FD-Modellierung und Analyse der seismischen Wellenausbreitung am Vøring Plateau im Kontext der CO$_2$-Sequestrierung in Offshore-Basaltformationen

In the present bachelor’s thesis, a seismic survey at the Vøring Plateau is simulated and subsequently
analyzed. For this purpose, the viscoelastic wave propagation is modeled using the SOFI2D software
based on the finite-difference method. The subsurface model underlying the modeling is created
from borehole data of the ODP Hole 642.
This work is carried out in cooperation with the Geomar Helmholtz Centre for Ocean Research
Kiel (GEOMAR) within the PERBAS research project (Permanent sequestration of gigatons of
CO2 in continental margin basalt deposits). The project aims at developing CO2 storage in offshore
basalt formations as a future CCS technology. Such basalt formations are abundantly present at
the Vøring Plateau. More specifically, a nearly 800 m thick basalt series consisting of 120 individual
basaltic lava flows interbedded by sedimentary layers, drilled at ODP Hole 642, is at the core of this
study. The evaluation of the modeling results and, in particular, the elaboration of the characteristic
wave field properties will be used by GEOMAR to optimize the acquisition geometry of a marine
3D seismic survey at the Vøring Plateau.
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First, a one-dimensional subsurface model of the physical parameters wave velocities vp and vs,
bulk density ρ, and intrinsic seismic attenuation Q is created from the data of ODP Hole 642.
In the modeling with SOFI2D, solving the two-dimensional viscoelastic wave equation, synthetic
seismograms, simulating the use of ocean-bottom seismometers, are generated. These are then
analyzed in detail using the basic seismic reflection and refraction method. Visual representations
of the calculated wave field propagation serve as a guiding tool. A detailed characterization of the
properties of the wave field is performed.
The basalt series causes both a reflected and guided wave field in the modeling. The former generates
stronger signal amplitudes (about an order of magnitude stronger for comparable ray paths) and
allows for the imaging of deeper structures throughout the basalt series, while the observed head
waves are generated exclusively within the upper 250 m. The highly variable velocity and density
structure of the basalt series create a complex wave field characterized by superimpositions. This
limits the horizontal distance over which coherent reflective signals form, to 500 m or at most 1000 m
for all deeper structures. The conversion between P- and S-waves also plays a role, resulting in
different types of signals which are classified in the study. Multiple reflections are observed, with
the strongest ones originating from the sea surface causing a quasi-periodic repetition of the seismic
response of the subsurface. However, they do not limit the maximum depth that can be explored
with primary reflections for small offsets.
The findings of this study provide qualitative insights into the wave field characteristics, but due to
the limitations of the one-dimensional model, quantitative deviations are to be expected depending
on the specific location of GEOMAR’s planned seismic survey. This could be addressed by a
continuation of this work using a two-dimensional subsurface model.