Methods for the seismic exploration of geological fault zones
Geological fault zones are considered to be potential geothermal reservoirs for heat and power generation because permeability can be enhanced in these zones compared to the surrounding rock masses. Faults are usually very complex and heterogeneous, and therefore associated with a high risk of successfully penetrating permeable structures underground. Risks can be reduced by using the seismic techniques already used by the oil and gas industry. The use of modern seismic methods by this industry, such as 3D seismic for improved structural visualisation of the underground rock formations, and the analysis of seismic attributes, which support the interpretation of the stratigraphy and lithology of rocks, are now standard practise.
The gebo-G1 project will investigate geological fault zones on the basis of the available and new seismic P-wave and S-wave surveys to look at (1) the structural relations, (2) their elastic properties and (3) possible transmissivities. The aim of the project is to adapt existing methods and to develop new methods to estimate the geothermal potential and relevant parameters of fault zones to reduce the exploration and drilling risks.
A suitable survey area was found in the first project phase on the eastern margin of the Leine Valley Graben near Nordheim (south Niedersachsen), where a fault system representative of the North German Basin crops out at the surface in the geological units of the Triassic (in Bunter Sandstone). Focusing on a shallow fault system allows more detailed characterisation of the area around the faults and the internal fault structure than would be possible by exploring a deeper fault system. By generating a broad-band source signal, with higher maximum frequencies, this enables much higher resolutions to be achieved than is usually the case in oil and gas exploration (Musmann & Buness, 2010).
A benchmark model was elaborated on the basis of the geology in the survey area. Horizon thicknesses, structural geology, and physical parameters were taken from already existing databases (geology, seismic and wells) and compared with details in other reference sources. The model was used to plan the acquisition geometry for the surveys to be carried out especially for the project, as well as the influence of topography and the fault zone on the seismic lines.
The project ran its own survey in 2010 with the aim of determining the structural geology of the faults at the edge of the graben in the survey area. Two high-resolution P-wave lines of around 2.8 km and 1.8 km length were recorded. The latter was extended into a 1-fold to 2-fold overlying 3D seismic line (1.3 km * 0.9 km), to provide additional information on the spatial relationships of the underground structures. The seismic source used was the P-wave mini vibrator MHV 2,7 from LIAG (Buness, 2007), which generates a frequency range of 25 − 180 Hz over 16 s. The recordings were made with 5 m geophone and 10 m source point separations and were therefore relatively closely spaced; the CMP separation was therefore 2.5 m. 360 active channels were available in total.
The initial processing was oriented to "classic" standard processing for vibro data with NMO/DMO corrections, post-stack migration, and subsequent depth conversion. Special attention was given to the reconstruction of the frequency content of the source signal, and accurate determination of the velocity model. To this end, the velocity field was determined using a tight semblance analysis, and supplemented in shallow zones by velocities acquired from refraction tomographic inversion.
The stacked sections visualised the geological units of the Triassic from base Keuper at a depth of ~ 50 m, down to the Zechstein at a depth of ~ 1 km (Fig. 1). A comparison of the seismic survey results with the structural geological models of the region reveals that the fault system at the edge of the Leine Valley Graben is much more complex than previously thought. A complex pattern of steep and in some parts shallow dipping faults is revealed in front of the main graben edge faults (Musmann et al. 2011): the visualisation of the downfaulted block reveals very strong faulting. The seismic in this zone has only very diffuse reflectivity so that individual reflectors cannot be clearly identified, which is interpreted as indicating a mechanically very strongly disrupted zone. The upthrown block, however, can be more clearly seen in the seismic. The Muschelkalk and Bunter Sandstone units in this block also reveal signs of strong tectonic stress as revealed by the different reflectivities and continuities of the seismic signals, and the seismic velocities. The main graben edge fault itself is also seen in the velocity field. Although the transition over the graben edge is generally associated with an increase in velocity, the zone marking the main graben edge fault is actually characterised by a slight decrease in velocity (2 – 7 %). This is derived from the semblance analysis as well as the refraction tomography.
These lines (and particularly the parts covering the fault zones) are to be investigated in the second half of 2011 by LIAG's own S-wave survey. The first test measurements have already revealed that this method is suitable for application in the area of investigation. After evaluating the S-wave survey results, the findings will be compared to the P-wave survey results with respect to the structures revealed, the resolution and the seismic velocities. And finally, the elastic properties defined from the combination of both surveys are to be used to draw conclusions on possible transmissivities.
Musmann, P. & Buness, H. (2010), High-resolution seismic imaging of near-surface fault structures within the Upper Rhine Graben, Germany, in Richard D. Miller; John H. Bradford & Klaus Holliger, ed., 'Advances in near-surface seismology and ground-penetrating radar', SEG Geophysical Developments Series No. 15, Tulsa, pp. 281-296.
Buness, H. (2007), 'Improving the processing of vibroseis data for very shallow high-resolution measurements', Near Surface Geophysics, 173-182.
Musmann, P.; Thomas, R. & Buness, H. (2011), Seismische Erkundung von geologischen Störungszonen am Beispiel des Leinetalgrabens: Erste Ergebnisse, in '71. Jahrestagung der Deutschen Geophysikalischen Gesellschaft', Köln, 21.-24. Februar 2011.