gas hydrate - CCOP

Estimation of Q-Factors from VSP Data
in Gas Hydrate-Bearing Zone
Joong-Moo Byun 1) , Dong-Geun Yoo 2) , and Ho-Young Lee 2)
1) Hanyang University, Seoul, Korea
2) Korea Institute of Geoscience and Mineral Resources, Daejon, Korea
ABSTRACT: The Q-factor that quantifies the attenuation, the intrinsic property of
the material, is a very important factor required in extracting useful attributes from
seismic data. The spectral ratio method has been widely used in computing the
frequency-independent Q’s from the zero-offset VSP data because of its ease and
fastness. The algorithms of the spectral ratio and the modified spectral ratio method
have been developed, which provides the frequency-dependent Q’s, and applied to
synthetic zero-offset VSP data sets and the zero-offset VSP data acquired at Mallik
3L-38 gas hydrate research well. The Q-factors calculated from the synthetic data
by the spectral ratio method approach more closely the true values for the medium
with low Q-factor than the medium with high Q-factor. The changes in the Q-factors
extracted from the Mallik zero-offset data by the spectral ratio method agree well
with the boundaries of the layers, including the gas hydrate zone, depicted in the
reflection image. The results of applying the modified spectral ratio method show
very unstable Q-factors for some layers in the synthetic data and the Mallik data.
Further analysis and research are required to obtain more precise Q-factors by the
modified spectral ratio method.
Keywords: Q-factor, spectral ratio method, modified spectral ratio method, gas
hydrate zone, zero-offset VSP.
INTRODUCTION
Besides the attenuation by geometric spreading, an additional amount of intrinsic attenuation
occurs in amplitude of the seismic energy propagating outward, due to dissipation
mechanisms whereby elastic wave energy converts to heat. It has been found by experiment
that these losses often are nearly proportional to frequency. Generally, the absorption property
of the medium is characterized as the Q-factor of the medium, and the medium with a lower
Q-factor yields more absorption.
Attenuation has a considerable effect on the amplitude and wavelet shape of recorded seismic
data. Thus AVO analysis becomes much more complicated where attenuation effects are
superimposed on the AVO signature. Moreover, Q is a very important required factor in
extracting attributes such as lithological information, porosity, permeability, viscosity, and the
degree of the saturation, because Q is more sensitive to some of these parameters than the
velocity (Jeng et al., 1999).
New Energy Resources in the CCOP Region - Gas Hydrates and Coalbed Methane 35