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1.
E. RIETSCH 《Geophysical Prospecting》1977,25(3):541-552
Computerized evaluation of Vibroseis similarity test data is the logical consequence of the increasing quality requirements for signal reproducibility and for synchronization of vibrators. The differences in start time and phase as well as an indication of local phasing problems between any two Vibroseis signals can be obtained from an analysis of the difference of their respective phase spectra. This method appears to be accurate and stable with respect to harmonics which usually plague the signals from transducers monitoring the motion of the vibrators'base plates. 相似文献
2.
In certain areas continuous Vibroseis profiling is not possible due to varying terrain conditions. Impulsive sources can be used to maintain continuous coverage. While this technique keeps the coverage at the desired level, for the processing of the actual data there is the problem of using different sources resulting in different source wavelets. In addition, the effect of the free surface is different for these two energy sources. The approach to these problems consists of a minimum-phase transformation of the two-sided Vibroseis data by removal of the anticipation component of the autocorrelation of the filtered sweep and a minimum-phase transformation of the impulsive source data by replacement of the recording filter operator with its minimum-phase correspondent. Therefore, after this transformation, both datasets show causal wavelets and a conventional deconvolution (spike or predictive) may be used. After stacking, a zero-phase transformation can be performed resulting in traces well suited for computing pseudo-acoustic impedance logs or for application of complex seismic trace analysis. The solution is also applicable to pure Vibroseis data, thereby eliminating the need for a special Vibroseis deconvolution. The processing steps described above are demonstrated on synthetic and actual data. The transformation operators used are two-sided recursive (TSR) shaping filters. After application of the above adjustment procedure, remaining signal distortions can be removed by modifying only the phase spectrum or both the amplitude and phase spectra. It can be shown that an arbitrary distortion defined in the frequency domain, i.e., a distortion of the amplitude and phase spectrum, is noticeable in the time section as a two-sided signal. 相似文献
3.
Sinem Yavuz Roman Pevzner Sofya Popik Konstantin Tertyshnikov Todd Wood Michelle C. Robertson Julia Correa Barry Freifeld 《Geophysical Prospecting》2022,70(1):108-120
Time-lapse seismic is one of the main methods for monitoring changes in reservoir conditions caused by production or injection of fluids. One approach to time-lapse seismic is through permanent reservoir monitoring, whereby seismic sources and/or receivers are permanently deployed. Permanent reservoir monitoring can offer a more cost-effective and environmentally friendly solution than traditional campaign-based surveys that rely on temporarily deployed equipment while facilitating more frequent measurements. At the CO2CRC Otway Project, surface orbital vibrators were coupled to a buried geophone array to form a permanent reservoir monitoring system. These are fixed position seismic sources that provide both P and S waves using induction motor-driven eccentric masses. After an initial injection of CO2 in February 2016, five months of continuous seismic data were acquired, and reflection imaging was used to assess the system performance. Analysis of the data showed the effects of weather variations on the near-surface conditions and the sweep signatures of surface orbital vibrators. Data processing flows of the continuous data was adapted from Vibroseis four-dimensional data processing flows. Ground roll proved a significant challenge to data processing. In addition, variations in the surface wave pattern were linked to major rainfall events. For the appraisal of surface orbital vibrators in imaging, a Vibroseis four-dimensional monitor survey data with similar geometry was also processed. Surface orbital vibrators are observed to be reliable sources with a potential to provide a repeatable signal, especially if the ground roll should fall outside the target window of interest. To guide future permanent reservoir monitoring applications, a repeatability analysis was performed for the various key data processing steps. 相似文献
4.
Since its introduction in the late 1950s, hydraulic vibrators have become the dominant source for land seismic surveys. The hydraulic vibrators typically used for commercial land seismic acquisition, however, are large, costly to operate and expensive to purchase. This inhibits their use for small-scale and short-duration surveys as well as Vibroseis research. In this paper we describe, in detail, the construction of a portable vibrator from commercially available components for a cost of less than $US2,000. Data shows that the vibrator is able to successfully transmit sweeps from 15 to 180 Hz with different spectral contents. The vibrator produces a stronger signal than a sledgehammer and we estimate its output to be around 1 kN. The frequency content of the data was concentrated at lower frequencies (<100 Hz) and the ground-roll was far more energetic than that produced using a sledgehammer. 相似文献
5.
H. EDELMANN 《Geophysical Prospecting》1966,14(Z2):455-469
In order to obtain high resolution correlograms, it is of importance amongst other things to get reflection signals with large bandwidth. An advantage of the VIBROSEIS *** *** Trade Mark and Service book of the Continental Oil Company.
method is that the frequencies radiated by the vibrators can be matched to the transmission response of the subsurface involved. By choosing the right frequency range, the highest possible amplitude and most favourable form may be given to the reflection signals. In a reflection correlogram, individual signals cannot be considered in isolation. Signals of different origin are interfering with one another. They very often have different amplitudes, so that it may be desirable in many cases to filter out events of certain apparent velocity. With the VIBROSEIS method this may be achieved quite simply. All frequencies of the noise signal are uniformly suppressed. The advantage is that noise signals, e.g. refraction signals, which cannot be sufficiently attenuated by wavelength filtering, may be completely eliminated by this velocity filtering without affecting the bandwith of the desired signal. The total dynamic range of the tape recording can be used for the registration of wanted events. To perform this kind of filtering several vibrators are necessary in the field; each of them is controlled by an individual signal. There is an unavoidable error of static and dynamic corrections which causes the results of reflection measurements to deteriorate when using multiple coverage. High frequency components especially are seriously affected by destructive interference. This difficulty can be avoided by using a VIBROSEIS signal with high frequency component amplitudes supported. For the probability of error of corrections a normal distribution is assumed. A smoothed amplitude characteristic may be achieved after stacking. The amplitude characteristic of seismic devices is commonly reduced to about 100 cps bandwidth. For further improvement of resolution of VIBROSEIS correlograms it is necessary to apply special filtering methods. This is of particular interest when any kind of gain control is used to display weak events more clearly. With increasing amplification the sidelobes of the strong signals may reach the size of the weak events. In order to eliminate this effect, the amplitude characteristic of the VIBROSEIS signal is adjusted for optimum suppression of side-lobes. 相似文献
method is that the frequencies radiated by the vibrators can be matched to the transmission response of the subsurface involved. By choosing the right frequency range, the highest possible amplitude and most favourable form may be given to the reflection signals. In a reflection correlogram, individual signals cannot be considered in isolation. Signals of different origin are interfering with one another. They very often have different amplitudes, so that it may be desirable in many cases to filter out events of certain apparent velocity. With the VIBROSEIS method this may be achieved quite simply. All frequencies of the noise signal are uniformly suppressed. The advantage is that noise signals, e.g. refraction signals, which cannot be sufficiently attenuated by wavelength filtering, may be completely eliminated by this velocity filtering without affecting the bandwith of the desired signal. The total dynamic range of the tape recording can be used for the registration of wanted events. To perform this kind of filtering several vibrators are necessary in the field; each of them is controlled by an individual signal. There is an unavoidable error of static and dynamic corrections which causes the results of reflection measurements to deteriorate when using multiple coverage. High frequency components especially are seriously affected by destructive interference. This difficulty can be avoided by using a VIBROSEIS signal with high frequency component amplitudes supported. For the probability of error of corrections a normal distribution is assumed. A smoothed amplitude characteristic may be achieved after stacking. The amplitude characteristic of seismic devices is commonly reduced to about 100 cps bandwidth. For further improvement of resolution of VIBROSEIS correlograms it is necessary to apply special filtering methods. This is of particular interest when any kind of gain control is used to display weak events more clearly. With increasing amplification the sidelobes of the strong signals may reach the size of the weak events. In order to eliminate this effect, the amplitude characteristic of the VIBROSEIS signal is adjusted for optimum suppression of side-lobes. 相似文献
6.
H. EDELMANN 《Geophysical Prospecting》1966,14(4):455-469
In order to obtain high resolution correlograms, it is of importance amongst other things to get reflection signals with large bandwidth. An advantage of the VIBROSEIS *** *** Trade Mark and Service book of the Continental Oil Company.
method is that the frequencies radiated by the vibrators can be matched to the transmission response of the subsurface involved. By choosing the right frequency range, the highest possible amplitude and most favourable form may be given to the reflection signals. In a reflection correlogram, individual signals cannot be considered in isolation. Signals of different origin are interfering with one another. They very often have different amplitudes, so that it may be desirable in many cases to filter out events of certain apparent velocity. With the VIBROSEIS method this may be achieved quite simply. All frequencies of the noise signal are uniformly suppressed. The advantage is that noise signals, e.g. refraction signals, which cannot be sufficiently attenuated by wavelength filtering, may be completely eliminated by this velocity filtering without affecting the bandwith of the desired signal. The total dynamic range of the tape recording can be used for the registration of wanted events. To perform this kind of filtering several vibrators are necessary in the field; each of them is controlled by an individual signal. There is an unavoidable error of static and dynamic corrections which causes the results of reflection measurements to deteriorate when using multiple coverage. High frequency components especially are seriously affected by destructive interference. This difficulty can be avoided by using a VIBROSEIS signal with high frequency component amplitudes supported. For the probability of error of corrections a normal distribution is assumed. A smoothed amplitude characteristic may be achieved after stacking. The amplitude characteristic of seismic devices is commonly reduced to about 100 cps bandwidth. For further improvement of resolution of VIBROSEIS correlograms it is necessary to apply special filtering methods. This is of particular interest when any kind of gain control is used to display weak events more clearly. With increasing amplification the sidelobes of the strong signals may reach the size of the weak events. In order to eliminate this effect, the amplitude characteristic of the VIBROSEIS signal is adjusted for optimum suppression of side-lobes. 相似文献
method is that the frequencies radiated by the vibrators can be matched to the transmission response of the subsurface involved. By choosing the right frequency range, the highest possible amplitude and most favourable form may be given to the reflection signals. In a reflection correlogram, individual signals cannot be considered in isolation. Signals of different origin are interfering with one another. They very often have different amplitudes, so that it may be desirable in many cases to filter out events of certain apparent velocity. With the VIBROSEIS method this may be achieved quite simply. All frequencies of the noise signal are uniformly suppressed. The advantage is that noise signals, e.g. refraction signals, which cannot be sufficiently attenuated by wavelength filtering, may be completely eliminated by this velocity filtering without affecting the bandwith of the desired signal. The total dynamic range of the tape recording can be used for the registration of wanted events. To perform this kind of filtering several vibrators are necessary in the field; each of them is controlled by an individual signal. There is an unavoidable error of static and dynamic corrections which causes the results of reflection measurements to deteriorate when using multiple coverage. High frequency components especially are seriously affected by destructive interference. This difficulty can be avoided by using a VIBROSEIS signal with high frequency component amplitudes supported. For the probability of error of corrections a normal distribution is assumed. A smoothed amplitude characteristic may be achieved after stacking. The amplitude characteristic of seismic devices is commonly reduced to about 100 cps bandwidth. For further improvement of resolution of VIBROSEIS correlograms it is necessary to apply special filtering methods. This is of particular interest when any kind of gain control is used to display weak events more clearly. With increasing amplification the sidelobes of the strong signals may reach the size of the weak events. In order to eliminate this effect, the amplitude characteristic of the VIBROSEIS signal is adjusted for optimum suppression of side-lobes. 相似文献
7.
Vibroseis is a method that imparts coded seismic energy into the ground. The energy is recorded with geophones and then processed using the known (coded) input signal. The resulting time‐domain representation of vibroseis data is an impulsive wavetrain with wavelet properties consistent with the coded input signal convolved with the earth's reflectivity series. Historically, vibratory seismic surveys collect data from one source location at a time, summing one or more sources at each location. We present a method of designing orthogonal sweeps using the concept of combisweeps. The orthogonal sweeps allow simultaneous recording and later separation of two or more unique source locations. Orthogonality of sweeps permits separation of the data into unique source‐location field records by a conventional correlation procedure. The separation power of the orthogonal sweeps is demonstrated by a comparison between separated data and data acquired with one vibrator. Separation noise was at a negligible level for our demonstration data sets when two vibrators were located 50 m to 200 m apart. Coincident generation and recording of two vibroseis sweeps at different locations would allow almost double the amount of data to be recorded for a given occupation time and requires only half the storage medium. 相似文献
8.
—Seismic data processing mostly takes into account the statistics inherent in the data to improve the data quality. Since some years the deterministic approach for processing shows many advantages. This approach takes into account e.g., the source signature, with the knowledge of its amplitude and phase behavior. The transformation of the signal into an optimized form is called wavelet processing. By this step an optimal input for deconvolution can be produced, which needs a minimum- delay signal to function well. The interpreter needs a signal which gives the optimum resolution, which is accomplished by the zero-phase transformation of the input signal. The combination of different input sources such as Vibroseis and Dynamite requires a phase adoption. All these procedures can be implemented via Two-Sided-Recursive (TSR-) filters. Spectral balancing can be accomplished very effectively in time domain after a minimum delay transform of the input signals. The DEKORP data suffer from a low signal/noise ratio, so that special methods for the suppression of coherent noise trains were developed. This can be done by subtractive coherency filtering. Multiple seismic reflections also can be suppressed by this method very effectively. All processing procedures developed during recent years are now fully integrated in commercial software operated by the processing center in Clausthal. 相似文献
9.
Nonlinear sweeps have often successfully been employed in the 1960s. However, this area of sweep technology has been neglected since the introduction of digital recording techniques in the Vibroseis system. Now the advent of computerized recording instruments yields a new economical possibility of forming approximately nonlinear sweeps by combining several linear sweeps with or without time gaps to a “Combisweep”. The total duration of a Combisweep may be as long as the maximum available recording time, for example 32 s. Beside the attenuation of correlation noise, the new method has further merits, such as the weighting of predetermined frequency ranges, in order to effect a certain kind of optimum filtering on the emitter side, or in order to compensate to some degree for frequency dependent absorption. In all these applications the Combisweep is considered as one signal in the correlation process. But by correlating with the individual sweeps or a partial combination of them and by applying automatic switching at predetermined times within the gaps between the individual sweeps additional possibilities arise, such as obtaining in one run with a twenty-four channel recording unit twenty-four traces with small distances between vibrators and geophones for shallow reflections and another twenty-four traces with larger distances for deeper reflections. Various Combisweeps and their applications are presented. 相似文献
10.
电磁驱动式可控震源在城市浅层地震勘探中所面临的最突出的困难是微弱的反射信号常常淹没在很强的背景噪声之中.为了提高地震记录的信噪比,可以利用多台可控震源阵列实施相位控制形成定向地震波束以增强地震波的能量.本文讨论这种相控阵震源的波束形成机制.引入了地震波场的边际能量密度的概念,利用地震波场的时间切片技术,对模型空间各个方向上的能量强度进行了定量分析.用有限差分法对相控阵震源Chirp信号扫描的地震响应进行了数值模拟.当定向地震波束的汇聚带与观测排列的空间范围相一致时,相控阵震源合成地震记录的能量强度要显著高于单个可控震源情形的能量强度,波形振幅的均匀性要明显优于常规组合激发震源情形波形振幅的均匀性. 相似文献
11.
Optimum pilot sweep 总被引：3，自引：0，他引：3
The successful application of high-resolution seismic methods requires evaluating each element in the seismic system and ensuring that each part of the system contributes optimally to the success of the method. Unfortunately, unlike data processing, seismic signal generation is not carefully optimized. The purpose of our study was to optimize the source signal in order to better coordinate field operations with subsequent data processing to achieve their common objective. We developed an iterative method for a rational frequency distribution of the energy of a seismic source. The method allows the optimum amplitude spectrum of a source signal to be calculated, thus providing the best data quality at the end of the processing. We assume that the source signal is affected by a total transfer function, by the reflectivity function of a target interval, and by ambient noise, whose characteristics, if not known, can be estimated or measured in practice. The transfer function includes data processing other than the correlation stage and the final trace-optimizing filter. The variance of a reflectivity estimate is considered to be a measure of the data quality and improvement of the characteristic corresponds to a decrease in the variance. For this reason, a constrained Wiener deconvolution filter is used as the final trace-optimizing filter. It not only minimizes the variance of a reflectivity estimate but also ensures a specific signal-to-noise ratio. The method is made feasible by following the Vibroseis technique, primarily because of the versatility of the technique in controlling the signal spectrum. With the optimum amplitude spectrum obtained, the corresponding optimum pilot sweep can be readily calculated. Examples using synthetic data are presented to illustrate the method. 相似文献
12.
The use of certain codes for the Vibroseis system of exploration has been investigated. The codes considered are well-known in communication theory. The effect of coding the Vibroseis input signal is the suppression of correlation noise and therefore an improvement of the signal to noise ratio. It is believed that, during the course of research, a new property of one of the codes used has been discovered. 相似文献
13.
B. M. GURBUZ 《Geophysical Prospecting》1982,30(4):432-443
An analytical relationship for the autocorrelation function of an upsweep with high-frequency attenuation is used in the construction of synthetic seismograms. Field experiments were conducted in two areas to investigate the attenuation of upsweep where the near-surface materials were different. The results showed that the attenuation of high frequencies occurs at the source point depending on the near-surface lithology. The attenuation effect is usually neglected in the construction of the input wavelet of synthetic seismograms for Vibroseis data. In this study, the high-frequency attenuation of upsweep was considered in the construction of the input wavelet for the synthetic seismogram in an area where the Vibroseis technique was used. The synthetic seismogram generated in this manner had a better correlation with the Vibroseis section than that of corresponding synthetics using minimum-phase and the unattenuated autocorrelation wavelet of the upsweep. 相似文献
14.
Modelling and modal analysis of seismic vibrator baseplate 总被引：1，自引：0，他引：1
Zhouhong Wei 《Geophysical Prospecting》2010,58(1):19-32
The vibroseis method must be extended to its limits as the search for oil and gas continues on land. To successfully improve vibroseis data quality, it is crucial to evaluate each element in the vibroseis data acquisition system and ensure that the contribution from each element is successful. Vibroseis systems depend greatly upon the ability of vibrators to generate synchronous, repeatable ground-force sweeps over a broad frequency range. This requires that the reaction mass and the baseplate of the vibrator move as rigid bodies. However, rigid-body motion is not completely true for high- frequency vibrations, especially for the vibrator baseplate. In order to accurately understand the motion of the vibrator baseplate, a finite element analysis model of the vibrator baseplate and the coupled ground has been developed. This model is useful for simulating the vibrator baseplate dynamics, evaluating the impact of the baseplate on the coupled ground and vibrator baseplate design. Model data demonstrate that the vibrator baseplate and its stilt structure are subject to six significant resonant frequencies in the range of 10–80 Hz. Due to the low rigidity of the baseplate, the baseplate stilt structure experiences severe rocking motions at lower frequencies and the baseplate pad experiences severe flexing motions at higher frequencies. Flexing motions cause partial decoupling, which gives rise to increased levels of harmonic distortion and less useable signal energy. In general, the baseplate pad suffers more bending and flexing motions at high frequencies than low frequencies, leading less efficiency in transmitting the useable energy into the ground. 相似文献
15.
MT data from a detailed 84-site grid array (28 × 12 km) in the Paraná Basin (Brazil) reveal a high degree of frequency-independent and parallel amplitude distortion. The Palaeozoic sediments across the survey area are covered by 1.2 km of flood basalts. A deep well-log provides some control regarding the emplacement of thin diabase sills but information regarding vertical feeder dikes is non-existent. The degree of parallel behaviour is identified using anisotropy ratios which quantify the extent and bandwidth of the distortion characteristics. A 2D modelling study is carried out using the concept of a horizontally layered (1D) basin with superimposed, small-scale inhomogeneities. Two likely geological distortion structures are considered. The first represents at- or near-surface (0 to 100 m) inhomogeneities. The second represents thin but vertically elongate dikes. The two distortion effects are found to produce different and characteristic behaviour in the H-polarization (TM) mode model data. The modelled E-polarization (TE) mode data are undistorted and provide control. The model developed to represent vertical dikes gives rise to a characteristic frequency dependence which is distinct from that of the undistorted response. This model also generates subparallel distortion effects which give rise to a form of variance in the response estimates as a function of location. The behaviour occurs, importantly, in both the amplitude and phase of the distorted data. A comparison of the normalized amplitude data, observed and modelled, indicates a high degree of correspondence down to a low frequency limit (< 0.1 Hz) where additional and deep contributions to the observed response become apparent. The modelling results and data characteristics indicate that the survey area is underlain by a series of closely parallel, thin intrusive dikes (a dike swarm). 相似文献
16.
RAINER TONN 《Geophysical Prospecting》1991,39(1):1-27
Ten methods for the computation of attenuation have been investigated, namely: amplitude decay, analytical signal, wavelet modelling, phase modelling, frequency modelling, rise-time, pulse amplitude, matching technique, spectral modelling and spectral ratio. In particular, we have studied the reliability of each of these methods in estimating correct values of Q using three synthetic VSP seismograms for plane P-waves with different noise contents. The investigations proved that no single method is generally superior. Rather, some methods are more suitable than others in specific situations depending on recording, noise or geology. The analytical signal method has been demonstrated to be superior if true amplitude recordings are available. Otherwise spectral modelling or, in the ‘ noise-free’ case the spectral ratio method, is optimal. Finally, two field VSPs in sediments are investigated. Only in the case of the highest quality VSP can significant information be deduced from the computed attenuation. 相似文献
17.
E. RIETSCH 《Geophysical Prospecting》1981,29(2):178-188
Due to non-linear effects, the swept frequency signals (sweeps) transmitted into the subsurface by vibrators are contaminated by harmonics. Upon correlation of the recorded seismograms, these harmonics lead to noise trains which are particularly disturbing in the case of down-sweeps. The method described in this paper—which can be regarded as a generalization of Sorkin's approach to the suppression of even order harmonics—allows elimination, from the final vibratory source seismogram, of harmonics of the sweep up to any desired order. It requires that not one single signal but rather a series of M signals is employed where each signal has an initial phase differing from that of the previous one of the series by the phase angle 2πM. Prior to stacking, the seismograms generated with the different signals have to be brought into the form they would have if they had been generated with the same signal. The method seems also to be capable of reducing the correlation noise if sign-bit recording techniques are used. 相似文献
18.
The quality of Vibroseis survey data can be improved by continuously monitoring the vibrator's baseplate and reaction mass accelerations. Equipment failures can be detected as they occur, rather than relying on similarity trials at the beginning and end of the day's production. Equipment faults can then be corrected as they happen and thus would not have a detrimental effect on the quality of the survey data. Source efficiency can be optimized by monitoring the amount of harmonic distortion generated by the vibrator at different drive levels on the different surfaces which may be encountered during a survey. Phase problems introduced by poor coupling of the baseplate to the ground can also be identified and addressed in the field. Rapid analysis of vibrator signals is required if continuous monitoring is to be useful. Frequency-time (f-t) analyses of vibrator signals are often used in processing centres, but are slow and require a large storage capacity which makes the technique unsuitable for a field analysis system. The two methods proposed to analyse vibrator signals entail the use of hodograms and time-varying notch filters. Hodograms provide a qualitative analysis of harmonic distortion and vibrator performance. A fast, time-varying notch filter gives quantitative and qualitative information about the harmonic distortion present in the signal and can be used to identify problems with vibrator behaviour. Both the hodogram and fast, time-varying notch filter methods can analyse the vibrator's reaction mass and baseplate accelerations as it progresses through its sweep and can present automatically interpreted results to the operator before moving to the next vibrator point. 相似文献
19.
E. RIETSCH 《Geophysical Prospecting》1980,28(4):531-550
The amplitude of the signal and the energy of the noise on each of at least three traces can be estimated provided that the signal has the same form (but not necessarily the same amplitude) on these traces and that the noise on any trace is correlated with neither the signal nor the noise on any other trace. This estimation of signal amplitude and noise energy can be achieved by a rather simple algorithm. The accuracy of the estimate depends, of course, on the degree to which the assumption that signal and noise on the different traces are mutually uncorrelated is actually met. The accuracy tends to improve with increasing number of traces. 相似文献
20.
M. H. SAFAR 《Geophysical Prospecting》1984,32(3):392-405
It is well recognized that in order to realize the full potential of the Vibroseis technique, one needs to ensure accurate phase locking and a meaningful cross-correlation. To achieve these two important objectives we require an accurate estimate of the compressional stress wave radiated by the vibrator into the ground. In this paper a simple method (subject of a patent application) is developed for predicting the compressional stress waves radiated by a vertical vibrator. The main feature of the proposed method is that it involves the field measurement of the acceleration of the reaction mass and the baseplate, respectively. The method is illustrated by computing the compressional stress waves generated by a typical vertical vibrator radiating into ice, chalk, sand, and mud. It is shown that for a seismic vibrator radiating into hard ground the pressure of the downgoing P-wave is 180° out of phase with the baseplate velocity. It is also shown that when the driving force of the seismic vibrator has a flat amplitude spectrum, the amplitude spectrum of the downgoing P-wave falls off by 6 dB/octave towards low frequencies. 相似文献