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1.
A new approximate method to calculate the space-time acoustic wave motion generated by an impulsive point source in a horizontally layered configuration is presented. The configuration consists of a stack of fluid layers between two acoustic half-spaces where the source and the receiver are located in the upper half-space. A distorted-wave Born approximation is introduced; the important feature of the method is the assumption of a background medium with vertical varying root-mean-square acoustic wave speed. A closed-form expression for the scattered field in space and time as a function of the contrast parameters is deduced. The result agrees closely with rigorously calculated synthetic seismograms. In the inverse scheme the wave speed and mass density can be reconstructed within a single trace. Results of the inversion scheme applied to synthetic data are shown.  相似文献   

2.
In the present work, the waveforms of reflected wave sonic log for open and cased boreholes are calculated. Calculations are performed for a borehole containing an acoustic multipole source (monopole, dipole, or quadrupole). A reflected wave is more efficiently excited at resonant frequencies. These frequencies for all source types are close to the frequencies of oscillations of a fluid column located in an absolutely rigid hollow cylinder. It is shown that the acoustic reverberation is controlled by the acoustic impedance of the rock Z = Vp ρs for fixed parameters of the borehole fluid, where Vp is the compressional wave velocity in the rock, and ρs is the rock density. This result is correct for all types of acoustic sources (monopole, dipole, or quadrupole). Methods of the waveform processing for determining parameters characterizing the reflected wave are discussed.  相似文献   

3.
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.  相似文献   

4.
Attenuation of seismic waves, quantified by the seismic quality factor Q, holds important information for seismic interpretation, due to its sensitivity to rock and fluid properties. A recently published study of Q, based on surface seismic reflection data, used a modified spectral ratio approach (QVO), but both source and receiver responses were treated as isotropic, based on simple raypath arguments. Here, this assumption has been tested by computing apparent attenuation generated by frequency-dependent directivity of typical marine source and receiver arrays and acquisition geometries. Synthetic wavelet spectra were computed for reflected rays, summed over the first Fresnel zone, from the base of a single interval, 50–3000 m thick and velocity 2000 m/s, overlying a 2200 m/s half-space, and for offsets of 71–2071 m. The source and receiver geometry were those of an actual survey. The modelled spectra are clearly affected by directivity, most strongly because of surface ghosts. In general, the strong high-frequency component, produced by the array design, leads to apparently negative attenuation in individual reflection events, though this is dependent on offset and target depth. For shallow targets (less than 400–500 ms two-way traveltime (TWT) depth), apparent Q-values as extreme as ?50 to ?100 were obtained. For deeper target depths, the directivity effect is far smaller. The implications of the model study were tested on real data. QVO was applied to 20 true-spectrum-processed CMPs, in a shallow (405–730 ms TWT) and a deeper (1000–1300 ms TWT) interval, firstly using a measured far-field source signature (effectively isotropic), and secondly using computed directivity effects instead. Mean interval Q?1-values for the deeper interval, 0.029 ± 0.011 and 0.027 ± 0.018 for conventional and directional processing, respectively, suggested no directivity influence on attenuation estimation. For the shallow interval (despite poor spectral signal-to-noise ratios and hence scattered attenuation estimates), directional processing removed directivity-generated irregularities from the spectral ratios, resulting in an improvement from Q?1int = ?0.036 ± 0.130 to a realistic Q?1int = 0.012 ± 0.030: different at 94% confidence level. Equivalent Q-values are: for the deeper interval, 35 and 37 for conventional and directional processing, respectively, and ?28 and 86 for the shallow interval. These results support the conclusions of the model studies, i.e. that source/receiver directivity has a negligible effect except for shallow targets (e.g. TWT depth ≤ 500 ms) imaged with conventional acquisition geometry. In such cases directivity corrections to spectra are strongly recommended.  相似文献   

5.
Single-well, or uni-well, imaging uses an acoustic source and an array of receivers located in the same borehole to image local geological structure. Due to the intrinsic attenuation of the formation it is likely that a source emitting frequencies in the typical cross-well range would be necessary to illuminate structure at distances above 100 m from the borehole. At these frequencies a significant proportion of the source energy is converted into tube-waves which are, for the purpose of these surveys, noise. This paper reports the results of a modelling study designed to assess the feasibility of using existing cross-well hardware, i.e. a piezo-electric source and hydrophone array, modified to run in a single borehole, to perform single-well surveys. In particular we study the case of an open borehole in a gas-filled, low-permeability sandstone reservoir. Our results suggest that the amplitudes of the tube-wave reverberations generated by calliper variations are such that reflections of interest arriving in the time window after the first tube-wave arrival will not be visible. However, reflections may be visible in the time window preceding the first tube-wave arrival provided tube waves from previous shots are not still present and the long source–receiver offsets required to make observations in this window can be incorporated into the tool design.  相似文献   

6.
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.  相似文献   

7.
The receiver function method was originally developed to analyse earthquake data recorded by multicomponent (3C) sensors and consists in deconvolving the horizontal component by the vertical component. The deconvolution process removes travel path effects from the source to the base of the target as well as the earthquake source signature. In addition, it provides the possibility of separating the emergent P and PS waves based on adaptive subtraction between recorded components if plane waves of constant ray parameters are considered. The resulting receiver function signal is the local PS wave's impulse response generated at impedance contrasts below the 3C receiver.We propose to adapt this technique to the wide‐angle multi‐component reflection acquisition geometry. We focus on the simplest case of land data reflection acquisition. Our adapted version of the receiver function approach consists in a multi‐step procedure that first removes the P wavefield recorded on the horizontal component and next removes the source signature. The separation step is performed in the τ?p domain while the source designature can be achieved in either the τ?p or the t?x domain. Our technique does not require any a priori knowledge of the subsurface. The resulting receiver function is a pure PS‐wave reflectivity response, which can be used for amplitude versus slowness or offset analysis. Stack of the receiver function leads to a high‐quality S wave image.  相似文献   

8.
Summary The problem of a point source in an isotropic, inhomogeneous fluid medium is discussed. It is assumed that the density of the fluid is constant and the acoustic velocity varies with depth asc=c 0(1 +m z) wherem is a constant andc 0 is, the velocity at the level of the origin. An approximate expression for the field due to a point source in such a medium is obtained when the medium is infinite as well as when it is semi-infinite. It is found that the results obtained agree with the WKB solution of the problem.  相似文献   

9.
三维随钻反射声波成像测井的数值模拟   总被引：2，自引：0，他引：2       下载免费PDF全文
采用有限差分方法对基于圆弧片状声源和接收器的三维随钻反射声波成像测井进行模拟,研究了反射信号的幅度、相位等参数随方位和源距的变化规律,着重分析了利用该方法在水平井中对地层界面进行探测的可行性.研究结果表明,钻铤的存在使得圆弧片状声源能够向固定方位辐射声场,其主瓣三分贝角宽窄,旁瓣级低,向目的方位辐射的纵波场的幅度约为传统环状声源的0.6倍,适用于三维随钻反射声波成像测井;对于本文计算的井孔模型,反射波信号约为井孔导波信号的1/100;随着源距的增加,反射纵波幅度逐渐减小,转换波(P-SV、SV-P)的幅度先增加后减小,反射SV波的幅度增加,建议在实际应用中,选择合适源距的波形进行处理,并对其他非目的波动进行压制,以期获得更好的成像效果.本文模拟了在水平井中对地层上下界面进行探测的例子,结果显示,反射波纵波信号有较好的方位分辨率,能够准确获得井外波阻抗不连续界面的方位,而且对上、下界面的成像互不影响.  相似文献   

10.
A new azimuthal acoustic receiver sonde with a body and corresponding circuits was designed for a downhole tool. The 64‐sensor receiver sonde holds eight receiver stations that can be combined into at least 64 three‐sensor receiver subarrays. As a result, the receiver sonde can use different sensor combinations instead of different transducer types to produce multiple modes, including a phased azimuthal reception mode and conventional monopole, dipole, and quadruple modes. Laboratory measurements were conducted to study the performance of the azimuthal acoustic receiver sonde for a downhole tool, and the experimental results indicate that the receiver sonde provides a consistent reception performance. Individual sensors receive similar time‐domain waveforms, and their corresponding frequency bands and sensitivities are consistent within the measurement errors of around 5%. The direction of the reception main lobe is approximately parallel to its exterior normal direction. In addition, a receiver subarray with three sensors receives waveforms that have higher energy and narrower beamwidths. For individual sensors, the angular width of the dominant reception lobe is 191.3° on average, whereas that of the individual receiver subarrays is approximately 52.1° on average. The amplitude of the first arrival received by the receiver subarray centred at the primary sensor directly pointing to the source is approximately 2.2 times the average amplitude of the first arrivals received by the other receiver subarrays in the same receiver station. Thus, the maximum amplitude of the waveforms received by the receiver subarrays can be used to determine the direction of the incident waves. This approach represents a promising method for determining the reflector azimuth for acoustic reflection logging and three‐dimensional acoustic logging.  相似文献   

11.
We developed a new marine controlled‐source electromagnetic receiver for detecting methane hydrate zones and oil and gas reservoirs on the seafloor, which is not imaged well by seismic reflection surveys. To determine the seafloor structure, the electromagnetic receiver should have low noise, power consumption, clock drift error, and operating costs while being highly reliable. Because no suitable receiver was available in our laboratory, we developed a new marine controlled‐source electromagnetic receiver with these characteristics; the receiver is equipped with acoustic telemetry modem and an arm‐folding mechanism to facilitate deployment and recovering operations. To demonstrate the applicability of our new receiver, we carried out a field experiment offshore of Guangzhou in the South China Sea, where methane hydrates have been discovered. We successfully obtained controlled‐source electromagnetic data along a profile about 13 km long. All six new receivers were recovered, and high‐quality electromagnetic data were obtained. Relatively high apparent resistivity values were detected. The results of the offshore field experiment support the claim that the electromagnetic data obtained using the new receiver are of sufficient quality for the survey target.  相似文献   

12.
Three methods for least-squares inversion of receiver array-filtered seismic data are investigated: (1) point receiver inversion where array effects are neglected; (2) preprocessing of the data with an inverse array filter, followed by point receiver inversion; (3) array inversion, where the array effects are included in the forward modelling. The methods are tested on synthetic data generated using the acoustic wave equation and a horizontally stratified earth model. It is assumed that the group length and the group interval are identical. For arrays that are shorter than the minimum wavelength of the emitted wavefield, and when the data are appropriately muted, point receiver inversion (first method) gives satisfactory results. For longer arrays, array inversion (third method) should be used. The failure of the inverse array filter (second method) is due to aliasing problems in the data.  相似文献   

13.
A polyaxial (true‐triaxial) stress‐loading system, developed originally for determining all nine components of P‐ and S‐wave velocities and attenuation and fluid permeability for 50.8 mm‐side cubic rock specimens tested to failure, has been modified to permit the measurement of acoustic emission events associated with the failure process. Results are reported for Crosland Hill sandstone tested to failure under loading conditions leading to the formation of sets of aligned microcracks, achieved by maintaining the minor principal stress at a low value while increasing the two other principal stresses until failure of the rock. An ultrasonic survey associated with the test has been employed to map the transversely‐isotropic velocity structure created by through‐going parallel fractures resulting from the sets of aligned microcracks. This velocity structure has then been employed to locate acoustic emission events recorded during the test by four acoustic emission sensors located in each of the six specimen loading platens. A selection of acoustic emission events associated with one of the fractures has been processed for moment tensor analysis information, in order to determine the source type and orientation of microcracking as the fracture grows. The mechanisms indicate tensile behaviour during initial fracture propagation. Shear failure, however, appears to dominate as the fracture finally approaches the opposite face of the cubic specimen. The work presented here has, in part, led to the development of new rock testing systems and geophysical monitoring and processing technologies that will enable laboratory study of rock behaviour under conditions better resembling those experienced in situ.  相似文献   

14.
K. Obana  H. Katao  & M. Ando 《Island Arc》1999,8(2):245-258
The present study describes a new system designed and developed for observing crustal deformation on the sea floor. The system consists of two parts, the kinematic positioning by global positioning system (GPS) and acoustic ranging techniques. Since the location of a site at the ocean bottom relative to a reference site on land cannot be determined directly, the procedure was divided into two steps. First, the position of a vessel was determined using differential and kinematic GPS techniques, and then the position of a reference point at the ocean bottom was located relative to the vessel using the acoustic ranging technique. Thus, the location of the ocean-bottom station is determined relative to the reference sites on land in the global reference system. The accuracy was tested in several ways. In one experiment, a buoy was used as the surface station which linked the positions of a GPS receiver to the acoustic transponder. Assuming a simple sound velocity profile of the seawater, the position of an ocean-bottom reference point was estimated with an accuracy of several meters. Thus, with the present system, it is difficult to observe ocean-bottom crustal deformations generated by typical plate motions. Methods are being investigated to improve the observation system for more accurate sea floor positioning.  相似文献   

15.
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.  相似文献   

16.
Point source reflection/transmission responses from planar interfaces based on the Sommerfeld-Weyl integral are causal and are given in the form of a solution integral with an infinite integration limit. They can be simply transformed to integrals with a finite limit by an approach that takes into account causality as well as time-symmetry aspects. To introduce the fundamental principles involved, the transformation is applied to the simple case of a point-source response from a planar acoustic interface. For a point source above a planar interface separating an upper and lower acoustic half-space of velocity c1and c2 the inhomogeneous plane waves in the Sommerfeld-Weyl source representation do not contribute to the final result if c1≤c2 and only a limited spectrum of inhomogeneous plane waves comes into play if c1c2.  相似文献   

17.
The multichannel analysis of surface wave (MASW) method has been effectively used to determine near-surface shear- (S-) wave velocity. Estimating the S-wave velocity profile from Rayleigh-wave measurements is straightforward. A three-step process is required to obtain S-wave velocity profiles: acquisition of a multiple number of multichannel records along a linear survey line by use of the roll-along mode, extraction of dispersion curves of Rayleigh waves, and inversion of dispersion curves for an S-wave velocity profile for each shot gather. A pseudo-2D S-wave velocity section can be generated by aligning 1D S-wave velocity models. In this process, it is very important to understand where the inverted 1D S-wave velocity profile should be located: the midpoint of each spread (a middle-of-receiver-spread assumption) or somewhere between the source and the last receiver. In other words, the extracted dispersion curve is determined by the geophysical structure within the geophone spread or strongly affected by the source geophysical structure. In this paper, dispersion curves of synthetic datasets and a real-world example are calculated by fixing the receiver spread and changing the source location. Results demonstrate that the dispersion curves are mainly determined by structures within a receiver spread.  相似文献   

18.
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。  相似文献   

19.
In anisotropic media, several parameters govern the propagation of the compressional waves. To correctly invert surface recorded seismic data in anisotropic media, a multi‐parameter inversion is required. However, a tradeoff between parameters exists because several models can explain the same dataset. To understand these tradeoffs, diffraction/reflection and transmission‐type sensitivity‐kernels analyses are carried out. Such analyses can help us to choose the appropriate parameterization for inversion. In tomography, the sensitivity kernels represent the effect of a parameter along the wave path between a source and a receiver. At a given illumination angle, similarities between sensitivity kernels highlight the tradeoff between the parameters. To discuss the parameterization choice in the context of finite‐frequency tomography, we compute the sensitivity kernels of the instantaneous traveltimes derived from the seismic data traces. We consider the transmission case with no encounter of an interface between a source and a receiver; with surface seismic data, this corresponds to a diving wave path. We also consider the diffraction/reflection case when the wave path is formed by two parts: one from the source to a sub‐surface point and the other from the sub‐surface point to the receiver. We illustrate the different parameter sensitivities for an acoustic transversely isotropic medium with a vertical axis of symmetry. The sensitivity kernels depend on the parameterization choice. By comparing different parameterizations, we explain why the parameterization with the normal moveout velocity, the anellipitic parameter η, and the δ parameter is attractive when we invert diving and reflected events recorded in an active surface seismic experiment.  相似文献   

20.
A theoretical study is presented of the effect of source/receiver patterns with fixed phase difference on the radiation/receiving characteristics of the single elements of the pattern in linear elastic media. The radiation characteristic of two vertical vibrators in phase and in counterphase is investigated in detail. By introducing source/receiver patterns the characteristics become dependent on dimension of the pattern, frequency and angle of observation in the horizontal plane. The types of waves generated by source patterns are, independent of phase differences, equal to those generated by single sources. With two vertical vibrators in phase radiation of P-waves can be improved relative to that of SV-waves. With two vibrators in counterphase radiation of SV-waves in vertical directions is enhanced relative to that of P-waves; this may be a valuable method of SV-shear wave generation in exploration seismics. SV-wave amplitudes larger than P-wave amplitudes can be expected at angles larger than 5°–10° with the vertical.  相似文献   

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