Experiences with Distributed Acoustic Sensing using both straight and helically wound fibers in surface-deployed cables -- a case history in Groningen, The Netherlands

Distributed Acoustic Sensing (DAS) has been limited in its use for surface-seismic reflection measurements, due to the fiber's decreased sensitivity when the fiber is deployed horizontally. Deploying the fiber in a helically wound fashion has the promise of being more sensitive to broadside waves (e.g. P-wave reflections) and less sensitive to surface waves than straight fiber. We examine these claims by burying a set of straight fibers (SF) and helically wound fibers (HWF) with different wrapping angles, using standard and engineered fibers. These fibers were buried in a 2 m deep trench in a farmland in the province of Groningen in the Netherlands. They are linked up to two interrogating systems and an electrically driven vibrator was used as a seismic source. We observe in our field data that using HWF has a destructive effect on the surface-wave amplitudes. Our data confirmed the effect of the wrapping angle on the polarity of the surface-wave arrival and the dampening effect of the helical winding, both behaving in quite a predictable fashion. Apart from the effect of the wrapping angle, the different design choices, e.g. cable filling and material type, did not show a significant effect on the amplitude of the signals. As for P-wave reflections, we observe that both engineered SF and HWF provide reflection images comparable to those obtained from the geophone data despite the straight fiber's decreased broadside sensitivity. A polarity reversal and an amplitude difference between SF and HWF fibers are observed. Finally, we show that the combined use of SF and HWF proved to be useful since SF showed better sensitivity in the shallower part and HWF in the deeper part.Comment: This manuscript has been submitted to GEOPHYSICS journa

Transdimensional surface wave tomography of the near-surface: Application to DAS data

Distributed Acoustic Sensing (DAS) is a novel technology that allows sampling of the seismic wavefield densely over a broad frequency band. This makes it an ideal tool for surface wave studies. In this study, we evaluate the potential of DAS to image the near-surface using synthetic data and active-source field DAS data recorded with straight fibers in Groningen, the Netherlands. First, we recover the laterally varying surface wave phase velocities (i.e., local dispersion curves) from the fundamental-mode surface waves. We utilize the Multi Offset Phase Analysis (MOPA) for the recovery of the laterally varying phase velocities. In this way, we take into account the lateral variability of the subsurface structures. Then, instead of inverting each local dispersion curve independently, we propose to use a novel 2D transdimensional surface wave tomography algorithm to image the subsurface. In this approach, we parameterize the model space using 2D Voronoi cells and invert all the local dispersion curves simultaneously to consider the lateral spatial correlation of the inversion result. Additionally, this approach reduces the solution nonuniqueness of the inversion problem. The proposed methodology successfully recovered the shear-wave velocity of the synthetic data. Application to the field data also confirms the reliability of the proposed algorithm. The recovered 2D shear-wave velocity profile is compared to shear-wave velocity logs obtained at the location of two boreholes, which shows a good agreement

Application of virtual seismology to DAS data in Groningen

In this report we investigate whether and under what conditions virtual seismology via the acoustic Marchenko method can be applied to DAS data from a survey in the province of Groningen, The Netherlands. Virtual seismology allows to retrieve the band-limited Green's function between a virtual source at an arbitrary focal point in the subsurface, while accounting for all orders of multiples. The method requires the reflection response at the surface and an estimate of the traveltime between the surface and focal point. However, in order to successfully apply the method the reflection response needs to be free from surface waves and other direct waves, and properly scaled in order for the Marchenko scheme to converge. These limitations severely complicate the application of the Marchenko method to field data, especially seismic surveys on land. This report considers a full 2D geophone survey as well as a 1.5D approximation for a DAS survey, and compares the results of the virtual sources with an actual dynamite source. The results show that virtual seismology can be used to recreate the reflections recorded at the surface from the dynamite source using either geophone or DAS data.Comment: 7 pages, 6 figure

Time-frequency analysis of seismic sequences

Conference PaperReflection patterns play an important role in seismic sequence stratigraphy, therefore making their quantitative description essential for the construction and validation of sequence stratigraphic models from seismic data. The characteristics of a seismic reflection pattern can be elicited from the data by representing the data as a joint function of time and frequency. Developments in the field of time-frequency analysis have led to t-f representations with better resolution than can be obtained with classical methods for local frequency analysis. This justifies a study of the application of these new representations to the analysis of seismic data. Some examples of the t-f representation of the seismic response of layered sequences are given. They clearly show the contribution of the stratigraphic sequence to the spectral content of the data. The construction of a sequence stratigraphic model from a t-f representation is demonstrated with a field data example where we match a pattern that was observed in the data to a sequence model

Extraction of Attributes from 3D Seismic data

Introduction The characterization of seismic data in terms of their spatial properties, such as horizon dip and azimuth, has greatly improved the interpretation of 3D seismic data (Hoetz and Watters 1991). In most cases the 3D seismic attributes are extracted at or between picked seismic horizons. In our paper we present an alternative method for 2D and 3D attribute extraction, that does not require the picking or tracking of horizons. We extend the relation between the 1D complex trace attributes and local spectra (Steeghs et al. 1995) to higher dimensions. Using the local 2D wavenumber-frequency spectrum we can extract dips and azimuths over time slices in 3D seismic data. Method and preliminary results Two-dimensional instantaneous attributes (Barnes 1994) can be related to a local spectrum. For a 2D seismic section the local power spectrum will be a 4D energy density function of the variables offset, x, time, t, and spectr