frequency component in seismic surveys and interpreting both the incident wave and the scattered wave patterns.     

Interceptor Trenches for Positive Ground Water Control

Interceptor trenches are an effective ground water control method at waste management sites. Trenches may be installed without disturbing the wastes, and the withdrawal of ground water recovers contaminants that have left the waste management perimeter. The rapid and steep depression of the piezometric surface on both sides of the trench is positive proof of a barrier to horizontal flow across the trench in the affected permeable units.
Historically, the construction of interceptor trenches has been very difficult. A new and efficient installation method has been developed and successfully utilized for several applications at a petrochemical facility on the Texas coastal plain. Rapid and cost-effective installation is made possible by innovations in sump and trench construction and the tie-in between the two.
The sump is constructed first using standard well construction techniques to drill a 96-inch diameter hole to contain the 42-inch diameter polyethylene pipe sump. A European designed and fabricated trenching machine then excavates the trench, inserts the drainage pipe and backfills with sand and/or gravel in one operation. A specially designed perforated pipe entry door built into the side of the sump barrel provides for efficient and safe connection of the drainage pipe to the specially designed collection sump. The effectiveness of interceptor trenches has been confirmed in full scale applications through the reversal of flow gradients and the prevention of continued horizontal migration of ground water contaminants.     

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。     

相控震源定向地震波信号分析

应用可控震源地震勘探,当环境噪声很强,采用组合震源工作仍不能满足信噪比要求时,引入能形成定向地震波的相控震源.由相控震源定向照明地震理论分析,主波束方向上3单元相控震源产生的反射地震波信号信噪比比单震源高6.53~9.54 dB,比组合震源高1.76~4.77dB.为研究相控地震实际效果,在同一测区进行了三种震源地震对比实验.由单炮地震记录和水平叠加时间剖面可知,相控震源反射波信号信噪比明显高于单震源情况,略高于组合震源情况.进一步对反射波信号功率谱特性做定量分析,得到如下结果：与单震源情况相比,相控震源使各道反射波信号信噪比提高了0.75~8.15 dB,平均提高3.65 dB;与组合震源情况相比,各道信噪比提高了0.93~3.17 dB,平均提高2.02 dB,实验结果与理论分析吻合.综上所述可知,基于相控震源的定向照明地震技术是可行的,可以有效提高地震信号的信噪比.     

CROSS-BOREHOLE OBSERVATION OF MODE CONVERSION FROM BOREHOLE STONELEY WAVES TO CHANNEL WAVES AT A COAL LAYER1

Conversion of borehole Stoneley waves to channel waves was observed in data from a seismic cross-borehole experiment conducted between wellbores penetrating a thin coal layer at 2022 m depth, near Rifle, Colorado. Traveltime moveout observations show that borehole Stoneley waves underwent partial conversion to channel waves at the coal layer. The channel waves were detected directly in an adjacent borehole 35 m away at receiver positions within the coal. Stoneley waves, subsequently produced by partial conversion of channel waves, were also detected at receiver positions located up to 50 m above and below the coal layer in the adjacent borehole. We infer the channel wave to be the first-higher Rayleigh mode by comparing the observed group velocity with theoretically derived dispersion curves. Identifying the conversion between borehole and stratigraphically guided waves is significant because coal penetrated by multiple wells may be detected without placing a transmitter or receiver in the coal itself.