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Quantifying the Seismic Response of Underground Structures via Seismic Full Waveform Inversion: Experiences from Case Studies and Synthetic Benchmarks
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Seismic full waveform inversion (waveform tomography) is a method to reconstruct the underground velocity field in high resolution using seismic data. The method was first introduced during the 1980’s and became computationally feasible during the late 1990’s when the method was implemented in the frequency domain. This work presents three case studies and one synthetic benchmark of full waveform inversion applications. Two of the case studies are focused on time-lapse cross-well and 2D reflection seismic data sets acquired at the Ketzin CO2 geological storage site. These studies are parts of the CO2SINK and CO2MAN projects. The results show that waveform tomography is more effective than traveltime tomography for the CO2 injection monitoring at the Ketzin site for the cross-well geometry. For the surface data sets we find it is difficult to recover the true value of the velocity anomaly due to the injection using the waveform inversion method, but it is possible to qualitatively locate the distribution of the injected CO2. The results agree well with expectations based upon conventional 2D CDP processing methods and more extensive 3D CDP processing methods in the area. A further investigation was done to study the feasibility and efficiency of seismic full waveform inversion for time-lapse monitoring of onshore CO2 geological storage sites using a reflection seismic geometry with synthetic data sets. The results show that waveform inversion may be a good complement to standard CDP processing when monitoring CO2 injection. The choice of method and strategy for waveform inversion is quite dependent on the goals of the time-lapse monitoring of the CO2 injection. The last case study is an application of the full waveform inversion method to two crooked profiles at the Forsmark site in eastern central Sweden. The main goal of this study was to help determine if the observed reflections are mainly due to fluid filled fracture zones or mafic sills. One main difficulty here is that the profiles have a crooked line geometry which corresponds to 3D seismic geometry, but a 2D based inversion method is being used. This is partly handled by a 3D to 2D coordinate projection method from traveltime inversion. The results show that these reflections are primarily due to zones of lower velocity, consistent with them being generated at water filled fracture zones.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2013. , 62 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1005
Keyword [en]
full waveform inversion, waveform tomography, CO2 monitoring, CO2 sequestration, time lapse seismic, Inversion
National Category
Earth and Related Environmental Sciences Geophysics
Research subject
Geophysics with specialization in Solid Earth Physics
Identifiers
URN: urn:nbn:se:uu:diva-187142ISBN: 978-91-554-8562-7 (print)OAI: oai:DiVA.org:uu-187142DiVA: diva2:575076
Public defence
2013-02-01, Hambergsalen, Geocentrum, Villavagen 16, Uppsala, 10:00 (English)
Opponent
Supervisors
Available from: 2013-01-10 Created: 2012-12-03 Last updated: 2013-02-11Bibliographically approved
List of papers
1. Cross-well seismic waveform tomography for monitoring CO2 injection: a case study from the Ketzin Site, Germany
Open this publication in new window or tab >>Cross-well seismic waveform tomography for monitoring CO2 injection: a case study from the Ketzin Site, Germany
Show others...
2012 (English)In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 189, no 1, 629-646 p.Article in journal (Refereed) Published
Abstract [en]

Geological storage of CO2 is one means of mitigating the effects of continued burning of fossil fuels for power generation. An important component in the storage concept is the monitoring of the CO2 distribution at depth. Seismic methods can play a significant role in this monitoring, in particular cross-well methods are of interest due to their high resolution. For these purposes, a series of cross-well seismic surveys were acquired within the framework of the CO2SINK project at Ketzin, Germany, at various stages of an injection test. We study here the potential of applying cross-well seismic waveform tomography to monitor the CO2 injection process. First, we test the method on synthetic data having a similar geometry to that of the real data. After successful application on the synthetic data, we test the method on the real data acquired at the Ketzin Site. Traveltime tomography images of the real data show no observable differences between the surveys. However, seismic waveform tomography difference images show significant differences. A number of these differences are artefacts that can probably be attributed to inconsistent receiver coupling between the different surveys. However, near the injection horizon, below the caprock, a velocity decrease is present that is consistent with that expected from the injection process.

Keyword
Inverse theory, Downhole methods, Controlled source seismology, Seismic tomography, Computational seismology
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-173647 (URN)10.1111/j.1365-246X.2012.05375.x (DOI)000301573800045 ()
Available from: 2012-05-03 Created: 2012-05-02 Last updated: 2017-12-07Bibliographically approved
2. Application of seismic waveform tomography to monitoring of CO2 injection: modeling and a real data example from the Ketzin site, Germany
Open this publication in new window or tab >>Application of seismic waveform tomography to monitoring of CO2 injection: modeling and a real data example from the Ketzin site, Germany
2013 (English)In: Geophysical Prospecting, ISSN 0016-8025, E-ISSN 1365-2478, Vol. 61, no Suppl.s1, 284-299 p.Article in journal (Refereed) Published
Abstract [en]

Seismic monitoring of the injected carbon dioxide (CO2) distribution at depth is an important issue in the geological storage of CO2. To help monitor changes in the subsurface during CO2 injection a series of 2D seismic surveys were acquired within the framework of the CO2SINK and CO2MAN projects at Ketzin, Germany at different stages of the injection process. Here we investigate using seismic waveform tomography as a qualitative tool for time-lapse seismic monitoring given the constraints of the limited maximum offsets of the 2D seismic data. Prior to applying the inversion to the real data we first made a number of benchmark tests on synthetic data using a similar geometry as in the real data. Results from the synthetic benchmark tests show that it is difficult to recover the true value of the velocity anomaly due to the injection, but that it is possible to qualitatively locate the distribution of the injected CO2. After the synthetic studies, we applied seismic waveform tomography on the real time-lapse data from the Ketzin site along with conventional time-lapse processing. Both methods show a similar qualitative distribution of the injected CO2 and agree well with expectations based upon more extensive 3D time-lapse monitoring in the area.

Place, publisher, year, edition, pages
Wiley-Blackwell, 2013
Keyword
CO2 monitoring, CO2 sequestration, time lapse seismic, seismic waveform inversion
National Category
Earth and Related Environmental Sciences Geophysics
Identifiers
urn:nbn:se:uu:diva-187092 (URN)10.1111/1365-2478.12021 (DOI)000320136500019 ()
Available from: 2012-12-03 Created: 2012-12-03 Last updated: 2017-12-07Bibliographically approved
3. A feasibility and efficiency study of seismic waveform inversion for time-lapse monitoring of onshore CO2 geological storage sites using reflection seismic geometry
Open this publication in new window or tab >>A feasibility and efficiency study of seismic waveform inversion for time-lapse monitoring of onshore CO2 geological storage sites using reflection seismic geometry
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The monitoring of the CO2 distribution at depth is very important for onshore geological storage of CO2. Seismic methods are effective monitoring tools during and after the injection process, but are generally expensive and time consuming to perform. In this paper we perform a series of synthetic experiments in order to compare the seismic waveform inversion method with conventional seismic monitoring methods for time-lapse monitoring at CO2 geological storage sites. We mainly focus our study on seismic data sets with typical reflection acquisition geometries, that is far offset data is limited. Three synthetic seismic data sets (consisting of one baseline and two repeats) were generated by a third party 2D forward modeling code to compare the waveform inversion method with the conventional method. The area into which CO2 is injected is on the order of a few hundred meters wide and a few 10s of meters high and can be considered as a leak. We compare the waveform inversion results and conventional time-lapse results to determine how sparse spatial sub-sampling affects the results by successively increasing both the shot and receiver spacing. Aside from testing the influence of the shot and receiver spacing on the results, we also use a coarse starting model and increase the starting frequency to make the test more realistic. Our results show that, under certain conditions concerning noise, it may be possible to use a combination of sparse spatial sampling geometries and seismic waveform inversion to monitor CO2 injection sites. Regardless, seismic waveform inversion may be a good complement to standard CDP processing when monitoring CO2 injection.

Keyword
CO2 monitoring, CO2 sequestration, time lapse, seismic waveform inversion
National Category
Earth and Related Environmental Sciences Geophysics
Identifiers
urn:nbn:se:uu:diva-187138 (URN)
Available from: 2012-12-03 Created: 2012-12-03 Last updated: 2013-02-11
4. Fracture zones or mafic sills?: Seismic waveform tomography at the Forsmark spent nuclear fuel repository, eastern central Sweden
Open this publication in new window or tab >>Fracture zones or mafic sills?: Seismic waveform tomography at the Forsmark spent nuclear fuel repository, eastern central Sweden
2012 (English)In: Society of Exploration Geophysicists. Expanded Abstracts with Biographies, ISSN 1052-3812, 1-5 p.Article in journal (Refereed) Published
Abstract [en]

The Swedish Nuclear Fuel and Waste Management Company (SKB) has been carrying out extensive studies at the Forsmark nuclear waste repository site in the eastern part of central Sweden since 2002. An important component of the studies has been reflection seismic surveys which are useful for locating and characterizing sites for storage of spent nuclear fuel in the crystalline bedrock. Identification of sub-horizontal to gently dipping seismic reflections is especially important since these may represent transport routes for radionuclides. Studies have shown that such reflections can be generated by water filled fracture zones that have a lower velocity than the surrounding bedrock. However, lithological changes, I.e. mafic sills, may also be responsible for reflections in some cases. At the Forsmark site, it is difficult to distinguish fracture zones from mafic sills in the standard reflection seismic processed sections. However, since mafic sills usually have a positive velocity contrast with the background velocity field while fractures zones have a negative one, the two possibilities could be differentiated if we could reconstruct the underground velocity field. Waveform tomography has the potential to perform this reconstruction, and allowing us to discriminate between fractures zones and mafic sills. In this study we test this potential of seismic waveform tomography and conclude that the reflections investigated most likely are generated at low velocity fracture zones.

Keyword
Inverse theory, Tomography, Seismic tomography, Controlled source seismology, Fractures and faults
National Category
Earth and Related Environmental Sciences Geophysics
Identifiers
urn:nbn:se:uu:diva-187132 (URN)10.1190/segam2012-0697.1 (DOI)
Available from: 2012-12-03 Created: 2012-12-03 Last updated: 2017-12-07Bibliographically approved

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