We present the results of the modeling of a 3-D differential attenuation
structure (Q-1) beneath the Northwest Geysers geothermal field. A
set of 480 high-quality microearthquakes distributed evenly throughout
the field and with a minimum of 10 P-wave arrivals was selected for
the study. We constructed spectral ratios by dividing each spectrum
with a reference spectrum from each observing station. The reference
spectrum was derived from the average spectrum of all events that
were recorded at that station to correct for the strong site dependence
of the observed spectra. We then estimated the differential attenuation
operator from the slopes of the spectral ratios. The velocity models
and the raypaths for all events are known from a previous velocity
inversion study. The inversion for the differential attenuation structure
was carried out using a modified progressive inversion method. The
observed Q-1 structure correlates well with mapped geologic units.
High Q-1 and lower velocities correlate with Franciscan melange,
while lower attenuation and higher velocities correspond to metagraywacke
units. High P-wave Q-1 also underlies the southern region between
2 and 3 km depth where low Vp/Vs values suggest undersaturation of
the reservoir rocks. Most of the steam entries also occur within
this region and probably delineate the steam reservoir. These anomalies
may be explained by high rock temperatures and the presence of steam
and other gases.
%0 Journal Article
%1 romero_etal:1997
%A Romero, Arturo E.
%A McEvilly, Thomas V.
%A Majer, Ernest L.
%D 1997
%I SEG
%J Geophysics
%K geophysics seismology
%N 1
%P 149--167
%R 10.1190/1.1444114
%T 3-D microearthquake attenuation tomography at the Northwest Geysers
geothermal region, California
%U http://dx.doi.org/10.1190/1.1444114
%V 62
%X We present the results of the modeling of a 3-D differential attenuation
structure (Q-1) beneath the Northwest Geysers geothermal field. A
set of 480 high-quality microearthquakes distributed evenly throughout
the field and with a minimum of 10 P-wave arrivals was selected for
the study. We constructed spectral ratios by dividing each spectrum
with a reference spectrum from each observing station. The reference
spectrum was derived from the average spectrum of all events that
were recorded at that station to correct for the strong site dependence
of the observed spectra. We then estimated the differential attenuation
operator from the slopes of the spectral ratios. The velocity models
and the raypaths for all events are known from a previous velocity
inversion study. The inversion for the differential attenuation structure
was carried out using a modified progressive inversion method. The
observed Q-1 structure correlates well with mapped geologic units.
High Q-1 and lower velocities correlate with Franciscan melange,
while lower attenuation and higher velocities correspond to metagraywacke
units. High P-wave Q-1 also underlies the southern region between
2 and 3 km depth where low Vp/Vs values suggest undersaturation of
the reservoir rocks. Most of the steam entries also occur within
this region and probably delineate the steam reservoir. These anomalies
may be explained by high rock temperatures and the presence of steam
and other gases.
@article{romero_etal:1997,
abstract = {We present the results of the modeling of a 3-D differential attenuation
structure (Q-1) beneath the Northwest Geysers geothermal field. A
set of 480 high-quality microearthquakes distributed evenly throughout
the field and with a minimum of 10 P-wave arrivals was selected for
the study. We constructed spectral ratios by dividing each spectrum
with a reference spectrum from each observing station. The reference
spectrum was derived from the average spectrum of all events that
were recorded at that station to correct for the strong site dependence
of the observed spectra. We then estimated the differential attenuation
operator from the slopes of the spectral ratios. The velocity models
and the raypaths for all events are known from a previous velocity
inversion study. The inversion for the differential attenuation structure
was carried out using a modified progressive inversion method. The
observed Q-1 structure correlates well with mapped geologic units.
High Q-1 and lower velocities correlate with Franciscan melange,
while lower attenuation and higher velocities correspond to metagraywacke
units. High P-wave Q-1 also underlies the southern region between
2 and 3 km depth where low Vp/Vs values suggest undersaturation of
the reservoir rocks. Most of the steam entries also occur within
this region and probably delineate the steam reservoir. These anomalies
may be explained by high rock temperatures and the presence of steam
and other gases.},
added-at = {2012-09-01T13:08:21.000+0200},
author = {Romero, Arturo E. and McEvilly, Thomas V. and Majer, Ernest L.},
biburl = {https://www.bibsonomy.org/bibtex/2ebeb649bcc212b2eba19af9e018c10b2/nilsma},
day = 1,
doi = {10.1190/1.1444114},
interhash = {87c14cd3bf2beca54b3bd1472861ece8},
intrahash = {ebeb649bcc212b2eba19af9e018c10b2},
journal = {Geophysics},
keywords = {geophysics seismology},
month = jan,
number = 1,
pages = {149--167},
publisher = {SEG},
timestamp = {2021-02-09T13:27:05.000+0100},
title = {3-D microearthquake attenuation tomography at the Northwest Geysers
geothermal region, California},
url = {http://dx.doi.org/10.1190/1.1444114},
volume = 62,
year = 1997
}