Seismic interferometry experiment in a shallow cased borehole using a seismic vibrator source‡

We present the results of a seismic interferometry experiment in a shallow cased borehole. The experiment is an initial study for subsequent borehole seismic surveys in an instrumented well site, where we plan to test other surface/borehole seismic techniques. The purpose of this application is to improve the knowledge of the reflectivity sequence and to verify the potential of the seismic interferometry approach to retrieve high‐frequency signals in the single well geometry, overcoming the loss and attenuation effects introduced by the overburden. We used a walkaway vertical seismic profile (VSP) geometry with a seismic vibrator to generate polarized vertical and horizontal components along a surface seismic line and an array of 3C geophones cemented outside the casing. The recorded traces are processed to obtain virtual sources in the borehole and to simulate single‐well gathers with a variable source‐receiver offset in the vertical array. We compare the results obtained by processing the field data with synthetic signals calculated by numerical simulation and analyse the signal bandwidth and amplitude versus offset to evaluate near‐field effects in the virtual signals. The application provides direct and reflected signals with improved bandwidth after vibrator signal deconvolution. Clear reflections are detected in the virtual seismic sections in agreement with the geology and other surface and borehole seismic data recorded with conventional seismic exploration techniques.     

Tomographic inversion of near-surface <Emphasis Type="Italic">Q</Emphasis> factor by combining surface and cross-hole seismic surveys

The estimation of the quality factor Q plays a fundamental role in enhancing seismic resolution via absorption compensation in the near-surface layer. We present a new geometry that can be used to acquire field data by combining surface and cross-hole surveys to decrease the effect of geophone coupling on Q estimation. In this study, we drilled number of receiver holes around the source hole, each hole has different depth and each geophone is placed geophones into the bottom of each receiver hole to avoid the effect of geophone coupling with the borehole wall on Q estimation in conventional cross-hole seismic surveys. We also propose a novel tomographic inversion of the Q factor without the effect of the source signature, and examine its stability and reliability using synthetic data. We estimate the Q factors of the near-surface layer in two different frequency bands using field data acquired in the Dagang Oilfield. The results show that seismic absorption in the near-surface layer is much greater than that in the subsurface strata. Thus, it is of critical practical importance to enhance the seismic solution by compensating for near-surface absorption. In addition, we derive different Q factors from two frequency bands, which can be treated, to some extent, as evidence of a frequency-dependent Q.     

套管井偶极弯曲波频散向高频偏移的特性

对套管井偶极弯曲模式波的频散特性进行了系统的数值考察、实例对比和分析.发现套管井弯曲波频散曲线随地层横波速度的降低,特别是地层横波速度小于2000 m·s^-1以下,会迅速移向高频区,偶极弯曲波基础模式主频散区（或截止频率）可出现在13 kHz以上,以致超出了现行低频偶极子声波测井仪的激发与接收频带,这是一过去没有被研究者注意到的现象,并进一步被现场实例所证实.研究表明控制套管井弯曲波频散曲线主频散区位置的主要是钢套管的厚度和地层横波速度.对地层横波速度大于井孔流体声速的快速地层,在钢套管壁厚一定（8 mm）的情况下,频散曲线主频散区可移至11 kHz以上,可能出现的最大可能频域位置是同一井孔内径,井外全钢时的频散曲线上等于、小于地层横波速度那一段,这对各种地层和套管参数都是适用的.对地层横波速度小于等于井孔流体声速（1500 m·s^-1）的慢速地层,弯曲波频散曲线随地层横波速度的降低移向高频区的特点更为明显,可能移至16 kHz以上;而套管厚度的影响,也比快速地层大的多,对地层横波速度小于1380 m·s^-1的慢速地层,无论用多高的频率激发,都不能在现行使用的各类套管井（壁厚6~12 mm）中用偶极声波测井仪测到弯曲模式波.     

IMAGING CAPABILITY OF CROSS-HOLE SEISMIC REFLECTION SURVEYS1

In seismic migration, it is important to sample a range of dips around the local structural dip at each image point. Meaningful images are obtained only where this condition holds. For cross-hole seismic reflection surveys, the distribution of dips sampled at each image point is controlled principally by the survey geometry, including source and receiver array lengths and their element spacings. Using a real data set as an example, we show how survey geometry can limit imaging capability close to the boreholes and even in the middle of the section between the boreholes. At the processing stage, effective removal of direct waves and accurate estimation of the velocity field are essential for optimizing image quality. For migration, we propose a generalized Berryhill (GB) scheme which is based on the Kirchhoff integral and takes into account both the near-field and far-field terms. This should improve the ability to image close to source and receiver arrays, provided that the element spacing in the nearby array is small enough.     

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.     

MODELLING GUIDED WAVES IN CROSS-HOLE SURVEYS IN UNCRACKED AND CRACKED ROCK1

The theoretical behaviour of guided waves in cross-hole seismic surveys is examined with dispersion curves and synthetic seismograms. The modelling suggests, and field observations confirm, that the dominant energy in many cross-hole seismic surveys propagates as guided waves, as well as body waves, with the energy tied to an interface, or combination of interfaces, whenever there are interfaces with significant velocity contrasts continuous between the wells. The dispersion and waveforms of such modal solutions carry detailed information about the internal structure, particularly the stress-aligned structure of fluid-filled inclusions in the immediate neighbourhood of the waveguide. Since the information content can be controlled to some extent by the choice of frequency and source type and the level of source and receiver in the two wells, such guided waves in cross-hole surveys may be important for monitoring changes in nearly horizontal reservoirs during hydrocarbon production processes. Initially, we investigate the basic properties of propagation in isotropic strata, and then examine the behaviour of guided waves in the stress-aligned fluid-filled inclusions.     

Cased borehole effects on downhole seismic measurements1

Both approximate and exact formulations for the interaction of an incident elastic wave with a cased borehole are presented. In the approximate method, simple and explicit formulae are derived for the pressure in fluid at low frequencies. In the exact method, elastic potentials in each annulus are represented as a superposition of fundamental solutions to the Helmholtz equations. Continuity of displacements and stresses across layer boundaries are used to determine unknown coefficients. A global matrix algorithm is employed to compute simultaneously these coefficients in individual layers. Calculations show that, in cased boreholes, the borehole effects on downhole seismic measurements are more significant than in open boreholes. A strong resonance occurs in the fluid for SV-wave incidence from a soft formation. This resonance is prominent even at very high frequencies because the tube-wave velocity is raised well above the formation shear velocity by the steel pipe. At a particular angle of incidence of a plane P-wave, the pressure in the fluid is near zero at low frequencies (the cased borehole screening phenomenon). For hard formations and frequencies above 1 kHz, the cased borehole influence on a downhole geophone measurement is significant, especially at grazing incidence. For soft formations, both the pressure in the fluid and the solid displacement on the borehole wall show strong dependence on frequency and angle of incidence, even at low frequencies.     

井间高分辨率纵横波层析成像研究井间油藏

采用视速度-偏振井间波场分离法分离得到纵波和横波波场,使用跨孔地震走时层析成像级联方法计算井间纵波和横波速度结构以及泊松比空间分布.综合纵横波速度、泊松比和测井参数,分析两井间岩性和储层特性.数值模型试验表明该方法垂直分辨率达到主频波长的1/4,并能准确分辨倾斜断层和垂直断层.处理相同纵波资料结果表明本文方法分辨率远远高于以往的井间走时层析成像方法,比井间声波层析成像分辨率更高.使用该方法处理垦71区井间资料,得到同台同源纵波和横波速度结构和泊松比分布.泊松比的低/高变化与测井参数指示的砂/泥岩层基本吻合.根据泊松比分布,参考岩石地震物理学测试数据,可以区分井间介质中的泥岩层、砂岩层,以及砂岩层中的饱水和饱油区,确定储层的连通性,圈定井间的剩余油藏.     

VERTICAL SEISMIC PROFILING—SEPARATION OF P- AND S-WAVES*

In vertical seismic profile's (VSP's) shot with a large source offset, rays from shot to receiver can have large angles of incidence. Shear waves generated by the source and by conversions at interfaces are likely to be recorded by both the vertical and the horizontal geophones. Varying angles of incidence may give strong variations in the recorded amplitudes. Separation of P- and SV-waves and recovery of their full amplitudes are important for proper processing and interpretation of the data. A P-S separation filter for three-component offset VSP data is presented which performs this operation. The separation filter is applied in the k-f domain and needs an estimate of the P- and S-velocities along the borehole as input. Implementation and stability aspects of the filter are considered. The filter was tested on an 1800 m offset VSP and appeared to be robust. Large velocity variations along the borehole could be handled and results were superior to those obtained by velocity filtering.     

偶极声源在裸眼井及套管井外的横波辐射特征

偶极横波远探测技术作为近年来发展起来的一门地球物理测井技术,一个重要应用前景就是在裸眼井和套管井测井时寻找井外地层中的油气构造.本文从井中偶极声场积分表达式的远场渐近解出发,对裸眼井及套管井中偶极声源的辐射场进行了对比分析,首次得到了偶极声源在不同声源频率和不同地层类型下的辐射特征.并深入研究了套管井中偶极声源的SH横波远场辐射特性,给出了不同耦合情况下的SH横波远场辐射特性的变化规律,对比了套前套后SH横波远场辐射的异同.结果表明：井中偶极声源的远场辐射特性受声源频率和地层类型影响较大;在套管井中,偶极声源辐射到地层中的横波能量也可以进行远探测,尽管与裸眼井情况相比,辐射能量会有所降低.本文的结果为偶极横波远探测技术的应用提供了理论支持.     

Imaging weak zones in the foundation using frequency domain attenuation tomography

Cross-hole imaging method using Time Domain (TD) and Frequency Domain (FD) parts of cross-hole radar tomography data acquired using Step Frequency Ground Penetrating Radar (SFGPR) was implemented. This method was adopted for imaging foundation of a dam to check if the foundation was free of geological weak zones. The dam site is characterised by massive and jointed-phyllites associated with major and minor shears. The cross-hole radar tomography data was acquired in the frequency bandwidth of 250 MHz, from the deepest level gallery up to a depth of 40 m in the foundation. In TD, first arrival time and amplitudes of radio waves were inverted using Simultaneous Iterative Reconstruction Technique (SIRT) resulting in velocity and attenuation tomograms. The tomograms showed nearly uniform velocity or attenuation structure in the respective tomographic plane. Subsequently, cross-hole radar tomography data was analysed in FD for a variation of spectrum-amplitude at different frequencies. Amplitudes picked at each single frequency were then inverted using SIRT for obtaining frequency domain attenuation tomogram (FDAT). The FDAT clearly showed presence of anomalous high attenuation zones in the depth range of 23–33 m of the tomographic plane. The anomalous zones in the attenuation tomogram are weak zones in the foundation. To validate the above observations, cross-hole seismic tomography was also done in the same boreholes. Cross-hole seismic tomography results showed low velocity (p-wave) zones around the same location corresponding to the high attenuation zone in FDAT, bringing the dormant weak zone to light. This enabled fine-tuning of the reinforcement design and strengthening the weak zone. This paper discusses the cross-hole radar tomography imaging method, the results of its application in imaging weak zones in the foundation and the comparison of cross-hole radar tomography results (in TD and FD) with the cross-hole seismic tomography results.     

The Suction Side Sample Catcher in Ground Water Quality Sampling

A suction side sample collector (SSSC) is a contrivance installed hydraulically ahead of the intake port of a pumping device. This paper describes construction and operational details of SSSCs fitted to a submersible pump with packer for use in a 6-inch cased borehole, an air lift pump with packer for use in a 1-inch or 2.5-inch cased borehole, a bladder pump for use in a casing of 2-inch or greater diameter, and a jet pump with packer for use in a 2-inch cased borehole.
Each form of SSSC has been thoroughly tested in ground water quality sampling for volatile organic chemicals. Comparative data for samples collected with the SSSCs and conventional sample collecting gear are presented. The SSSC is demonstrated to be superior to other methods of collecting volatile organic chemical samples owing to its freedom from contamination by the pump delivery line and to its mode of collecting the sample from a position in the well remote from disturbance by the pumping technique.
SSSCs are conveniently decontaminated, easily transported, and can be used to deliver samples to the laboratory while still at formation pressure. The air-lift pumps, described in this paper for use with SSSCs in 1- and 2.5-inch casings, have pumping capacities greater than obtained by other methods that can operate in these small casings. Discharge rates of up to 2 gpm are routinely achieved with the 1-inch model and higher rates are common With the 2.5-inch model. The use of packers with these pumps reduces the time needed to replace the water in the casing with fresh water from the formation.     

The Use of Wave-field Directivity for Velocity Estimation: Moving Source Profiling (MSP) Experiments at KTB

—Within the "Integrated Seismics Oberpfalz 1989 (ISO89)" a three-component Moving Source Profiling (MSP) experiment, also named walk-away VSP, was carried out at the drilling site of the "Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland (KTB)" in Germany. Analysis of transmitted waves traveling from the source locations at the surface down to the receiver array in the borehole reveals velocity information about the illuminated part of the subsurface. Complementary to the widely used evaluation of travel-time perturbations to locate velocity inhomogeneities we suggest the use of the directivity of transmitted wave types down in the borehole. To determine the wave-field directivity we focus on transmitted arrivals by employing principles of "Controlled Directional Reception (CDR)." We calculate local slant-stacks for three different depth positions as a function of the source offset, thus obtaining the variation of the vertical slowness (vertical ray parameter) of incident waves along the horizontal source profile and the vertical receiver array. The slowness data combined with travel times are interpreted by forward modeling taking into account geological information of the survey area. Our findings confirm results from gravity measurements which suggest the existence of large amphibolite/metabasite complexes in the vicinity of the borehole. The described method is also used to identify P-to-S converted energy originating from fracture zones above the receiver array and to locate the region in which conversion occurs.     

Geometrical spreading factor of head waves in cased and open boreholes

Factors (coefficients) of geometrical spreading of compressional and shear head waves are calculated for an impulse multipole source of elastic oscillations in boreholes. It is shown that the length of the logging tool (i.e., the distance between the source and the nearest receiver) used for sonic measurements and the velocities of elastic waves in the medium both contribute to the factor of geometrical spreading. For a high-velocity formation (the shear wave velocity in the rock is higher than the compressional wave velocity in the fluid that fills the borehole) and a sufficiently long sonic tool with a monopole source, the coefficient of geometrical spreading is approximated by asymptotic formula 1/Z [Roever et al., 1974; Krauklis and Krauklis, 1976], where Z is the length of the tool; i.e., the amplitude of the compressional head wave decreases proportionally to the distance between the source and the receiver. In acoustically soft formations, this approximation is inapplicable even for long tools with length Z > 4 m. Waveforms in cased boreholes have a significant frequency dispersion even in case of good-quality cementing, and the factor of geometrical spreading there depends considerably on the length of the tool and the elastic properties of the rocks.     

岷县漳县M_S6.6地震震源区地壳速度结构特征初步研究

利用距离2013年岷县漳县地震最近的固定台站岷县台2008-2009年的远震接收函数,确定了该地震震源区及临近区域的地壳厚度和波速比。结果表明:岷县台下方地壳速度结构的横向非均匀性较强,各方位接收函数差异较大,特别是震源区与临近区域存在明显的差别。临近区域的中下地壳存在明显的低速层,而震源区中下地壳中存在明显的高速区;且震源区地壳平均波速比为1.76,上地壳的波速比仅为1.62。据此推断:震源区是坚硬的上地壳覆盖在较软的中下地壳之上,岷县漳县地震破裂有可能是下地壳流的活动导致上地壳的破裂。     

Vertical seismic profiling using a daisy‐chained deployment of fibre‐optic cables in four wells simultaneously – Case study at the Ketzin carbon dioxide storage site

The geological storage of carbon dioxide is considered as one of the measures to reduce greenhouse gas emissions and to mitigate global warming. Operators of storage sites are required to demonstrate safe containment and stable behaviour of the storage complex that is achieved by geophysical and geochemical monitoring, combined with reservoir simulations. For site characterization, as well as for imaging the carbon dioxide plume in the reservoir complex and detecting potential leakage, surface and surface‐borehole time‐lapse seismic monitoring surveys are the most widespread and established tools. At the Ketzin pilot site for carbon dioxide storage, permanently installed fibre‐optic cables, initially deployed for distributed temperature sensing, were used as seismic receiver arrays, demonstrating their ability to provide high‐resolution images of the storage formation. A vertical seismic profiling experiment was acquired using 23 source point locations and the daisy‐chained deployment of a fibre‐optic cable in four wells as a receiver array. The data were used to generate a 3D vertical seismic profiling cube, complementing the large‐scale 3D surface seismic measurements by a high resolution image of the reservoir close to the injection well. Stacking long vibro‐sweeps at each source location resulted in vertical seismic profiling shot gathers characterized by a signal‐to‐noise ratio similar to gathers acquired using geophones. A detailed data analysis shows strong dependency of data quality on borehole conditions with significantly better signal‐to‐noise ratio in regions with good coupling conditions.     

A trial for monitoring temporal variation of seismic velocity using an ACROSS system

Abstract The temporal variation of seismic velocity near the Nojima Fault, which ruptured during the 1995 Kobe earthquake (Hyogo-ken Nanbu earthquake), was detected using an accurately controlled routine-operated seismic source (ACROSS). The source generates elastic waves by a centrifugal force of an eccentric mass rotating around an axis. The mass is driven with an AC servomotor whose angular position is accurately controlled with reference to a very accurate global positioning system (GPS) clock. The error of the mass' position is less than 0.002 radian and does not accumulate. As a result, the source generates sinusoidal waves of very narrow spectral peaks enabling their detection with an excellent signal-to-noise ratio. Although the stability of the rotation is quite excellent, a large daily variation was found, which seems to be caused by changes in atmospheric temperature. The daily variation was 10% in amplitude and 0.1 radian in phase of the signal observed at the 800 m borehole seismometer. A significant variation was found to be due to that of coupling between the rotational source and the foundation made of reinforced concrete in which the source was situated. In order to make a correction on the signal of the 800 m borehole seismometer, the vibration of the foundation was measured and modeled assuming a rigid body movement. The correction successfully reduced the daily variation by approximately 90%, resulting in a variation of 1% in amplitude and 0.01 radian in phase. The phase variation of 0.01 radian corresponds to 100 μs and less than 0.1% in velocity over 1000 m between the source and the receiver.     

套管井应力集中诱导井周波速各向异性的研究

针对在套管井中应用正交偶极声测井检测地层各向异性和确定异常构造应力的需要, 本文通过求解水平构造应力作用下套管井的应力场分布,应用声弹性理论说明应力集中对套管井井周弹性波速空间分布的影响,分析了波速空间分布对加套管后的正交偶极各向异性声测量的影响.井孔、套管、水泥环和地层系统的复杂性改变了原裸眼井情况地层中的应力场分布,使应力集中的范围明显缩小到井眼附近.对应不同方位径向偏振横波的交叉点向井孔附近移动.横波波速空间分布的方位极值规律发生变化,但纵波波速空间分布的方位极值规律不变,仍能提供最大远场构造应力施加的方向.     

A COMPARISON OF LABORATORY AND FIELD MEASUREMENTS OF P-WAVE ANISOTROPY1

P-wave velocity anisotropy determined from a cross-hole survey at the Imperial College borehole test site compares favourably with that measured in the laboratory on core from the holes. The holes penetrate a layered sequence of sandstones, shales and carbonates of the Namurian Upper Limestone Group. The Laboratory measurements of the vertical and horizontal velocities of core samples indicate that the shales exhibit P-wave anisotropies of over 20% but that the sandstones and limestones are only slightly anisotropic. These discrete measurements have been used in combination with wireline data to produce a log of P-wave anisotropy. Including the anisotropic information vastly improves the match between observed and synthetic traveltimes from the cross-hole data set. This implies that there is little frequency dependence of intrinsic P-wave anisotropy. Inversion of the cross-hole traveltimes highlights the need for good angular coverage in order to resolve the anisotropy parameters. The observed P-wave anisotropy of the field data is due to the combined effect of sedimentary layering and the intrinsic anisotropy of the rocks. The intrinsic anisotropy is found to be the dominant factor.