## Velocity estimation from seismic data by nonlinear inversion and characterization of gas hydrate deposits offshore Oregon

##### Abstract

Seismic attributes such as traveltimes and reflection amplitude variation with
offset contain information on the elastic parameters of subsurface rocks. The aim of
generalized inversion of seismic data is to estimate values of the elastic parameters such
as P-wave velocity, S-wave velocity and density for lithology discrimination and direct
detection of hydrocarbons. My dissertation research comprises two parts: development of
a method to improve the least-squares and the preconditioned conjugate gradient
algorithm, and estimation of detailed velocity structure of gas hydrate-bearing sediments
offshore Oregon from Ocean-bottom seismometers (OBS) and multi-channel streamer
(MCS) data.
I developed a new nonlinear inversion algorithm for estimating velocities from
fully stacked reflection data with application to a field data set consisting of well logs
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from Ocean Drilling Program (ODP) Leg 170 and multi-channel seismic reflection
(MCS) data offshore Costa Rica. Inversion of post-stack seismic data generally yields
reflection coefficients or impedance as a function of two-way reflection time. In this
experiment, fully stacked seismic data and density logs at selected locations along a 2-D
seismic line are inverted to estimate seismic velocities. Mathematically, generalized
inversion provides the best estimate of earth model parameters by minimizing the socalled
cost (or misfit between observed and computed seismic data) function, which is a
function of the data covariance matrix CD and the a priori model covariance matrix CM.
Matrices CD and CM (generally approximated by scalars σd and σm) introduce stability to
the process and robustness and thus have strong influence on the quality of the final
inversion solution. Based on the least-squares and the preconditioned conjugate gradient
algorithm, I have developed a 2-step procedure to solve this nonlinear inverse problem by
first determining the two matrices CD and CM using the two-step procedure that involves
mapping the sensitivity of model smoothness and data error to the parameters σd and σm
.I found that there always exits an area in the σdσm plane in which the low values of the
cost function lie, and hence a large 2-dimensional search space can be reduced to a
significantly smaller search region. This led to the easy application of this method.
The results from this experiment show that almost every identified reflector of
seismic data is very well matched by final synthetic seismograms and the density from
borehole log data, which confirms that my estimates of velocities are reliable.
Combination of the inverted velocity and density profiles allows identification of major
stratigraphic boundaries.
The improved inversion method is extended to the inversion of pre-stack seismic
data, which is applied to estimate seismic velocities of gas hydrate-bearing sediments,
offshore Oregon. Gas hydrates are recognized as a target for major future energy
reserves, are believed to be a potential source of an important greenhouse gas, and are
considered to be a possible cause of submarine geo-hazard. A simple indicator of gas
hydrate is a bottom-simulating reflector (BSR), which marks the transition between
hydrate-bearing sediments with high Vp above free gas with low Vp. A 3-D streamer and
ocean bottom seismometer (OBS) survey in the Hydrate Ridge, offshore Oregon was
conducted to image structures controlling the migration of methane-rich fluid and free
gas and to map the gas-hydrate distribution. Preliminary Vp and Vs profiles obtained from
OBS data by interactive analysis are used as a starting model to estimate Vp from the
streamer data.
The results of my inversion and interpretation study in Hydrate Ridge are
summarized below:
Both 3-D streamer and OBS data show a strong BSR indicating the presence
of gas hydrate above and free gas below.
Interactive P- and S-wave velocity analysis of OBS data allows us to identify
the presence of a “conversion surface” in the gas hydrate-bearing sediments.
The conversion surface separates the overlying low P-wave velocity layer and
underlying high P-wave velocity layer.
Inverted velocity profiles show a low-velocity layer existing below the sea
floor and above the normal gas hydrate, suggesting a new geological model of
gas hydrates.
Two types of hydrate fabrics, massive and porous hydrates, observed by deeptowed
video survey, were identified in the P-wave velocity profiles. Three
main layers of gas hydrate-sediments separated by the conversion surface and
BSR are distinguished. Below the free gas is the normal sedimentary section.
The profiles reflecting the physical properties of sediments, such as the Pwave
velocity, acoustic impedance and Poisson’s ratio profiles, are able to
map the distribution of gas hydrates and show very similar trends of lateral
variation of the main layers.
A series of faults in the accretionary complex under the ridge not only offer
pathways for methane and fluid ascending from deeper layers but also control
the distribution of the porous hydrates with low velocity below the seafloor.
Hornbach et al. (2003) suggest their results using velocity analysis of seismic
reflection data on the Blake Ridge is the first direct seismic detection of
concentrated hydrate confirmed by velocity analysis. My results of direct
inversion of seismic data extend these results to greater resolution of the entire
seismic data set. Further, my results may be the first seismic indication of a
visually observed porous hydrate zone.

##### Department

##### Description

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