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- Volume 58, Issue 1, 2010
Geophysical Prospecting - Volume 58, Issue 1, 2010
Volume 58, Issue 1, 2010
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How do hydraulic vibrators work? A look inside the black box
More LessABSTRACTIn order to have realistic expectations of what output is achievable from a seismic vibrator, an understanding of the machine's limitations is essential. This tutorial is intended to provide some basics on how hydraulic vibrators function and the constraints that arise from their design. With these constraints in mind, informed choices can be made to match machine specifications to a particular application or sweeps can be designed to compensate for performance limits.
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Modelling and modal analysis of seismic vibrator baseplate
By Zhouhong WeiABSTRACTThe 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.
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Developments in vibrator control
Authors D. Boucard and G. OllivrinABSTRACTHydraulic limitations, non‐rigidity of the baseplate as well as variable characteristics of the ground constantly distort the downgoing energy output by vibrators. Therefore, a real time feedback control must be performed to continuously adjust the emitted force to the reference pilot signal. This ground force is represented by the weighted sum of the reaction mass and the baseplate accelerations. It was first controlled with an amplitude and phase locked loop system, poorly reactive and sensitive to noise. Later on, new vibrator electronics based on a digital model of the vibrator were introduced. This model is based on the physical equations of the vibrator and of the ground. During an ‘identification’ process, the model is adjusted to each particular vibrator. Completed by a Kalman adaptive filter to remove the noise, it computes ten estimated states via a linear quadratic estimator. These states are used by a linear quadratic control to compute the torque motor input and to compare the ground force estimated from the states with the pilot signal. Test results using downhole geophones demonstrate the benefit of filtered mode operation.
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Review of vibroseis data acquisition and processing for better amplitudes: adjusting the sweep and deconvolving for the time‐derivative of the true groundforce
More LessABSTRACTThe goal of vibroseis data acquisition and processing is to produce seismic reflection data with a known spatially‐invariant wavelet, preferably zero phase, such that any variations in the data can be attributed to variations in geology. In current practice the vibrator control system is designed to make the estimated groundforce equal to the sweep and the resulting particle velocity data are cross‐correlated with the sweep. Since the downgoing far‐field particle velocity signal is proportional to the time‐derivative of the groundforce, it makes more sense to cross‐correlate with the time‐derivative of the sweep. It also follows that the ideal amplitude spectrum of the groundforce should be inversely proportional to frequency. Because of non‐linearities in the vibrator, bending of the baseplate and variable coupling of the baseplate to the ground, the true groundforce is not equal to the pre‐determined sweep and varies not only from vibrator point to vibrator point but also from sweep to sweep at each vibrator point. To achieve the goal of a spatially‐invariant wavelet, these variations should be removed by signature deconvolution, converting the wavelet to a much shorter zero‐phase wavelet but with the same bandwidth and signal‐to‐noise ratio as the original data. This can be done only if the true groundforce is known. The principle may be applied to an array of vibrators by employing pulse coding techniques and separating responses to individual vibrators in the frequency domain. Various approaches to improve the estimate of the true groundforce have been proposed or are under development; current methods are at best approximate.
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Source measurement effect on high fidelity vibratory seismic separation
Authors Nirupama Nagarajappa and David WilkinsonABSTRACTHigh Fidelity Vibratory Seismic (HFVS) acquisition and separation can play an important role in today's land acquisition schemes. The method – in which multiple vibrators are swept simultaneously using sweeps with known phase encoding and then the data are inverted and separated into individual records – can improve productivity in the field and at the same time improve signal characteristics in the data. It relies on the measured weighted sum of accelerations (base plate and reaction mass) to invert the acquired data and separate the individual vibrator responses. Separation can be sub‐optimal if the measured motions vary from the ‘true source’ input into the ground. Differences in true source and measured source can arise due to poor coupling between vibrators and ground, soil compaction or other factors. Using both a synthetic model and real data, we show that if the true source changes between sweeps but is not measured, vibrator responses can leak into adjacent vibrator responses upon separation. In a recent survey with HFVS acquisition, we observed a 25–30 dB separation between adjacent vibrators, which could be improved with greater reliability of the source measurement. The vibrator leakage can reduce the data quality considerably. We discuss the results of this survey and show that separation is affected by source measurement error. Further, we conclude that it is necessary either 1) to use source measurements that can capture the variability of the true source between sweeps or 2) to compensate for the source measurement variations in processing or in acquisition.
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On the accuracy of the ground force estimated in vibroseis acquisition
Authors Christos Saragiotis, Péter Scholtz and Claudio BagainiABSTRACTFor a linear elastic Earth the time derivative of the ground force is considered proportional to the far‐field wavelet. Under the assumption that the baseplate is stiff and the bending forces of the baseplate are negligible, the ground force is also approximated by the sum of the accelerations of the baseplate and the reaction mass weighted by the respective masses. Combining these two assumptions, the time derivative of the weighted sum is considered proportional to the far‐field wavelet. This result, often referred to as the far‐field wavelet assumption, although convenient and most often employed is not always valid. We explore its validity using the spectral harmonic ratios of recorded data, which are used extensively in data filtering and analysis of vibratory data. We show that the far‐field wavelet assumption fails particularly for harmonic components of even order. More compact soil after repeated shots further invalidates this assumption. Non‐linear modelling of the ground under the vibrator point may provide a direction towards solving this discrepancy. Finally, we describe a method for the estimation of the harmonic spectral ratios.
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Acquisition and processing of simultaneous vibroseis data
More LessABSTRACTNumerous methods have been developed in the past three decades to speed up vibroseis acquisition. This paper gives an overview of the most promising ones and proposes a classification of them into three categories: simultaneous shooting, cascaded sweeps and slip‐sweep. The principles upon which these methods are based, the processing techniques developed to separate the simultaneously acquired data and their main features are also discussed. Finally, some criteria for the selection of the most suitable methods for the acquisition and the separation stages are proposed.
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Vibroseis productivity: shake and go
Authors Christine Krohn, Marvin Johnson, Rachel Ho and Michael NorrisABSTRACTWe use both model and field data to compare three methods for increasing vibroseis productivity and decreasing acquisition costs. The first method, HFVS (high‐fidelity vibratory seismic), allows us to separate the responses from individual vibrators when multiple vibrators are operating simultaneously. The data quality of the separated records is superior to that of conventional correlated data because they are processed with measured ground‐force signals, but the number of sweeps must be greater than or equal to the number of vibrators. The second method, cascaded sweep, eliminates the listening time between multiple sweeps and partially mitigates harmonic noise observed at later times on near‐offset traces. Finally, a combined method, continuous‐HFVS (C‐HFVS), allows source separation with a single, long, segmented sweep. Separation is as good as with HFVS and interference noise is limited to times near the end of a sweep‐segment length. All three methods produce acceptable seismic images for post‐stack and prestack amplitude interpretation.
The choice of which option to use depends upon the area being investigated. HFVS has numerous benefits, especially when fine sampling is required to mitigate static problems and elevation changes. Due to the ability to separate individual responses, fine sampling can be achieved without sacrificing productivity. For deeper targets, cascaded sweep can be more efficient but data quality suffers from harmonic noise. C‐HFVS, which combines features of HFVS and cascaded sweep, has the potential to result in the highest productivity, without sacrificing either fine sampling or data quality.
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Distance separated simultaneous sweeping, for fast, clean, vibroseis acquisition
By Jack BouskaABSTRACTDistance separated simultaneous sweeping DS3 is a new vibroseis technique that produces independent records, uncontaminated by simultaneous source interference, for a range of offsets and depths that span all target zones of interest. Use of DS3 on a recent seismic survey in Oman, resulted in a peak acquisition rate of 1024 records per hour. This survey employed 15 vibrators, with a distance separation of 12 km between simultaneous active sources, recorded by 8000 active channels across 22 live lines in an 18.5 km × 11 km receiver patch. Broad distribution of simultaneous sources, across an adequately sized recording patch, effectively partitions the sensors so that each trace records only one of the simultaneous sources. With proper source separation, on a scale similar to twice the maximum usable source receiver offset, wavefield overlap occurs below the zone of interest. This yields records that are indistinguishable from non‐simultaneous source data, within temporal and spatial limits. This DS3 technique may be implemented using a wide variety of acquisition geometries, optimally with spatially large recording patches that enable appropriate source separation distances. DS3 improves acquisition efficiency without data quality degradation, eliminating the requirement for special data processing or noise attenuation.
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Volumes & issues
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Volume 72 (2023 - 2024)
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Volume 71 (2022 - 2023)
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Volume 70 (2021 - 2022)
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Volume 69 (2021)
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Volume 68 (2020)
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Volume 67 (2019)
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Volume 66 (2018)
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Volume 65 (2017)
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Volume 64 (2015 - 2016)
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Volume 63 (2015)
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Volume 62 (2014)
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Volume 61 (2013)
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Volume 60 (2012)
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Volume 59 (2011)
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Volume 58 (2010)
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Volume 57 (2009)
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Volume 56 (2008)
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Volume 55 (2007)
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Volume 54 (2006)
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Volume 53 (2005)
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Volume 52 (2004)
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Volume 51 (2003)
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Volume 50 (2002)
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Volume 49 (2001)
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Volume 48 (2000)
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Volume 47 (1999)
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Volume 46 (1998)
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Volume 45 (1997)
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Volume 44 (1996)
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Volume 43 (1995)
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Volume 42 (1994)
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Volume 41 (1993)
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Volume 40 (1992)
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Volume 39 (1991)
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Volume 38 (1990)
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Volume 37 (1989)
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Volume 36 (1988)
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Volume 35 (1987)
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Volume 34 (1986)
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Volume 33 (1985)
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Volume 32 (1984)
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Volume 31 (1983)
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Volume 30 (1982)
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Volume 29 (1981)
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Volume 28 (1980)
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Volume 27 (1979)
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Volume 26 (1978)
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Volume 25 (1977)
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Volume 24 (1976)
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Volume 23 (1975)
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Volume 22 (1974)
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Volume 21 (1973)
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Volume 20 (1972)
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Volume 19 (1971)
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Volume 18 (1970)
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Volume 17 (1969)
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Volume 16 (1968)
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Volume 15 (1967)
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Volume 14 (1966)
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Volume 13 (1965)
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Volume 12 (1964)
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Volume 11 (1963)
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Volume 10 (1962)
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Volume 9 (1961)
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Volume 8 (1960)
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Volume 7 (1959)
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Volume 6 (1958)
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Volume 5 (1957)
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Volume 4 (1956)
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Volume 3 (1955)
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Volume 2 (1954)
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Volume 1 (1953)