New
at IDA
IDA
recently redesigned the project's
Web site to be more informative and responsive to user's needs.
The site is divided into the following sections:
- About
IDA contains a description of the project's purpose, history
and relation to the Green's who have provided substantial financial
support throughout the 26 years of Project IDA's existence.
- IDA
Network contains a network map, alphabetical list of station
links, and a description of the Data Collection Center's work.
- Stations
provides direct access to the station list. Through the links
on these pages, users gain access to a physical inventory of
what equipment is at a site, schematics of the station layout,
photographs, contact information, and a list of what data has
been processed by the IDA DCC.
- Telemetry
contains a description of the means IDA uses to telemeter data
from GSN stations back to the user community as well as links
to a map of telemetry topology and more detailed descriptions
of the NRTS software and the AutoDRM.
- What's
New currently has descriptions of stations under development,
new hardware, and topics of immediate interest such as a description
of the filtering problem discovered on the IDA MK7 data logger.
- Reports
contains links to equipment technical reports as well as to
quarterly reports of the past five years.
- Contact
Information lists whom to contact for more information.
We
are quite excited about this site and hope users will take the
opportunity to explore the extensive information available.
Pete
Davis, IDA Project, University of California at San Diego
New
at Harvard University Waveform Quality Center
WQC
Noise Analysis Identifies Flaw in GSN Datalogger
The main task of the Harvard Waveform Quality Center (WQC) is
to identify and report on problems with data from GSN stations.
We often discover such problems when data from a particular station
are unusable in the routine analysis of global seismicity (the
Harvard
CMT Project). The types of problems that we can identify this
way (through comparison with synthetic waveforms) are typically
related to malfunctioning seismometers or erroneous system response
information, such as incorrectly specified gain or seismometer
orientation.
During 2001 the WQC has also been developing methods to identify
less obvious data problems, such as deteriorations over time in
the quality of data from a given station. We were particularly
interested in investigating the long-period noise levels at GSN
stations over the last 10 years, since some of us at the Harvard
WQC had the impression that background noise levels had increased,
especially at some of the historically quiet GSN sites. An increase
in background noise at previously quiet stations directly affects
our ability to successfully analyze smaller earthquakes, which
are typically well recorded only at a small number of stations.
We are currently at the tail end of a systematic analysis of all
long-period data collected by the GSN since 1990. We process all
LH- and VH-channel seismograms to determine monthly estimates
of background seismic noise, here defined as the rms signal level
not exceeded 10% of the time. The resulting monthly estimates
at some particular period are then plotted as a function of time
to identify significant changes in the background noise levels;
any sudden change is likely to indicate an instrument problem.
The details of the analysis are described on our web
site.
For many stations, the background noise levels are remarkably
constant; one example is the Chinese Network station WMQ.
A seasonal component is clearly visible for many stations that
resolve ground noise, reflecting the variations in meteorological
or surf conditions, as at KIP.
At some stations we discovered large and abrupt temporal changes
in the background noise, in particular at some traditionally quiet
IRIS/IDA sites such as ERM,
ESK,
NNA,
and SUR.
In the mid-1990s, the background noise level at 100 seconds period
increases abruptly by more than 15 db for these stations. At shorter
periods (around 30 seconds), the increase in background noise
is even greater.
After further analysis, and a confirmation of our results by Bob
Uhrhammer at Berkeley, we also discovered that the abrupt deterioration
of long-period data, which is clearly seen for several more IRIS/IDA
stations, correlates with the time of upgrade to the IDA Mark
7 datalogger. We notified the IDA group, who, following their
own investigation, confirmed that the increase in long-period
noise is caused by a design error in the digital signal processing
(DSP) unit of the Mark 7 datalogger. A report
that describes the problem and possible remedies was subsequently
prepared. Some of the long-period signal lost in the DSP can be
recovered by a reprocessing of the broad-band (BH) data stream.
An effort to reprocess all of the affected IRIS/IDA data is underway,
as is a program to repair the Mark 7 dataloggers in the field.
The WQC has prepared a web
site with the long-period background-noise histories for all
GSN stations, and we are planning to develop this site further.
Though the Mark 7 problem was the first "signal" that caught our
attention in this analysis, it is clear that there are several
other temporal changes in noise that await an explanation.
Gšran
Ekstršm, Department of Earth and Planetary Science, Harvard
University
