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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.

Marine seismic exploration involves the use of airgun source arrays for emitting sound energy into the earth. In this paper, we discuss the benefits of using a new airgun design in marine seismic exploration, by way of the reduction in risk of potential injury to marine mammals. This minimizes the potential impact of marine seismic exploration to sea life without affecting the quality of the final seismic image produced.

Heterodyne distributed vibration sensing (hDVS) and, more generically, distributed acoustic sensing (DAS) are technologies that rely on traditional fibre-optic cables to provide inline strain measurement. In recent years, hDVS has found application in seismic monitoring; in particular vertical seismic profiling (VSP) greatly benefits from the advantage of using permanent fibres cemented behind casing (Mateeva et al., 2013). VSP measurements were also demonstrated using fibre installed on production tubing (Barberan et al., 2012) and deployed within a wireline cable (Hartog et al., 2014). Another potential application for hDVS is flow monitoring, where, owing to the fibre’s large spatial coverage, it is possible to monitor vibrations and waves along the length of the well. Baihly et al. (2015) offered an insight on this use of hDVS. Furthermore, Molteni et al. (2016) evaluated microseismic monitoring as another compelling hDVS application. This paper evaluates the viability of using hDVS for distributed dynamic pressure sensing in marine seismic acquisition while reviewing the readiness of hDVS technology for this application. An initial concept of including hDVS in a streamer was disclosed by Kragh et al. (2012). Here, a possible design of a towed streamer using hDVS was discussed along with experimental results showing the potential of the technology to acquire pressure measurements in marine seismic surveys. We show the results of using hDVS to measure the pressure wavefield in two different experiments, and the results are compared with reference data acquired with traditional hydrophones. The use of a distributed measurement in towed streamers would provide a new type of measurement. The characterization of the pressure wavefield would be continuous along the cable with a denser spatial sampling. Similarly, straight fibres deployed along the streamer could characterize the tugging noise with increased spatial resolution. Furthermore, the use of cost-effective and lightweight fibres would greatly reduce the electronics embedded in the streamer and the total weight of the cable. The use of hDVS would also have new challenges – for example in how to segment the streamer into shorter sections without the fibre-optic connectors compromising the quality of the backscattered hDVS signal.

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 Landro, 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 (Ozbek 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.

The high-frequency sound emission from air-guns is of no benefit to imaging. Historically, the design goal for airguns was for ever-higher peak pressures and this has led to pulses with unnecessarily steep rise times. The new design (Figure 1) releases its air in a carefully designed way such that the steepness of the rising edge can be controlled. This acts like a low-pass filter, but one that is applied physically rather than in processing.

Reducing the volume of a marine seismic source array can cause a significant reduction in the effect upon and interaction with marine fauna. Small sources are favourable in environmentally sensitive areas; however, the conventional understanding is that larger sources are required for improved signal-to-noise ratio and deep penetration. Results from a number of reduced source-strength tests suggest that this is only partly true; at the lower end of the seismic bandwidth, we can benefit from more power (we are governed by non-shot-generated noise), but for the majority of the seismic bandwidth, we can indeed accept less power (we are governed by shot-generated noise). We look at the system as a whole and use a 4C towed streamer that is towed deep. This amplifies the low frequencies and compensates for the lack of low-frequency signal from the smaller source array. This allows us to acquire data with a small source array (typically three or four times smaller than normal) while maintaining the data quality of a conventional towed-streamer system using a larger source array. The high-frequency limit of the conventional pressure only streamer is also overcome by the inherent notch filling of the 4C measurements.

We address the low-frequency deficiency of marine airgun source arrays. The output well below the lowest bubble resonance frequency is determined by the total quantity of air released, not how the air is distributed between guns in the array. However, the output at and above the lowest bubble frequency depends on the details of the array. Increasing the low-frequency output requires a bigger bubble. This will shift the bubble resonance frequency downwards. We design the large bubble using a ‘hypercluster’ of conventional airguns, which are placed so the released air behaves as one large bubble oscillating with the period associated with the combined air quantity. The airguns are placed close enough for the released air to frequency-lock, but not necessarily so close that the bubbles coalesce. Of course, we could generate the same resonance frequency by using a single enormous airgun, but we do not want to do that because this would severely limit the mid-frequency output, and it would also pose significant operational challenges. Frequency-locking enables array designers to utilize more of the total high-pressure air capacity for generating low-frequency sound.

We address the question of whether or not there always is a “bandwidth benefit” in non-uniform spatial sampling of geophysical data. Answering this question is, for example, important in the context of random sampling of seismic data, as it recently has been shown that there can be such a benefit under certain assumptions on the spectral structure of the data. Assume that a fixed number of sensors are placed either uniformly (i.e., on a regular grid) or nonuniformly (either randomly distributed or following any suitable non-uniform sampling scheme). The bandwidth supported by uniform sampling is that of the Nyquist wavenumbers corresponding to the sampling distances on the regular grid. The bandwidth supported by the nonuniform sampling we propose here refers to the maximum bandwidth of data that could be reconstructed by a linear operator at arbitrary sampling locations within the survey area without unacceptably high reconstruction error. Without making further assumptions on the spectral structure of the data, i.e., especially without assuming sparseness of the data spectrum, we will argue that we see no such bandwidth benefit in non-uniform sampling in the examples we have investigated.

We discuss how to design a marine source array that emits the same pulse shape in all directions. We do this by analysing the relative contribution of the three main components of the farfield source spectrum: the ghost response, the aperture smoothing function, and the monopole spectra of the individual source elements. Counter to intuition, reducing the lateral array dimensions, such that it approximates a point aperture, will not result in an isotropic source; it will create a dipole because of the sea-surface reflection. For single depth arrays, the lateral geometry must be tailored such that the aperture smoothing function offsets the angular variation of the ghost response, in particular at frequencies close to the first ghost notch. Multi-depth arrays can be made insensitive to the lateral geometry by combining the source depths, such that the first ghost notch is far beyond the highest frequency of interest. This results in smaller spectral variation with take-off angle than that of single depth arrays. There is no advantage in using more than two layers.