Stefan Buske

Fresnel volume migration of multicomponent data

If the aperture of a seismic reflection experiment is strongly limited, Kirchhoff migration suffers from strong artifacts attributable to incomplete summation. This can be overcome by restricting the migration operator to the region that physically contributes to a reflection event. Examples of such limited-aperture experiments include data acquisition in boreholes, tunnels, and mines. We present an extension to three-component (3C) Kirchhoff prestack depth migration, where the migration operator is restricted to the Fresnel volume of the specular reflected raypath. We use the measured polarization direction at a 3C receiver to determine points of specular reflection. In homogeneous media, the polarization angle of 3C data can be used directly to decide whether a certain image point belongs to the Fresnel volume of a specular reflection. In heterogeneous media, the Fresnel volume around an image point is approximated by means of paraxial ray tracing. The method is tested on a synthetic vertical seismic profiling experiment with strongly limited aperture. Migration artifacts and crosstalk effects from converted waves are strongly reduced compared with standard migration schemes. The method is successfully applied to seismic data acquired in a tunnel.

Fast location of seismicity: A migration-type approach with application to hydraulic-fracturing data

We propose a new approach for the location of seismic sources using a technique inspired by Gaussian-beam migration of three-component data. This approach requires only the preliminary picking of time intervals around a detected event and is much less sensitive to the picking precision than standard location procedures. Furthermore, this approach is characterized by a high degree of automation. The polarization information of three-component data is estimated and used to perform initial-value ray tracing. By weighting the energy of the signal using Gaussian beams around these rays, the stacking is restricted to physically relevant regions only. Event locations correspond to regions of maximum energy in the resulting image. We have successfully applied the method to synthetic data examples with 20%–30% white noise and to real data of a hydraulic-fracturing experiment, where events with comparatively small magnitudes (<0) were recorded.

Fresnel volume migration of single-component seismic data

Standard implementations of Kirchhoff prestack depth migration (PSDM) distribute the recorded wavefield along two-way-traveltime isochrons and an image is generated by constructive interference of these isochrons along the actual reflector elements. Beside the recent developments of wave-equation-based approaches, Kirchhoff PSDM is still considered widely as a state-of-the-art technique in obtaining high-quality images of the subsurface, particularly for highly irregular survey layouts and target-oriented imaging tasks. However, for sparse sampling or limited aperture, the resulting image is affected by significant migration noise as a result of limited constructive interference of the back-propagated wavefield. Some modifications have been proposed to reduce these artifacts. These modifications include constructing a specular path of wave propagation, derived from estimates of the emergent angle of coherent phases in the seismogram section, and the mainly heuristic restriction of the imaging operator to the neighborhood of that wavepath. Our approach uses Fresnel volumes to restrict the migration operator in a physically frequency-dependent way. Using the emergent angle at the receiver, determined by a local slowness analysis, a ray is propagated into the subsurface; the back-propagation of the wavefield is restricted to the vicinity of this ray according to its approximated Fresnel volume. This so-called Fresnel volume migration approach enhances image quality significantly compared with standard Kirchhoff PSDM because of the inherent focusing and the restriction of the back-propagation to the region around the actual reflection point.

Broad depth range seismic imaging of the subducted Nazca Slab, North Chile

Abstract In this paper, we present a compilation of modern seismic and seismological methods applied to image the subduction process in North Chile, South America. We use data from active and passive seismic experiments that were acquired within the framework of the German Collaborative Research Center SFB267 ‘Deformation Processes in the Andes’. The investigation area is located between 20° and 25°S and extends from the trench down to 100 km depth. In the depth range between the sea bottom and 15 km, we process an offshore seismic reflection profile using a recently developed velocity-model-independent stacking procedure. We find that the upper part of the subducting oceanic lithosphere in this depth range is characterized by a horst-and-graben structure. This structure supports an approximately 3 km thick coupling zone between the plates. In the depth range between 15 and 45 km, we analyse the spatial distribution of aftershocks of the Antofagasta earthquake (1995). The aftershock hypocenters are concentrated in an approximately 3 km thick layer. Finally, in the depth range between 45 and 100 km, we apply Kirchhoff prestack depth migration to the onshore ANCORP profile. A double reflection zone is observed between 45 and 60 km depth, which may represent the upper and lower boundary of the subducted oceanic crust. Over the whole range down to more than 80–90 km depth, we obtain an image of the subducting slab. At that depth, the hypocenters of local earthquakes deviate significantly in the direction perpendicular to the slab face from the reflective parts of the slab. Consequently, our results yield a complete seismic image of the downgoing plate and the associated seismic coupling zone.

Seismic Images of Accretive and Erosive Subduction Zones from the Chilean Margin

Modern seismic imaging methods were used to study the subduction processes of the South American convergent margin. The data came from reflection and from wide-angle/refraction experiments acquired within the framework of the Collaborative Research Center SFB267 ‘Deformation Processes in the Andes’. Two areas of differing character and subduction type were investigated: an erosive margin to the north (19–26° S) and an accretionary margin to the south (36–40° S). Results from different seismic models yield three main transects that give an overall impression about the internal structure below the Chilean margin. At the erosive margin, we find that the upper part of the subducting oceanic lithosphere is characterized by a horst-and-graben structure that coincides with the coupling zone between the plates. Strong coupling between oceanic crust and fore-arc in the case of a horst-continent collision is also indicated by plate-parallel faults beneath the lower continental slope, which we interpret as the upper parts of the subduction channel. In this context, the subduction channel represents the downgoing Nazca Plate as well as those portions of the continental crust which moved landward. Low seismic velocities below the coastline also represent parts of the subduction channel and of the hydrofractured base of the upper crust near the plate interface. Between 45 and 60 km depth, a double reflection zone marks the upper and lower boundary of the subducted oceanic crust. Off southern Chile, the ocean bottom is characterized by relatively smooth morphology. In contrast, in the south, the trench is filled with sediments and contains an axial channel (Figs. 7.16 to 7.18) extending in N-S direction along the trench axis within the investigation area. The periodicity of the reflected seismic signal within these sediments correlates with the main glacial cycle during the Quaternary. The recent accretionary wedge is built up from strongly heterogeneous unconsolidated sediments. Frontal accretion takes place within the southern working area except for the region around the Arauco Peninsula, which shows uplift due to basal accretion and antiformal stacking. Below the Coastal Cordillera, the heterogeneity of the modern accretionary wedge and the antiformal stack structure of the Permo-Triassic accretionary wedge complicate imaging at depths greater than about 30 km. Thus, we obtain an image of the top of the subduction channel as a thin reflector segment only to about 25 km depth.

Three-dimensional focused seismic imaging for geothermal exploration in crystalline rock near Schneeberg, Germany

We present the results of a 3D seismic survey acquired near the city of Schneeberg in the western Erzgebirge (Germany). The main objective of this survey was to use reflection seismic exploration methods to image a major fault zone in crystalline rock, which could serve as a geothermal reservoir at a target depth of about 5 km–6 kmwith expected temperatures between 160◦C–180◦C. For this purpose, a high-resolution 3D Vibroseis survey was performed in late 2012 covering an area of about 10 km × 12 km. The 3D survey was complemented by a wide-angle seismic survey for obtaining velocity information from greater depths using explosives along ten profile lines radially centred at the target area. The region itself is dominated by the northwestsoutheast striking Gera-J ´achymov fault system and the southwest–northeast striking L¨ ossnitz–Zw ¨ onitz syncline. The main geological features in the survey area are well known from intensive mining activities down to a depth of about 2 km. The seismic investigations aimed at imaging the partly steeply dipping fault branches at greater depths, in particular a dominant steeply northeast dipping fault (Roter Kamm) in the central part of the survey area. In addition to this main structure, the Gera–J´achymov fault zone consists of a series of steeply southwest dipping conjugate faults. For imaging these structures, we used a focusing pre-stack depth migration technique, where the wavefield coherency at neighbouring receivers is used for weighting the amplitudes during migration. This method delivers a clear, focused image of the 3D structures within the target area. A 3D velocity model for depth imaging was obtained by first-arrival tomography of the wide-angle survey data. With this approach, we were able to image several pronounced structures interpreted as faults within the crystalline rock units, which partly reach the target depth where the temperatures for a geothermal usage would be sufficient. In general, the results show a complex three-dimensional image of the geological structures with different reflection characteristics, which can serve as a basis for a detailed characterization of the potential deep geothermal reservoir.

Microseismic Imaging From a Single Geophone: KTB

We have applied our approach for microseismic imaging to the data obtained from the German Continental Deep Drilling program (KTB project). This is a continuous data stream containing induced microseismicity data recorded at a single 3C geophone located at approximately 3.5 km depth. Using P and S time picks we have located 58 microseismic events using data from the borehole geophone and from near-surface stations. Since microearthquakes occur not at the same time, we have managed to separate continuous data stream recorded at the receiver to the number of 3C traces containing waveforms from different events. Using these traces we have produced seismic gather for the microseismicity cloud. We apply our approach to the produced seismic gather and construct 3D images of the seismic data between Pand Sdirect waves. There are two focused reflectors revealed within the microseismicity cloud which belong to SE2 reflector. In order to check consistency of the obtained results, we compare our results with seismic attributes obtained from the surface seismic 3D depth migrated image. Obtained result is consistent with seismic attributes by the location and dip, furthermore it provides with more detailed image of the fault zone fine structure.

Divergence of geometrical optics series at the boundary of its applicability: An analytical example in elementary functions (2D Gaussian beam in free space)

An analytical example in elementary functions is presented (2D Gaussian beam diffraction in free space), which demonstrates the divergence of the geometrical optics (GO) series when the conditions for its applicability are violated. This example shows that accounting for higher terms in GO power series leads to divergence and therefore becomes completely useless beyond the boundaries of GO applicability.

Location of Seismicity Using Gaussian Beam Type Migration

We propose a new approach for precise location of seismic sources using a Gaussian beam type migration of multicomponent data. This approach requires only the preliminary picking of event time intervals and is much less sensitive to the picking precision than standard location procedures. Furthermore, this approach is characterised by a high degree of automation. The polarisation information of multicomponent data is used to perform initial-value ray tracing. By weighting the amplitudes using Gaussian beams around these rays the stacking of amplitudes is restricted to physically relevant regions only. Event locations correspond to regions of maximum amplitudes in the resulting image. We show a successful application of the method to a synthetic data example with a noise level of 20 per cent. Currently, the method is applied to real data from the German Continental Deep Drilling (KTB) Program.

Estimating statistical parameters of an elastic random medium from traveltime fluctuations of refracted waves

Traveltime fluctuations of diving-type refracted waves are studied in the framework of geometrical optics in order to estimate the statistical parameters of an elastic random medium. A stratified background medium is considered in which the velocity increases linearly with depth. Smooth and strongly anisomeric (statistically anisotropic) inhomogeneities are embedded in this medium. The covariance and the variance of traveltime fluctuations are derived and subsequently used to estimate the standard deviation of the medium fluctuations and the inhomogeneity scale lengths in horizontal and vertical directions. The theoretical estimation procedure is verified by performing numerical calculations and it is observed that, under the considered conditions, the traveltime variance decreases at large offsets. This new phenomenon has not been observed before either in acoustics and optics, or in radio wave propagation.