Land data are contaminated by energetic surface waves that have low apparent velocities relative to the seismic signal. The conventional land acquisition approach to noise attenuation is to deploy arrays of hard-wired geophones whose combined response acts as a spatial filter. Besides the shortcomings of a fixed array response that is sensitive to perturbations, using a large amount of equipment limits the field efficiency of array-based acquisition. The advent of point-receiver acquisition technology addresses some of these limitations by enabling measurement of each single geophone along a receiver line and removing the slowest energetic noise by velocity-based filtering. However, according to the classic sampling theorem, the proper sampling of noise requires sampling the shortest wavelength twice. Because surface waves have slow propagation velocities, it is not feasible to sample the noise properly and a compromise is made between quality and efficiency. We have developed two different methods to improve the efficiency of point-receiver technology by allowing for wider receiver spacing without compromising the coherent noise attenuation capabilities. First, we combine compressed sensing-based methods such as the matching pursuit approach with the gradient measurement inferred from a novel 5C sensor to dealias and attenuate source-generated noise. Second, we have developed a method to design optimized acquisition geometries by using the spectral characteristics of the surface waves, where the objective is to place a limited number of receivers at the most favorable locations to help attenuate the coherent noise. We determine the effectiveness of our methods through synthetic and field data examples in which we find that spatially aliased coherent noise can be reconstructed to a denser grid and attenuated.

Summary We present a method to improve the efficiency of point-receiver technology by allowing for a wider receiver spacing without compromising the noise attenuation performance. Using gradient measurement inferred from a novel 5C sensor, aliased source-generated noise can be attenuated by a matching-pursuit-based approach. The gradient measurement is a finite-difference approximation of the vertical spatial gradient of the horizontal component, estimated by taking the difference between two vertically spaced horizontal components placed in the same sensor housing. Following from the free-surface boundary condition, the measured vertical gradient of the horizontal component can be used as an approximation of the horizontal gradient of the vertical component that is required for the noise attenuation method. We demonstrate the effectiveness of the proposed approach on field data acquired using the 5C sensor. The recorded coherent noise modes (i.e., ground roll) are de-aliased and attenuated using the matching-pursuit-based method, combining the vertical component and its approximated spatial gradient.

Marine seismic vibrators are generally considered to be less intrusive than airguns from an environmental perspective. This is because they emit their energy spread out in time, rather than in a single, high-intensity pulse. There are also significant geophysical benefits associated with marine vibrators, and they stem from the ability to specify in detail the output acoustic waveform. The phase can be specified independently at each frequency. Such detailed control cannot be achieved with conventional airgun sources, where the phase can only be modified using simple overall time delays. The vibrator phase can be employed in several different ways: it can be applied to the overall source phase in a sequence so that it varies from one source point to the next; it can be applied to the individual vibrators within the source array so the source directivity is changed; it can be applied to the overall source phase of each source in a simultaneous source acquisition. Carefully designed phase sequences can attenuate the residual source noise, and this in turn allows extra source points to be interleaved between the conventional ones. For these extra source points, the relative phase of the vibrators within the array can be chosen to create a transverse gradient source, which illuminates the earth predominantly in directions out of the plane of the sail line without left/right ambiguity. If seismic vibrator data are acquired using interleaved conventional and transverse gradient sweeps, more information is collected per kilometre of vessel travel than is the case in conventional acquisition. This richer data acquisition leads to the possibility of acquiring all the necessary seismic data in a shorter time. Three-dimensional reconstruction techniques are used to recover the same image quality that would have been obtained using the conventional, more time-consuming acquisition. For a marine vibrator to be suitable for these techniques it must, in general terms, have 'high fidelity'. The precise device specifications are defined through realistic end-to-end simulations of the physical systems and the processing. The specifications are somewhat more onerous than for a conventional vibrator, but they are achievable. A prototype vibrator that satisfies these requirements has been built. In a simulated case study of a three-dimensional deep-water ocean bottom node survey, the seismic data could have been acquired using marine vibrators in one third of the time that it would have taken using airguns.

Summary High-fidelity marine seismic vibrators allow control of the phase of the emitted seismic wavefield. Phase control of the seismic source can allow new simultaneous-source encoding techniques. These techniques may allow highly efficient seismic acquisition without sacrificing data quality. One approach that uses phase control is phase sequencing, where the emitted source wavefield phase is changed in a non-random way from shot-to-shot, allowing manipulation of data in the frequency-wavenumber domain. Combined with wavefield reconstruction, phase sequencing allows separation of simultaneous sources with minimal residual crosstalk. An important consideration when evaluating such techniques is that marine seismic vibrator deployment and utilization will be different from an air-gun source. For example, a marine vibrator source array is likely to move while emitting energy, it may have different array elements distributed at different depths, it will emit swept rather than impulsive waveforms, and it may behave differently in the presence of a rough sea. We demonstrate the separation of high-multiplicity simultaneous sources using phase sequencing, before discussing marine-vibrator specific acquisition effects. The impact of these acquisition effects will be demonstrated in future publications.

Summary Reliable detection of weak signals is potentially a fundamental limitation for microseismic event detection and location, especially for sensors in the near-surface region, where large source distances and high noise levels affect the signal-to-noise ratio (SNR). Previously, a nonlinear stacking method was introduced that could increase the SNR for weak signals and included methods that are insensitive to changes in signal polarity. This paper introduces nonlinear filtering methods that are a generalization of the nonlinear stacking method where the stacking operation is replaced by a filtering operation. This method keeps the SNR and polarity benefits of the stacking methods, but we can now choose to modify the pass and reject bands. For instance, we can improve the SNR for weak signals, with polarity changes, and allow additional signal misalignment criteria to be included in the passband. We will show synthetic-signal and real-signal examples where the new method has clear benefits compared to conventional stacking or other nonlinear stacking methods.

We show how phase control, enabled by high-fidelity marine seismic vibrators, can be used to acquire and separate high-multiplicity simultaneous-source data. By changing the phase of the source wavefield from shot to shot and following a prescribed phase sequence, energy from selected sources can be moved into different parts of the frequency-wavenumber spectrum in the common-receiver domain. We use synthetic examples to demonstrate how this type of seismic acquisition, combined with wavefield reconstruction techniques, has the potential to provide a new method of high-multiplicity simultaneous-source acquisition and separation. Compared with the more conventional time-dithered air-gun approach the phase sequencing approach shows consistently lower simultaneous-source crosstalk. This suggests that the introduction of a marine vibrator with controllable phase could offer significant changes in marine seismic data quality and survey efficiency. Phase sequencing cannot be used with air guns because we have no control of the phase of the air-gun signal other than simple changes of amplitude and overall time delay. Presentation Date: Tuesday, September 26, 2017 Start Time: 9:45 AM Location: 371F Presentation Type: ORAL

Finding sparse solutions to underdetermined inverse problems has gained increased interests over the past few years. The matching pursuit (MP) algorithm is one of the most successfully deployed greedy algorithms in many applications due to its simplicity and robustness. In this work, we propose a novel greedy algorithm called iterative adaptive approach MP (IAA-MP) that outperforms MP in terms of higher resolution, lower sidelobe levels, superior interference rejection capabilities, better estimation of the coefficient levels, and higher dynamic range. The IAA-MP improves the performance in challenging scenarios where the MP performance is not optimal. We demonstrate the versatility and robustness of the proposed approach in different applications. Presentation Date: Wednesday, September 27, 2017 Start Time: 2:15 PM Location: 360A Presentation Type: ORAL

Summary In the context of surface microseismic processing, a nonlinear stack method, the phase weighted nth root stack, was evaluated and benchmarked against linear stack. From a synthetic analysis, the parameter choice was evaluated to improve the detectability of small amplitude microseismic events. The choice and impact of small exponent values for the nonlinear stack method were confirmed on one stage of a multiwell, multistage hydraulic fracturing in the Marcellus shale formation. Compared to the linear stack, up to 30 % more events were detected and located for this data example.

Summary Compared with a conventional airgun source, a marine seismic vibrator can allow for much greater control of the emitted seismic wavefield. One possibility with a marine vibrator is to control the phase of the wavefield. We consider the use of phase control to emit wavefields that change in phase from shot to shot, following a prescribed sequence, referred to as phase sequencing. Some phase sequences allow moving energy from selected sources into a normally empty part of the frequency-wavenumber spectrum in the common-receiver domain. We demonstrate how this type of seismic acquisition, combined with wavefield reconstruction techniques, has the potential to provide a new method of simultaneous-source acquisition and separation. We further describe how this technique may be extended to high-multiplicity simultaneous source acquisition, with potential implications for data quality and survey efficiency. Other phase sequences can be used to attenuate residual shot noise from marine seismic data. Phase sequencing cannot be used with airguns because we have no control of the phase of the airgun signal other than simple changes of amplitude and overall time delay.

In a marine seismic survey using airguns the reverberant noise left over from previous shots (the residual shot noise) typically mandates the use of shot-time intervals that are longer than would be ideal on shot spatial sampling grounds alone. When a marine vibrator seismic source is used the phase of the emitted signal can be chosen at will. In particular, it can change from one shot to the next in a ‘phase sequence’. Phase sequencing can be used to move energy about in the frequency-wavenumber space in the common-receiver domain. In this abstract we describe one application of phase sequencing in which the residual shot noise is moved into a part of the frequency-wavenumber domain where there is no signal. The noise can then be removed without affecting the signal. This allows the shot-time interval to be reduced. Phase sequencing techniques can also be employed to enable simultaneous sources of high multiplicity. Phase sequencing cannot be directly used with airguns because we have no control of the phase of the airgun signal; control is limited to simple changes of amplitude and overall time delay. Presentation Date: Tuesday, September 26, 2017 Start Time: 2:40 PM Location: 371F Presentation Type: ORAL

Marine seismic vibrators are considered to be more environmentally friendly than air guns and this has prompted a resurgence of interest in their use. However, these devices have novel features that are potentially of great benefit to seismic imaging and that have not yet been exploited - in particular, we can control the phase. With marine vibrators, the emitted waveform can be chosen freely, provided that it lies within the envelope of what the device can emit. Typically, the waveform is a swept-frequency sinusoid. In this paper, we show how the sweep phase can be used to suppress the residual shot noise (RSN) and to separate simultaneous sources of high multiplicity. This work is also described by Laws and Halliday (2013) and is closely related to that of Laws (2012). The control of phase is a benefit of marine vibrators that can be exploited to great effect. First, we look at the RSN situation. The conventional wisdom is that roughly a 10s shot-time interval is required in marine seismic data acquisition so that the RSN is acceptably small relative to the signal. Because both signal and RSN originate from the seismic source, the ratio of signal/RSN is not improved by using a larger source (pace Landrø, 2008). We show a simple case of RSN attenuation using alternating sine and cosine sweeps. That is to say, the sequence of source phases goes, for successive shots, [0° 90° 0° 90° 0° 90° 0° 90°... and so on]. We show schematically how this ‘phase sequencing’ leads to a dramatic attenuation of the RSN. It does so by moving the RSN into parts of the frequency-wavenumber spectrum where there is no signal. We then use frequency-wavenumber filtering to remove the RSN. This can be done with simple filtering up to the limit imposed by spatial aliasing and beyond that limit by using wavefield reconstruction methods, such as generalized matching pursuit (Özbek et al., 2010; Vassallo et al., 2010). Reconstruction methods decompose the seismic wavefield into a set of basis functions that allow the signal and RSN to be identified and separated. Then, we look at the simultaneous source situation. The RSN removal problem can be considered as a special case of the more general simultaneous source separation problem. As an example, in the case of two simultaneous sources, one source can be swept with consistent phase, while the other source is swept with alternating phase, i.e., the sequence of relative phases is [0° 180° 0° 180° 0° 180° 0° 180°... and so on]. The simultaneous source separation is demonstrated using the same type of reconstruction method as described above for the RSN. See also Moore et al. (2008) and Ji et al. (2012). The use of phase sequencing opens up possibilities for simultaneous source separation using large numbers of sources. By having complete control over the phases of all the sources it is possible, in certain domains, to make simultaneous source data look similar to aliased data from a single source. This opens the door to using many established wavefield interpolation and dealiasing-techniques to perform high-multiplicity simultaneous source separation.

Summary Noise attenuation is a key challenge for surface-acquired microseismic processing. A number of data conditioning tools have been proposed and applied with various degrees of success to improve the signal-to-noise ratio prior to detection and location of microseismic events. Random noise attenuation, trace-by-trace correlation with a large magnitude event, and nonlinear stacking techniques have all been shown individually to improve microseismic event detectability in surface-acquired microseismic datasets. This paper demonstrates how the combination of these approaches significantly increases the number of detected microseismic events while keeping the number of false triggers to a minimum. In particular, random noise attenuation and trace-by-trace correlation with a large magnitude event followed by nonlinear stacking at the stage of substack generation provide a data conditioning workflow that significantly attenuates the effects of statics, anisotropy, and, to some extent, 3D velocity variations. This work is a step towards an optimized data conditioning workflow for surface-acquired microseismic data.

The perforation of the borehole casing and cement is the final stage in a well-completion procedure to establish a connection with the reservoir for hydraulic fracturing purposes. Although they have been neglected to date, the generated seismic expressions of these explosions in the stimulation well display a very characteristic signature. A Fayetteville Shale stimulation concurrently monitored using a downhole and surface seismic array was analyzed to develop a better understanding of the perforation arrivals and to identify the influence of reservoir properties, using a combination of data processing and forward modeling. The perforation shots typically give rise to multiple P- and S-wave arrivals. Comparison of observed and modeled arrivals reveals the importance of incorporating two crucial geologic parameters: a 1.6° subsurface dip and a strong anisotropy in the Fayetteville Shale, typically between 40% and 50%. Of equal importance are shot-generated tube waves traveling through the treatment well, which convert into body waves after interaction with the perforation plugs that seal off the borehole and give rise to secondary high-amplitude, low-frequency seismic events. Hence, the waveforms related to source, propagation, and treatment operation effects become strongly interlinked and need careful separation and interpretation to allow for the correct identification of primary and secondary perforation arrivals and for the reliable derivation of velocity models for surface microseismic monitoring.

Summary A Fayetteville shale stimulation concurrently monitored using a surface seismic array and a wide aperture downhole array is analysed in order to develop a better understanding of the factors influencing the surface expression of perforation shots using a combination of data processing and forward modelling. The perforation shots typically give rise to multiple compressional and shear wave arrivals. A comparison of observed and modelled arrivals reveals the importance of two geological parameters: a 1.6 degree subsurface dip, which had been neglected in previous studies, and a strong Thomsen anisotropy in the Fayetteville shale, typically between 40% and 50%. Of equal importance are shot generated waves (tube waves) travelling through the treatment well which interact with plugs remaining in the well and give rise to secondary high amplitude, low frequency, seismic events. These arrivals, which are related to source, propagation and treatment operation effects, need careful separation and interpretation in order to allow the reliable derivation of velocity models for surface microseismic monitoring.

Summary Land seismic acquisition requires a dense receiver grid in order to deal with groundroll. Relaxing the spatial sampling requirement will help reduce the field effort and improve the efficiency of the seismic acquisition. In this paper, we show that the spatial sampling interval can be increased by up to 3 times Nyquist when the vertical wavefield and its spatial gradients are used for wavefield reconstruction. To test this approach, a field data set was acquired where spatial gradients were estimated by differencing the response between two geophones. These field data are decimated and subsequently reconstructed to a dense spatial grid using a matching pursuit based method. The results are discussed and evaluated to assess the order of aliasing that can be recovered when gradients are used.

ABSTRACTWave field reconstruction – the estimation of a three‐dimensional (3D) wave field representing upgoing, downgoing or the combined total pressure at an arbitrary point within a marine streamer array – is enabled by simultaneous measurements of the crossline and vertical components of particle acceleration in addition to pressure in a multicomponent marine streamer. We examine a repeated sail line of North Sea data acquired by a prototype multicomponent towed‐streamer array for both wave field reconstruction fidelity (or accuracy) and reconstruction repeatability. Data from six cables, finely sampled in‐line but spaced at 75 m crossline, are reconstructed and placed on a rectangular data grid uniformly spaced at 6.25 m in‐line and crossline. Benchmarks are generated using recorded pressure data and compared with wave fields reconstructed from pressure alone, and from combinations of pressure, crossline acceleration and vertical acceleration. We find that reconstruction using pressure and both crossline and vertical acceleration has excellent fidelity, recapturing highly aliased diffractions that are lost by interpolation of pressure‐only data. We model wave field reconstruction error as a linear function of distance from the nearest physical sensor and find, for this data set with some mismatched shot positions, that the reconstructed wave field error sensitivity to sensor mispositioning is one‐third that of the recorded wave field sensitivity. Multicomponent reconstruction is also more repeatable, outperforming single‐component reconstruction in which wave field mismatch correlates with geometry mismatch. We find that adequate repeatability may mask poor reconstruction fidelity and that aliased reconstructions will repeat if the survey geometry repeats. Although the multicomponent 3D data have only 500 m in‐line aperture, limiting the attenuation of non‐repeating multiples, the level of repeatability achieved is extremely encouraging compared to full‐aperture, pressure‐only, time‐lapse data sets at an equivalent stage of processing.