Rüdiger Giese

3D baseline seismics at Ketzin, Germany : The CO2SINK project

A 3D 25-fold seismic survey with a bin size of 12 by 12 m and about 12 km(2) of subsurface coverage was acquired in 2005 near a former natural gas storage site west of Berlin, as part of the fiv ...

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.

Rayleigh-to-shear wave conversion at the tunnel face — From 3D-FD modeling to ahead-of-drill exploration

For safe tunnel excavation, it is important to predict lithologic and structural heterogeneities ahead of construction. Conventional tunnel seismic prediction systems utilize body waves (P- and S-waves) that are directly generated at the tunnel walls or near the cutter head of the tunnel boring machine (TBM). We propose a new prediction strategy that has been discovered by 3D elastic finite-difference (FD) modeling: Rayleigh waves arriving at the front face of the tunnel are converted into high-amplitude S-waves propagating further ahead. Reflected or backscattered S-waves are converted back into Rayleigh waves which can be recorded along the sidewalls. We name these waves RSSR waves. In our approach, the front face acts as an S-wave transceiver. One technical advantage is that both the sources and the receivers may be placed behind the cutter head of the TBM. The modeling reveals that the RSSR waves exhibit significantly higher amplitudes than the directly reflected body waves. The excavation damage zone causes dispersion of the RSSR wave leading to multimodal reflection response. For the detection of geologic interfaces ahead, RSSR waves recorded along the sidewalls are corrected for dispersion and stacked. From the arrival times, the distance to the S-S reflection point can be estimated. A recurrent application, while the tunnel approaches the interface, allows one to quantify the orientation of the reflecting interfaces as well. Our approach has been verified successfully in a field experiment at the Piora adit of the Gotthard base tunnel. The distance to the Piora fault zone estimated from stacked RSSR events agrees well with the information obtained by geologic surveying and exploratory drilling.

Toolbox for Applied Seismic Tomography (TOAST)

TOAST (Toolbox for Applied Seismic Tomography) makes methods of full-waveform inversion of elastic waves available for the practitioner. The inversion of complete seismograms is an utmost ambitious and powerful technology. One of its strengths is the enormously increased imaging-resolution since it is able to resolve structures smaller than the seismic wave length. Further it is sensitive to material properties like density and dissipation which are hardly accessible through conventional techniques. Within the TOAST project algorithms available in academia were collected, improved, and prepared for application to field recordings. Different inversion strategies were implemented (global search, conjugate gradient, waveform sensitivity kernels) and computer programs for imaging the subsurface in 1D, 2D, and 3D were developed. The underlying algorithms for the correct numerical simulation of physical wave propagation have thoroughly been tested for artefacts. In parallel these techniques were tested in application to waveform data. They proved their potential in application to synthetic data, shallow-seismic surface waves from field recordings, and microseismic and ultrasonic data from material testing. This provided valuable insight to the demands on seismic observation equipment (repeatability, waveform reproduction, survey layout) and inversion strategies (initial models, regularization, alternative misfit definitions, etc.). The developed software programs, results of benchmark tests, and field-cases are published online by the OpenTOAST.de initiative.

Seismic prediction ahead of a tunnel face - Modeling, field surveys, geotechnical interpretation -

An important precondition for underground construction is a detailed knowledge of the soil and/or rock conditions in the area of the construction. In order to overcome existing limitations in classical exploration methods, research and development for exploration ahead of a tunnel face focuses on: hardware development for excavation integrated measurements, modelling and processing of data measured under these specific circumstances, and integrative interpretation of seismic results with other data from the excavation, from geological mapping, and from exploratory drilling, where available. Finite difference modelling of seismic wavefields around tunnels has shown the general feasibility of seismic measurements for imaging structures ahead of a tunnel face. The modelling results were confirmed by field measurements in various tunnel sites. The integrated interpretation of seismic data with all available geological and geotechnical information is currently in the state of development and aims, in the middle to long term perspective, at an “a priori” detection of structures ahead of the face.

Seismic Tomography and Monitoring in Underground Structures: Developments in the Freiberg Reiche Zeche Underground Lab (Freiberg, Germany) and Their Application in Underground Construction (SOUND)

The construction of large tunnels and underground infrastructures faces increasingly large dimensions and complex geological conditions. Under these conditions, exploration techniques are needed which enable for a detection of potentially hazardous structures during construction. Seismic sensors, integrated into rock anchors, and small seismic signal sources using defined pneumatic impulses or sweep signals generated by magnetostrictive actuators are the components of an exploration system which can be easily integrated into different types of underground excavation work and which can also be deployed for the long-term monitoring of already existing tunnels or caverns. However, for a continuous acquisition of seismic signals during tunnel excavation, the strong and broadband signal generated by a tunnel boring machine (TBM) may be used as a continuously operating source. Within the collaborative project SOUND, the seismic equipment at the Underground Lab of the Reiche Zeche Research Mine in Freiberg (Germany) has been used for a tomographic monitoring study during the excavation of an inclined gallery. A synthetic, but realistic seismic data set was simulated using a randomly heterogeneous velocity model which can be regarded as a realistic prototype of the velocity distribution in the real Gneiss block. The simulated acquisition geometry has been derived from the actual source and receiver point distribution in the Underground Laboratory. It can be shown that the analysis of the modelled seismic data by full waveform inversion (FWI) was able to reveal the lateral heterogeneity of the velocity model with significantly higher resolution compared to traveltime tomography of the direct P-wave arrivals. The analysis of field data from the Underground Laboratory has shown that there are complex interactions in close vicinity to the receiver location, and before FWI can be applied to this real data set, source and receiver dependant signatures need to be removed by inversion and deconvolution. A further field experiment, performed during gallery excavation in the Underground Laboratory, has shown that the setup of seismic receivers in rock anchors and a sparse array of adaptive vibro-sources is able to detect subtle changes in seismic wave propagation related to stress changes due to the excavation of an inclined gallery. After the deployment in the Underground Laboratory, a field survey was carried out on a tunnel construction site. A broadband seismic data set, using the tunnel boring machine could be acquired providing a basis for high resolution imaging of structures ahead of the construction site and geotechnical characterization of the imaged volume.

Reservoir and Cap Rock Monitoring

One aim of the CLEAN project was to develop and test monitoring methods for the reservoir cap rock and the reservoir itself. It is shown here that advanced injection and production profile evaluation can be achieved using a combination of pressure, temperature and spinner flow meter data. Using distributed temperature sensing, temperature profiles in gas-filled wells can be acquired, as the sensor cable can be stationary during the measurement allowing for simultaneous thermal equilibration along the entire logged profile. A first field test of the developed hybrid wireline logging system was successfully performed under static conditions and the feasibility of warm-back monitoring was shown based on the results of numerical simulations for a possible CO2 injection scenario. A combined approach using the developed hybrid system for enhanced production logging during injection followed by warm-back monitoring within a subsequent shut-in period would allow for accurate determination of the spatial extent and injectivity of individual CO2 injection intervals.

Theoretical Test Case of the Injection of 100,000 t of CO2 into the Altmark Depleted Gas Field

CLEAN was a scientific programme in support of a pilot Enhanced Gas Recovery (EGR) project and it was planned to inject nearly 100,000 t of carbon dioxide (CO2) into the Altmark natural gas field. Due to delays in the permitting process, injection did not occur within the time frame of the project. Therefore a test case was studied of the theoretical injection of CO2.

GFZ Underground Laboratory in the Research and Education Mine “Reiche Zeche” Freiberg

The GFZ Underground Laboratory is operated by the Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences. It is located in the research and education mine “Reiche Zeche” in Freiberg, Germany allows testing of geophysical and geotechnical tools and methods in boreholes and galleries. The lab is ideally suited for seismic system components such as receivers and sources for three-dimensional high resolution seismic imaging and tomography surveying. The lab layout of a basement rock block surrounded by galleries around a vertical as well as two horizontal boreholes enables the realization of various underground survey geometries e.g. well-to-well and well-to-gallery. The galleries are equipped with thirty 3-component geophone anchors installed in 1 m and 2 m depths for tomographic measurements or the recording of radiation pattern of seismic borehole sources.