Cedric Schmelzbach

Shallow 3D seismic-reflection imaging of fracture zones in crystalline rock

A limited 3D seismic-reflection data set was used to map fracture zones in crystalline rock for a nuclear waste disposal site study. Seismic-reflection data simultaneously recorded along two roughly perpendicular profiles (1850 and 1060 m long) and with a 400 x 400-m receiver array centered at the intersection of the lines sampled a 680 x 960-m(2) area in three dimensions. High levels of source-generated noise required a processing sequence involving surface-consistent deconvolution, which effectively increased the strength of reflected signals, and a linear tau-p filtering scheme to suppress any remaining direct SV-wave energy. A flexible-binning scheme significantly balanced and increased the CMP fold, but the offset and azimuth distributions remain irregular; a wide azimuth range and offsets 800 m) are only found at the edges of the site. Three-dimensional dip moveout and 3D poststack migration were necessary to image events with conflicting dips up to about 40 degrees. Despite the irregular acquisition geometry and the high level of source-generated noise, we obtained images rich in structural detail. Seven continuous to semicontinuous reflection events were traced through the final data volume to a maximum depth of around 750 m. Previous 2D seismic-reflection studies and borehole data indicate that fracture zones are the most likely cause of the reflections.

Prestack and poststack migration of crooked-line seismic reflection data: A case study from the South Portuguese Zone fold belt, southwestern Iberia

Crooked-line 2D seismic reflection survey geometries violate underlying assumptions of 2D imaging routines, affecting our ability to resolve the subsurface reliably. We compare three crooked-line ...

The shallow elastic structure of the lunar crust: New insights from seismic wavefield gradient analysis

Enigmatic lunar seismograms recorded during the Apollo 17 mission in 1972 have so far precluded the identification of shear-wave arrivals and hence the construction of a comprehensive elastic model of the shallow lunar subsurface. Here, for the first time, we extract shear-wave information from the Apollo active-seismic data using a novel waveform analysis technique based on spatial seismic wavefield gradients. The star-like recording geometry of the active seismic experiment lends itself surprisingly well to compute spatial wavefield gradients and rotational ground motion as a function of time. These observables, which are new to seismic exploration in general, allowed us to identify shear waves in the complex lunar seismograms, and to derive a new model of seismic compressional and shear-wave velocities in the shallow lunar crust, critical to understand its lithology and constitution, and its impact on other geophysical investigations of the Moons deep interior.

Advanced seismic processing/imaging techniques and their potential for geothermal exploration

Seismic reflection imaging is a geophysical method that provides greater resolution at depth than other methods and is, therefore, the method of choice for hydrocarbon-reservoir exploration. However, seismic imaging has only sparingly been used to explore and monitor geothermal reservoirs. Yet, detailed images of reservoirs are an essential prerequisite to assess the feasibility of geothermal projects and to reduce the risk associated with expensive drilling programs. The vast experience of hydrocarbon seismic imaging has much to offer in illuminating the route toward improved seismic exploration of geothermal reservoirs — but adaptations to the geothermal problem are required. Specialized seismic acquisition and processing techniques with significant potential for the geothermal case are the use of 3D arrays and multicomponent sensors, coupled with sophisticated processing, including seismic attribute analysis, polarization filtering/migration, converted-wave processing, and the analysis of the diffracted wavefield. Furthermore, full-waveform inversion and S-wave splitting investigations potentially provide quantitative estimates of elastic parameters, from which it may be possible to infer critical geothermal properties, such as porosity and temperature.

Wavefield Separation of Multicomponent Land Seismic Data Using Spatial Wavefield Gradients

Land seismic recordings at the Earth’s surface register the sum of the upgoing, downgoing reflected and downgoing mode-converted wavefields. However, for true amplitude and phase imaging and subsurface characterization, records of the upgoing wavefield only are desired. We present an approach to isolate the upgoing wavefield for individual densely spaced receiver groups based on the elastodynamic representation theorem. We make use of spatial wavefield gradient estimates derived from multicomponent sensor groups, which allows approximating the wavefield separation expressions with compact filters. Our method does not make any assumption regarding single or isolated arrivals. Compared to the traditional approach of multicomponent data interpretation, which assumes vertical wave propagation, our approach incorporating spatial wavefield gradient estimates significantly improves the separation results up to incidence angles of around 25° and 50° for the horizontal and vertical components, respectively.

Efficient Deconvolution of Ground-Penetrating Radar Data

The time (vertical) resolution enhancement of ground-penetrating radar (GPR) data by deconvolution is a long-standing problem due to the mixed-phase characteristics of the source wavelet. Several approaches have been proposed, which take the mixed-phase nature of the GPR source wavelet into account. However, most of these schemes are usually laborious and/or computationally intensive and have not yet found widespread use. Here, we propose a simple and fast approach to GPR deconvolution that requires only a minimal user input. First, a trace-by-trace minimum-phase (spiking) deconvolution is applied to remove the minimum-phase part of the mixed-phase GPR wavelet. Then, a global phase rotation is applied to maximize the sparseness (kurtosis) of the minimum-phase deconvolved data to correct for phase distortions that remain after the minimum-phase deconvolution. Applications of this scheme to synthetic and field data demonstrate that a significant improvement in image quality can be achieved, leading to deconvolved data that are a closer representation of the underlying reflectivity structure than the input or minimum-phase deconvolved data. Synthetic-data tests indicate that, because of the temporal and spatial correlation inherent in the GPR data due to the frequency- and wavenumber-bandlimited nature of the GPR source wavelet and the reflectivity structure, a significant number of samples are required for a reliable sparseness (kurtosis) estimate and stable phase rotation. This observation calls into question the blithe application of kurtosis-based methods within short time windows such as that for time-variant deconvolution.

Understanding the Impact of Karst on Seismic Wave Propagation - A Multi-method Geophysical Study

Karstified areas are known to be difficult ground for seismic exploration. We conducted a combined numerical-modeling and field-experiment study with the objectives to study the impact of karst on seismic wave propagation and to advance geophysical characterization of karst with seismic as well as non-seismic methods (electric and electromagnetic techniques). Finite-difference simulations using models with realistic topography illustrate the pronounced impact of topographic variations in high-velocity carbonate-bedrock environments on the scattered surface/guided waves. Wavefield complexities such as strong lateral changes in the strength of surface/guided waves, which were observed in a Vibroseis gather from a karst terrain in the Middle East, were also evident in our data recorded in Switzerland. In the latter case, amplifications of surface/guided waves could be correlated with low-velocity zones, which are probably due to more intensively karstified zones. Our study demonstrates that because of the strong heterogeneity of karst terrains, dense sampling is required to properly comprehend and disentangle the observed wavefield. Furthermore, we observed in our field study that the electrical-resistivity models correlate more closely with the mapped lithology, whereas karstification seems to more strongly affect the P-wave velocity models.

GPR imaging of shear zones in crystalline rock

GPR data were acquired at the Grimsel Test Site to improve the geological model prior to high-pressure water injections into shear zones. Data acquired in tunnels using shielded 160 MHz antennas are of exceptionally high quality and could image shear zones up to a distance of approximately 24 m from the tunnel. The interpretation is based on i) comparing modelled shear-zone arrivals to measured ones and ii) directly interpreting fully processed (i.e., migrated) data. Our results add significant detail to the geological model and agree with findings of an anisotropic seismic tunnel-to-tunnel traveltime inversion.

Spatial Wavefield Gradient Data in Seismic Exploration

Multicomponent seismic array data provide not only recordings of the 3D particle motion and direction of arrival but also important information on the spatial gradients of the seismic wavefield. Such gradient data enable numerous new possibilities to analyse land-seismic data. Benefits of seismic spatial wavefield gradient data include enhanced wavefield separation, local slowness estimation, local elastic-parameter determination, and coherent-noise attenuation. Therefore, long-standing problems in seismic exploration such as suppression of scattered surface waves, isolation of particular wave modes, and wavefield reconstruction from spatially under-sampled data can be addressed with gradient data. Interesting applications of spatial gradients arise for shear (S) wave imaging. Considering that rotational motion (curl of wavefield) is directly linked to the S-wave presence, rotational motion measurements facilitate the identification, isolation, and processing of S-waves. If placed at the Earth’s free surface, rotational rates can be estimated from three-component (3C, vector) sensor arrays. Such data enable capture of all six degrees of freedom (three components of translation and three component of rotation). It is straightforward to apply the same principles of array-derived gradients and rotations to arrays of closely-spaced vector (3C, directed forces) seismic sources, which enables the simulation of pure rotational sources and, thus, ‘pure’ S-wave sources.

Testing vertical seismic profiling (VSP) as a subsurface mapping method at the Krafla volcanic geothermal field in Iceland

Summary Vertical seismic profiling (VSP) was tested for mapping volcanic stratigraphy, fractures, dykes, fluid and steam in the geothermal area of Krafla in Iceland. Seismic imaging in magmatic environments is very challenging, largely due to the intense scattering of seismic waves traveling through the highly heterogeneous volcanic rocks. VSP offers means to image structures beneath and away from the well in complex volcanic environments. The VSP survey at Krafla was carried out in two wells, for each of which a zero offset, a far offset and a walk-away experiment were recorded. The zero offset data is of good quality, with the observed reflections corresponding to stratigraphic boundaries that can be explained by a simple 1D velocity model. The corridor stacks of the synthetic and field data look similar to each other, apart from a constant time shift and amplitude differences. High scattering in the subsurface leads to low amplitude reflections from deeper horizons. The walk-away data shows little coherent reflectivity. Furthermore, a complex 2D velocity model involving heterogeneities in the horizontal as well as vertical directions will be required to explain the observed seismograms.