LabEx G-EAU-THERMIE PROFONDE
Appel à projet 2014 / 2015
1
Fabienne Reiser (Ph.D), 1Cedric Schmelzbach, 1Stewart Greenhalgh, 2Adnand Bitri, 3Jean-Michel
Marthelot
1 Institute of Geophysics, Department of Earth Sciences, ETH Zurich
2 BRGM, 3 av Guillemin, 45060 Oléans
3 EOST, Géophysique expérimentale
1. Abstract
In recent years, the seismic method has been only sparingly applied to geothermal exploration and
production compared to its extensive use in the oil and gas industry. Various sophisticated seismic
imaging techniques have been developed for hydrocarbon exploration and development and it needs
to be investigated how these techniques can be adapted and applied to geothermal sites. Vertical
seismic profiling (VSP), which entails sources on the Earth’s surface and geophones placed down the
well, can be applied to identify reflections and trace them to their points of origin in the subsurface,
provide information about their orientation and exact location when they intersect the borehole, and
tie borehole geology to surface seismic data. Since permeability (and attendant) water flow is mostly
associated with dipping fault and fracture zones, it is important to map dipping structures over
geothermal sites. VSP provides a convenient geometry for mapping gently and steeply dipping
interfaces in 3D away from the well, especially for multi-offset and multi-azimuth surveys.
Additionally, valuable reflectivity, velocity and anisotropy information can be extracted from VSP
data, which aids surface seismic reflection exploration. The Soultz VSP dataset from 2007 provides a
unique opportunity to test novel seismic imaging techniques, as it consists of triaxial recordings in
two wells at various source offset and azimuth positions. The condition of use of the dataset is
currently discussed with GEIE and the Labex, and will be materialized in a convention.
2. Context : VSP imaging as an exploration method for geothermal
resource assessment
Fabienne Reiser is currently doing her PhD on the use of active seismic imaging over geothermal sites
under the direction of Prof. Stewart Greenhalgh and Dr. Cedric Schmelzbach at ETH in Zurich. She
has just completed the first year of candidature. The principal goal of her PhD thesis is to test and
assess advanced seismic imaging methods on geothermal seismic data. It needs to be studied how
these methods can contribute to a better reservoir understanding and can help to improve the
planning and development of geothermal reservoirs. The VSP dataset from Soultz offers a great
opportunity to test advanced seismic imaging techniques over a geothermal site. She started to
process a subset of the Soultz data and have carried out some supporting numerical modelling
investigations based on the 3D fracture model from Sausse et al. (2010). However, at the moment
She only has access to data from 6 out of the 24 source positions.
In the recent (2013) PhD thesis of P. Lubrano from University of Strasbourg, which is entitled
« Traitement des données de sismique de puits acquises en 2007 sur le site de Soultz-sous-Forêts
pour la caractérisation de la fracturation du réservoir géothermique. », the interest and possibilities
of the Soultz dataset were explored in detail, at least as far as P-wave imaging was concerned. He
emphasized the difficulty to image sub-vertical faults with the given geometry of acquisition, the
limitation due to the heterogeneity of the shallow velocity model and the limited number of shots
resulting in weak redundancy in the data. Nevertheless, previous work has shown that the seismicdata signal-to-noise ratio is good, and further work on, for example, filtering, seismic wavelet
deconvolution, and refining the velocity model may help to extract more information from the
dataset. S-wave (converted wave) imaging and diffraction mapping offer additional possibilities, as
does full elastic waveform inversion.
Fabienne Reiser PhD thesis will last another 2-3 years and will include surface reflection imaging
developments and applications. The VSP component will occupy much of the coming 12 months and
so the duration of this project will be 1 year.
BRGM and EOST participation will be less intensive, focused in the technical discussion of the
methods and results. Their expertise relies on their knowledge of the existing seismic experiment
conducted in the Soultz area, and previous experience in VSP acquisition and processing. In addition,
A. Bitri and JM Marthelot have extensive experience in deep seismic (ECORS program for instance)
including the Rhine Graben.
3. Proposal
So far, a 2D visco-elastic modelling code that is based on finite difference method (Bohlen, 2002) was
employed that also includes P-to S-wave conversions, an arbitrary heterogeneous background
velocity model and anelastic absorption. The response for the zero offset source position and
fracture zones based on Sausse et al. (2010) is shown in Figure 1. Clear direct P- and S-waves can be
identified, the reflection from the sedimentary/ basement interface and the effects from the steeply
dipping fracture zones. From analyzing the divergence and curl of the recorded particle velocity field,
which effectively separates the P- and S-wave components, it was observed that most of the energy
that is reflected at the fracture zones corresponds to P-to S-wave conversions. Hence the processing
of P-to S-converted waves will be an important task in the future processing.
Figure 1: Horizontal and vertical component responses of the fractured model shown based on Sausse
et al. (2010). Apart from the direct P-and S-waves, several reflected events can be observed.
Basic processing that included geometry setup, geophone rotation from raw recorded to radial,
transverse and vertical components, and separating up- and downgoing wavefields with an f-k filter,
was applied. Polarization analysis along with hodogram display of the particle motion were used to
aid identification of the different wavefields and to retrieve directional information. A
multicomponent migration algorithm (Sollberger, 2013) was tested for the zero- offset data to obtain
P-P and P-S-wavefield migrated images. However, the multicomponent PP, multicomponent PS and
the single component migration images look all quite different in the sense that all show different
dipping interfaces (Figure 2). This is probably due to use of a single source position. More source
positions from different offsets and azimuths need to be included to better migrate the data.
Figure 2: P-P and P-S-wavefield multicomponent migration images on the left and single component
migration images on the right after application of multicomponent migration algorithm (Sollberger,
2013). The absolute dip of the fracture zones vary for the different migrations, probably due to a
single source position. More sources need to be included to better migrate the data.
During the next year she would like to analyse reflections from different modes (P-P, P-S, S-P, S-S
waves), apply multicomponent migration, assess the potential for shear wave birefringence analysis
(to decipher fracture anisotropy) and implement the rather novel and diagnostic diffraction imaging
procedure for delineating subtle structural features not amendable to standard specular reflection
analysis. Apart from zero offset VSP data, recordings from multiple offset source points (up to several
kms) and at different azimuths relative to the borehole can be processed. The separation of P-, P-toS-converted and S-waves will be critical, for example, for obtaining accurate P- and S- velocity models
that can then be used in a multicomponent migration algorithm to obtain P-P and P-S images
(Sollberger, 2013). The application of the multicomponent migration algorithm was tested on real
data from Soultz but only for the zero offset source position with minor success. She intend to
include more source positions in the migration algorithm for a better illumination of the subsurface.
The separation of P- and S-waves can also be carried out using controlled direction reception filtering
(using a code developed by Sollberger (2013) based on Greenhalgh et al. (1990)), which operates in
-p space. If a clear separation of P- and S-wavefields can be achieved, separate P- and S-wave
images can be obtained that can yield complementary information. Wavefield separation is also
critical for shear wave birefringence analysis. The data requirements for a successful study of shear
wave splitting are high if there is a good signal-to-noise ratio, a large amount of S-wave energy
present and a clear identification of differently polarized fast and slow S-waves. The VSP dataset
from Soultz-sous-Forêts can be studied to check whether these requirements are fulfilled. The VSP
dataset can also be studied in relation to assessing possibilities of diffraction imaging (Schmelzbach
et al., 2008).
4. Budget
Funding is sought to cover travel costs associated with the project. These fall into two categories:
1. Attendance at meetings of the project partners to discuss and present results in France. There will
be 2 such meetings, at 6 monthly intervals:
Zurich-Orléans train tickets
2 x 300 Euros = 600 Euro
Hotel accommodation and meal expenses: 2 x 200 Euros = 400 Euros
2. Presentation of results at a major international conference (e.g. SEG 2015 or EAGE 2016):
Air fare, hotel and living expenses, conference registration at student rate: Total 2000 Euros
Basic maintenance costs and consumables for computing and drafting will be met by ETH Zürich.
Participation of BRGM staff, travel costs will be covered by BRGM funding.
Budget requested: 3000 euros.
5. Bibliography
Bohlen, T. (2002). Parallel 3-D viscoelastic finite-difference seismic modelling. Comput. And Geosci.,
28(8), 887–899.
Greenhalgh, S. A., Mason, I. M., Lucas, E., Pant, D., & Eames, R. T. (1990). Controlled direction
reception filtering of P-and S-waves in τ-p space. Geophysical Journal International, 100(2), 221-234.
Sausse, J., Dezayes, C., Dorbath, L., Genter, A. & Place, J. (2010). 3D model of fracture zones at Soultzsous-Forets based on geological data, image logs, induced microseismicity and vertical seismic
profiles, Comptes Ren- dus Geoscience, 342(7–8), 531–545.
Schmelzbach, C., Simancas, J. F., Juhlin, C., & Carbonell, R. (2008). Seismic reflection imaging over the
South Portuguese Zone fold‐and‐thrust belt, SW Iberia. Journal of Geophysical Research: Solid Earth
(1978–2012), 113(B8).
Sollberger, D. (2013). Multicomponent Seismic Processing for Coherent Noise Suppression and
Arrival Identification. (Master's thesis, ETH Zürich).