Quanterra Timing Errors
July 1, 2000
This description of the Quanterra timing errors is primarily taken from the 1999-2000 Annual Report of the Berkeley Seismological Laboratory.
In late 1998, Quanterra provided the first release of MultiSHEAR, an enhanced version of its data acquisition software that was year 2000 compliant, and updated components of the OS/9 operating system to address the year 2000 problem. MultiSHEAR contained a number of enchancements, especially in the area of multi-site data collection, and introduced a totally new configuration procedure. The BSL worked with Quanterra during 1999 to enhance the configuration procedures to address the diverse needs of the BDSN and NHFN station configurations. During November and December 1999, all of the BSL Quanterra dataloggers were updated to MultiSHEAR with the corresponding OS/9 modifications, which addressed the year 2000 problems.
One significant feature of MultiSHEAR that affected the BSL were the correction of a systematic timing error of the decimated channels from SHEAR and UltraSHEAR software in the Quanterra datalogger.
The timing description in the following section is provided courtesy of the USGS Albuquerque Seismological Laboratory (ASL), which provides data collection and quality control procedures for the portion of the GSN that uses Quanterra dataloggers.
The Quanterra digitizers initially sample at very high rates. In firmware the data are introduced to a filter cascade of a various number of stages wherethey are low-pass FIR filtered and decimated multiple times. Depending on the specific system the data are further FIR filtered and decimated by configurable software. Each applied FIR filter introduces to the data a nominal delay of half the FIR filter width which then requires subsequent corrections to the data time tags.
For these Quanterra systems the calculation of the time tag applied to the data is more complicated than the first order correction associated with the half widths of the FIR filters. There is a very small correction term associated with data buffering and a more substantive subjective correction to account for the delay in reading the 'first break' during signal onset. This second term attempts to bridge the gap between impulsive and steady state signals. The size of this term has been a function of each filter's transition band and is generally 1.5-2.0 output samples. The cumulative effect of these corrections has mistimed most of the seismic channels. The mistiming is obvious when a sine wave is input into the Quanterra system and time series from the various channels are overplotted.
For more information on this problem from the ASL, please see their Web site on this subject.
These timing errors apply both to the raw data and to the phase readings of the BDSN and NHFN. Neither the raw data nor the phase readings have been corrected. The following tables provide the timing corrections required for the various Quanterra datalogger configurations used by the BSL.
The BDSN/NHFN Quanterra dataloggers were upgraded to MultiSHEAR software at the end of 1999. The exact date of the upgrade and the datalogger model are listed in this table. Data recorded after these dates do not have the systematic timing errors.
|Quanterra Data Loggers|
|Date of MultiSHEAR Upgrade|
|MIN||Closed before upgrade||Q980|
|STAN||Closed before upgrade||Q680|