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OCR for page 23
ISMS DATA CHARACTERISTICS
This chapter addresses the first charge to the panel, involving the data
characteristics for ISMS monitoring stations as proposed by the Group of Scientific
Experts (GSE). After providing introductory background, the chapter presents
comments and recommendations on the technical requirements of ISMS stations and
follows that with a discussion of the data streams.
The ISMS data will be an important contribution to the total seismic data needed
by diverse parts of the research community. Conversely, much data from current
networks can provide essential input to the monitoring effort. It is important, therefore,
that the data characteristics of the new ISMS stations be as compatible with the broad
needs of seismology in general as is consistent with the objectives of CTBT monitoring.
Furthermore, timely general access to the data streams from broadband
three-component instruments at the primary and auxiliary stations of the ISMS will
allow the best utilization of the data in earthquake monitoring, research, and treaty
monitoring activities to benefit the country. Specific suggestions about ensuring rapid
and full access will be discussed in Chapter 4.
Introduction and Background
The panel was asked to address the following charge:
Data Characteristics. The Group of Scientific Experts (GSE) has written draft
requirements for an IS\IS-standard station that specify characteristics such as sample
rate, passband, dynamic range, and sensitivity. They have also proposed a primary
network configuration and rough requirements for signal detection, parameter
extraction, and event location. What types of data (raw and/or processed) are sought by
the seismological community for use in test-ban monitoring research and in other types
of basic research?
23
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24
Comprehensive Test Ban Monitoring System
The draft requirements for an ISMS-standard three-component station were
extracted from the Progress Report to the Conference on Disarmament CD/121 ~ and
were circulated to the panel with a request for broad technical feedback. Our comments
in this chapter are confined to the specifics of instrumentation in the context of multiuse
potential of the seismic data streams. We do not address the CD strategy of a tiered
seismic network of primary and auxiliary stations, but we do consider the broad
implications ofthe instrument specifications that have been proposed for ISMS standard
stations. It is assumed that these technical specifications apply to the primary stations
and that the auxiliary station requirements may not be exactly the same. We will note
some distinctions in design requirements for broadband stations versus short-period
array components, given the different opportunities for noise-suppression processing.
The issue of what types of data are sought by the seismological community is
complex because many components of this community have distinct data requirements,
and no single network can service all functions. As a consequence, an extensive and
multifaceted national and international infrastructure exists for collecting local,
regional, and global seismic data with varying technical specifications (see Chapter 21.
This infrastructure supports research and monitoring functions associated with
earthquake hazard mitigation, earthquake engineering, fundamental earthquake
investigations, local and global earth structure investigations, and earthquake and
tsunami warning systems, as well as nuclear test monitoring systems. Many federal,
state, university, private, and international organizations are involved. Although each
seismological application has its own special data requirements, there are significant
intersections in requirements, and the U.S. and international seismic communities have
extensive multiagency, multiuniversity, bilateral, and multinational agreements in place
to take advar~tage of the entire data acquisition effort. The panel's perspective, therefore
is that it is most efficient and cost effective to optimize instrument and data compatibili-
ty to the extent possible to enable multiple uses of the seismic data.
The ISMS operation will provide both improved real-time access to some
existing international seismic data and access to totally new seismic data, which can
potentially complement the existing data used in diverse seismic research and
monitoring activities. From the research community's point of view, it is critical that
international planning for ISMS stations be well coordinated with the existing
seismological infrastructure servicing other areas of national need. Feedback to the
panel indicates that the broad seismological community agrees that ISMS primary
station data can contribute to many of these areas outside the treaty monitoring arena.
as long as suitable access is provided.
The panel's response to the first charge addresses proposed attributes of the
technical instrumentation of TSMS standard stations, indicating ways in which minor
modifications to the technical requirements will optimize multiple-use applications of
OCR for page 25
ISMS DATA CHARACTERISTICS
25
the seismic data streams. Then, we address the prioritization of the data generated by
the ISMS system for both research and earthquake monitoring applications.
Discussion of TSMS Station Technical Requirements
TSMS instrument specifications emphasize the short-period end of the
seismological spectrum, essential for recording small events, whereas the research
community emphasizes recording a very broad spectrum of seismic signals with a
dynamic range sufficient to resolve ground noise and to record, on scale, the largest
signals. The pane} has not addressed the seismological objectives that drive the
technical requirements for the ISMS stations, but they generally appear to be consistent
with the needs of a treaty monitoring system. Fortunately, modern seismic
instrumentation has expanded the bandwidth and dynamic range of commonly available
sensors. ISMS technical specifications for the three-component broadband sensors to
be deployed at each primary station are fairly close to those of the instrumentation
deployed by the U.S. Geological Survey (USGS), Incorporated Research Institutions for
Seismology (IRIS), and other members of the Federation of Digital Seismographic
Networks (FDSN) for a broad range of seismological applications.
The pane! strongly endorses the planned inclusion of at least one
three-component broadband set of instruments in a low-noise environment at each
primary station. The resulting continuous three-component broadband data from the
TSMS would have maximum impact in basic research investigations. The broadband
channels, with the specified passband, can readily be incorporated into the USGS's
earthquake monitoring and analysis procedures, extending the data available for use in
near real-time. However, the "Station Requirements for an ISMS Standard Station,"
listed in Table ~ of CD/121 I, are somewhat restrictive and limit broader applications
of the data stream. Below, we discuss the relationship of the technical requirements
listed in CD/121 ~ and indicate their relationship to the types of seismic data sought by
the seismological corrunuruty. We note that the GSE has actively been considering some
of the recommendations made below (based on the panel's first preliminary report), and
new station requirements are proposed in more recent CD working documents, some of
which accommodate our suggested charges.
a. Passband 60. 02 to 20 Hid. The low-frequency cutoff of this passband wall
preclude recording of very-Iong-period surface waves and free oscillations, which are
of extensive use in basic research on earthquake sources and earth structure. It is now
technologically straightforward to extend the low-frequency response to 0.003 Hz (the
lowest frequency of seismological interest) without significant impact on the cost of the
OCR for page 26
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Comprehensive Test Ban Monitoring System
broadband instrumentation. This change would replicate the Tow-frequency response
capabilities of many ofthe IRIS, USGS, and FDSN stations. Since many ofthe stations
initially designated as primary or auxiliary stations for the GSETT-3 already have very
broadband sensors (STS-! or KS54000) that intrinsically achieve a low-frequency
response, down to 0.003 Hz, it is straightforward to broaden the specified bandwidth,
but it is not possible at present to meet the full desired bandwidth using a single instru-
ment. The pane} recognizes that it may be desirable to use only a single sensor when
possible. At the very least, instruments such as the STS-2 should be utilized to extend
the Tow frequency response to 0.01 Hz while still having adequate response in the
short-period range. ISMS installations with both short-period arrays and a single
broadband sensor should have some flexibility in the response criteria ofthe broadband
system to ensure that low-frequency response is not sacrificed unnecessarily. The panel
was very concerned to see that some of the updated working documents of the GSE
specified low-frequency response down to only 0.04 Hz. This would greatly diminish
the broader applications of the data and would even jeopardize the application of the
ISMS data for routine functions such as computation of Ms. the surface wave
magnitude, which is important for event discrimination.
The pane! recommends that the low-frequency end of the nassband o f ISMS
broadband systems extend to 0.003 Hz wherever possible.
~. - -
There are some current applications for the seismic energy above 5 Hz in the
earthquake monitoring and basic research communities, and the availability of globally
distributed high-firequency data from quiet sites offers new potential for research on
earthquakes and earth structure. Many current stations of the Uris, USGS, and FDSN
networks readily can be (or have been) modified to achieve the high-frequency
bandwidth of the ISMS specifications at relatively minor cost, and they will
complement the primary stations.
b. Seismometer Noise (10 dB Below Peterson's Low Earth Noise Mode!
([NATE. If this criterion is intended for the entire passband (0.02 to 20 Hz), it requires
new instrument development, as we know of no broadband seismometer with a noise
figure that is 10 dB below the ENM over the filet range. A combination of STS-! or
STS-2 and GS- ~ 3 instrumentation can achieve this requirement over the passband 0.0 ~
to 10 Hz, and a combined KS540001 and GS-13 can achieve this requirement over the
passband 0.03 to ~ O Hz. Certainly, this requirement, if achieved, is compatible with all
applications of the seismic data.
It would notseriously impact the research communi~'s current use of the data
if the requirement were relaxed in the 10 to 20 Hz range to allow use of existing
state-of-the-ar' equipment in fee ·SMS, and the pane! so recommends.
OCR for page 27
ISMS DATA CHARACTERISTICS
27
c Calibration (within S percent in amplitude and 5 degrees in phase) This
is satisfactory for most uses of the ISMS data. It is satisfied by current IRIS, USGS, and
FDSN stations.
No change is recommended.
d. Sample Rate 640 sps + 50 ms). This sample rate is suitable for most
multiuse applications of ISMS data, as is the lower sampling rate of 20 sps as proposed
in more recent GSE documents. The panel notes that in practice the 20-Hz
high-frequency response objective is incompatible with the specified sample rate (40
sps). It may be desirable to oversample and filter to remove 50 to 60 Hz noise, and then
resample to obtain the final desired high-frequency response. This could reduce the
spectral density of the noise, which would benefit all applications of the short-period
energy. Although the panel has not chosen to address the monitoring motivation behind
the specification, some members noted that the preferred sampling rate is too low for
research on and application of spectral characteristics to assist in discriminating
between mining blasts and single explosions. Sampling intervals as much as four times
smaller than the time between individual blasts in the mining explosions are needed,
indicating sampling at up to ~ 00 sps. It is not currently realistic to achieve this sampling
rate for continuous ISMS data at all stations; however, it may be desirable for certain
stations, especially near mining areas. This high-sample-rate data could be saved on-site
and accessed on demand.
The pane! recommends that the sample rate be reevaluated
e. Resolution (~S dB below Peterson's [N~ This requirement is driven
by the desire to resolve very low amplitude signals that can be enhanced via stacking
array elements. While this resolution can be achieved at existing FDSN stations using
combined very broadband (VBB) and very short period (VSP) sensors, for isolated
three-component broadband stations this level of resolution significantly departs from
the needs of present seismological applications. The panel sees no advantage to
digitizing deeply into the noise for single broadband stations. Furthermore, there are
significant negative consequences. Large earthquakes at teleseismic distar~ces, and even
moderate ones at regional distances, produce signals that exceed the finite dynamic
range of a 24-bit system, leading to clipping if the system digitizes deeply into the
noise.
The pane! recommends that this requirement be relaxed for broadband
t/'ree-component stations The pane' also recommends that the resolution levetbe tied
to local noise. Since the [NM is rarely achieved, a more expeditious use of t/`e
dynamic bandwidth wouldF be to base t/'efloor of t/'e resolution on a site's actual
noise levels.
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Comprehensive Test Ban Monitoring System
Alternatively, the suggested CD noise resolution cart be achieved by the abolition
of triggered broadband strong-motion sensors at the broadband stations. This can be
implemented at low cost (as is a common practice at IRIS stations in seismically active
regions), and it will ensure that ISMS data are not depleted in the very signals of
greatest multipurpose use.
f. Sensitivity f200 counts/nm at 3 Hz). There is no known nuantizer that can
achieve the desired resolution over the entire passband with a sensitivity of 200
countsinm at 3 Hz. (The sensitivity would have to be set to 800 counts/nm to meet the
resolution requirement at 20 Hz.) Sensitivity is adjustable depending on site noise, but
for nuclear test discrimination at regional distances the critical passband appears to be
in the range of 5 to g Hz.
The pane! recommends specifying sensitivity goals at slightly Weigher
frequencies or over a range offrequencies.
g
System Noise (10 dB below Peterson's LNM). The seismometer noise and
sensitivity setting determines the ability to meet this requirement.
While arrays can take advantage of low system noise to beat down natural
background noise, this is not viable for individual fhree-component systems, so this
requirement could be relaxedfor the latter.
A more relevant reference point for system noise requirements is the local site
noise, not the ENM.
The pane! recommends that the system noise requirement should clearly
specify the frequency band of importance.
h. Dvnamic Range c~26 dB). The pane! interprets this design goal to apply
to the digitizer capability. Existing widely used 24-bit quantizers achieve this range, so
this requirement is compatible with multiuse applications of the data stream. However,
if dynamic range is defined as the range from the ENM to the clip level, for the desired
sensitivity (200 counts/nm at 3 Hz) this system will achieve no more than ~ ~ ~ dB at 20
Hz and 96 dB at ~ Hz. More recent GSE documents propose a more realistic 96 dB
requirement.
No specif c recommendation at this time.
1.
Linearity ego dB over the passband). This is fully compatible with
general seismic data requirements for diverse applications of the data.
No change recommended.
OCR for page 29
ISMS DATA CHARACTERISTICS
29
i. Jiming.4ccuracv Al msecJ. This is Filly compatible with general seismic
.
data requirements for diverse applications of the data.
No change recommended.
k. Operating Temperature (-10° to 45°C). A low-temperature requirement
is probably needed oniv for certain sites and can be attained for most broadband systems
with special insulating techniques and temperature controllers. Provided appropriate
power is available, the proper environmental control system can increase the range of
station locations, as is desirable for multiuse applications of the seismic data.
No specif c recommendation at this time.
Authentication Required. This is generally not needed for research
applications. To the extent consistent with the monitoring goals, measures that are
implemented should be such that they do not affect general use of the data stream. Some
authentication procedures could involve significant modifications of existing
instrumentation. The associated costs may limit the number of stations participating in
the ISMS, which has negative implications for system performance.
No specif c recommendation al this time.
m. State of Health (at [east cilock statue. calibration status. ancivaultstatusJ.
Such information is routine and desirable.
No change recommencied.
n. Format (one of the formats of the Group of Scientific Experts'. The broad
international seismological community has established a standardized digital seismic
data-exchange format Standard for the Exchange of Earthquake Data (SEED), which
is now widely used in the FDSN.
The pane! recommends that mechanism be established that would provide
·SMS data in SEED format in addition to other formats that might be used.
O. Protocol (Telecommunication Protocol/Information Protocol (TCP/IP).
This is compatible with other systems.
No change recommended.
p. Delav in Transmission (< 15 see). This is compatible with the needs of
other systems. For global tsunami warning and earthquake hazard assessment, access
within a few minutes is desirable. However, regional earthquake monitoring benefits
from delay times of no more than a few seconds.
No change recommended.
OCR for page 30
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Comprehensive Test Ban Monitoring System
q. Data Frame Length (<] sect. This is shorter than ir1 many existing
stations, and it is not clear that such frames are an advantage. It is not needed for other
uses of seismic data, which typically have data frame lengths of 2. ~ 5 to 8.6 sec for 20
sps and 1.075 to 4.3 see for 40 sps. Shorter frame lengths would negatively impact the
data compression schemes used in many existing stations. More recent GSE documents
have relaxed this requirement to < 60 sec.
The panel recommends reconsideration of data frame lengtI, requirement.
r. Data Access (Priority to International Data Center (ID C). then National
Data Center (NDCJ). This item pertains to priority for communication with the station,
not end-use distribution of the data. Provision of this access is technologically
straightforward and can be implemented on existing systems such as those of the FDSN.
it is very desirable that all data recorded at ISMS stations be made available promptly
to the general research community.
No specify c recommendation.
s. Disk Buffer (7 days). This is readily achieved with current technology.
No change recommended.
t.
Data Availabililv (>99 percent). This high percentage of reliability is
driven by the needs of the nuclear monitoring function. This requirement diverges in
practice from many other data acquisition systems because it is not cost effective. The
panel expects that a 99 percent data availability requirement will lead to high operation
and maintenance costs, thus limiting the funds available to support a large number of
stations in the monitoring system, particularly in the auxiliary network. Even the 95%
availability recommended for the auxiliary network in some recent GSE documents is
likely to prove ur~duly restrictive. For example, the overall average IRIS network data
availability is approximately 90 percent. The stated requirement could preclude use of
these high-quality stations as part of the ISMS unless new funds are provided for the
necessary level of maintenance. Academic and earthquake monitoring efforts typically
prefer data from relatively dense and widely distributed networks, tolerating delayed
access to some data and some gaps from individual stations, rather than the ISMS
concept of data from a sparse network of highly reliable stations in near real time. With
respect to auxiliary stations, tolerating reduced data availability, say at the 90 percent
level, from an enhanced number of stations would provide data that would still achieve
the overall desired availability and that would be of greater use to the seismological
research community because of the expanded coverage. For example, given that the
signal-to-noise ratio is such that two stations with 90% reliability have recordable
ground motion, the probability that at least one of them will actually record is 99%.
OCR for page 31
ISMS DATA CHARACTERISTICS
31
Verification researchers also would benefit from the denser coverage and improved
understanding of the regional geology and the wave propagation characteristics in
interpreting data from an extended seismic network.
The pane! recommends that separate and realistic data availability require-
ments be establishedfor the primary and auxiliary networks.
u. Timely Data Transmission (~98 percent). The availability of real-time
data will not only benefit ISMS operations but will also extend the real-time data
available to the USGS earthquake monitoring and basic research communities.
Experience with four Global Test Seismic Network (GTSN) stations indicates a
long-hauT communications link availability of 75 to 97 percent, depending on the
station, so implementing this level of performance of real-time transmission for all
primary stations will be very challenging.
No specif c recommendation at this time.
v. Station Location (within 100 m. array elements within ~ m relative J. This
is a routine requirement, although a specific reference frame for location, precision, and
accuracy should be given.
No specif c recommendation.
w. Seismometer Orientation (known within ~ degree). This is an extraordi-
narily high accuracy, not routinely achieved with any borehole instrument (KS360001
and 54000T orientation is +/- 3 degrees), and vault-type instruments can be oriented this
accurately only if a suitably accurate survey mark is provided in the vault. More recent
GSE documents suggest that 3° is an acceptable specification.
The pane! recommends relaxation of the orientation tolerance.
Desired Raw and Processed ISMS Data Streams
This issue watt be taken up in detail in the next chapter, but some initial response
is warranted in the context of the first charge. There is wide enthusiasm for timely and
straightforward access to the broadband three-component data streams from the
(continuous) primary and (segmented) auxiliary stations of the ISMS, as well as to
continuous data from the auxiliary stations, which will not be collected by the ISMS.
The continuous primary data can be directly incorporated into real-time analyses
conducted by the USGS global earthquake monitoring system, complementing data
collected through other networks. In addition, the broad seismological research
community has numerous applications of real-time seismic data analysis, and timely
OCR for page 32
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Comprehensive Test Ban Monitoring System
access to the ISMS data would enable maximum utilization of the data in diverse
applications. Segmented auxiliary station data, which will be acquired within several
hours of events of interest, also will be valuable for rapid analysis by the USGS and
basic research communities. Specific suggestions as to how to ensure rapid and full
access to the complete waveforms from the broaciband sensors in the primary array, as
well as all data from the auxiliary network, will be provided in the next chapter.
The ISMS processing will include automated and analyst-reviewed measure-
ments of phase arrival times, slowness measurement from the primary arrays, and array
beams, f-k spectra, and event locations. The USGS indicates that the phase and Ut/~^
measurements and bulletins from the TDC, both automatic and reviewed, would be
useful, but not essential, resources for their operational program. The USGS also
indicates it is unlikely that it will have serious use for products such as f-k spectra in the
near term. USGS operations do not currently place a high emphasis on global array data,
either raw data or formed beams, but this situation could change in the future.
There is interest from the general research community in obtaining short-perioc}
data from the primary arrays. The primary interest is in the original array data for events
of mb > 4 for the several-minute time window encompassing teleseismic phases. Access
to data from the individual array elements, rather than stacked signals, is likely to be of
interest to members of the research community, particularly with respect to
discrimination and deep earth structure research. However, the vast quantity of data
involved is such that it would be costly to duplicate the archive of the full data set.
Developing a procedure for accessing the ISMS data archive for tailored user requests
of array data seems to be the most attractive option. There does not appear to be a
general requirement in the research community for intermediate products such as the
~ 00 continuous beams formed by each array or f-k spectra as long as access to the raw
array data is established by some convenient procedure.
This chapter has considered data characteristics proposed by the GSE for the
TSMS seismic stations in light of research requirements both for general seismology and
for earthquake and nuclear test monitoring and detection. The general objective of the
panel's recommendations is to ensure that the TSMS seismic data are as inclusive and
as broadly applicable as possible. The pane! has therefore recommended some changes
in Tow-noise level requirements, sample rate requirements, sensitivity goals at higher
frequencies, and data frame length. The pane] also recommends adopting methods for
ensuring rapid and full access to data streams. These are discussed at greater length in
the next chapter.
This chapter has dealt with the data characteristics of the proposed permanent
ISMS monitoring stations. Characteristics of portable instruments have not been
covered, but the pane] notes that use of appropriate portable apparatus would increase
coverage temporarily in an area of particular interest.
Representative terms from entire chapter:
seismic data
