This document describes some suggested changes to the standard parameter settings used in the Earthworm version of Rex Allen's P-Picker algorithm. These changes have improved the picker's performance at two Earthworm installations where they have been implemented as of this writing: the University of Utah Seismograph Stations (UUSS) and the University of Washington. I consider these recommended parameter changes to be provisional because I am still working on optimizing the parameters and because I have not yet done a systematic comparison between the picker performance with and without these parameter changes. Nevertheless, I have been encouraged to make these provisional recommendations available to other Earthworm users who may wish to implement them. The standard parameter settings were originally developed by Rex Allen at the U.S. Geological Survey office in Menlo Park, California. These parameters have been in use for many years in Rex Allen P-Picker installations operating on data from the Northern California Seismic Network (NCSN), the Southern California Seismic Network (SCSN), and elsewhere. While the performance of the picker with these parameters has apparently been satisfactory for some networks, such as the NCSN and SCSN, that has not been the case for other networks including the UUSS network. The problem appears to be that the P-Picker algorithm with the standard parameters does not do a very good job of picking relatively low-amplitude, emergent first arrivals. Consequently, for networks with average station spacings significantly larger than those of the California networks, the picker is unable to pick enough arrival times from most of the smaller seismic events to enable them to be located. I believe that the picker performance can be significantly improved by changing just four of the program's parameters--five for broadband stations--as detailed below. These recommended changes are based primarily on detailed analyses of the reasons for the picker's failure to pick arrival times on several seismic records with small but clear P-wave first arrivals. I will first give a summary of the recommended changes to the picker parameters and then give a brief explanation about what these changes do and why I think they are needed. The reasons for the first three changes are fairly obvious once it is understood what the program does. The last two recommended changes are not as straightforward, and are subject to revision pending further work. SUMMARY OF SUGGESTED CHANGES: The 2-letter parameter names given below are those used in the original Earthworm version of the code. The longer names are those used in the 1997 and later versions of the code written by Will Kohler of the USGS. (1) Change i8 = MinPeakSize from 60 to 20 counts (2) Change i9 = MinCodaLen from 3 to 0 seconds (3) FOR BROADBAND STATIONS ONLY Change c1 = RawDataFilt from 0.985 to 0.939 (4) Change c3 = StaFilt from 0.6 to 0.4 (5) Change c4 = LtaFilt from 0.03 to 0.015 EXPLANATION OF SUGGESTED CHANGES: (1) Change i8 = MinPeakSize from 60 to 20 counts In one of the quality checks performed by the picker, the absolute values of the amplitudes of the first three extrema (peaks or troughs) are compared to a threshold value. This value was formerly called i8 but is called MinPeakSize in more recent versions of the code. At least one of these extrema must be greater than or equal to i8 for the pick to be considered valid. For UUSS data, and probably data from other networks as well, the standard threshold amplitude of i8 = 60 counts used for the NCSN is unnecessarily high. I chose to reset this parameter to 20, which is roughly twice the zero-to-peak noise level for quiet UUSS stations. (2) Change i9 = MinCodaLen from 3 to 0 seconds It is very important to change this parameter to a value less than 2 sec, even though for some versions of Earthworm this change will require recompiling some code. Here's why. The signal duration is defined as the difference between the P-wave arrival time and the time when the average absolute value of a 200-sample-long window drops below a threshold value. Except for "noisy" stations, this threshold is c7 = CodaTerm. The USGS uses a value of c7 = 49.14 counts for both historical and practical reasons. For some P-wave arrivals with good signal-to-noise ratio on the UUSS network, the average absolute value over the first 200 samples after the pick is less than 49.14 counts. Arrival time picks for such arrivals will be rejected after 2 sec if the sampling rate is 100 Hz and the minimum signal duration, i9, is set to 3 sec (the standard NCSN value). Setting i9 = MinCodaLen = 0 sec solves this problem. This problem could also be solved by lowering the threshold value, c7 = CodaTerm, perhaps to some value representative of the background noise levels. This solution is less desireable because it would lower the maximum magnitude that could be determined from the signal duration measurements which, in the interests of timeliness, are currently limited to 144 sec. The parameter i9 = MinCodaLen was unfortunately hardcoded to a value of 3 sec in the picker software supplied with Earthworm versions 2.8 (May 1997) through 3.1 (April 1998). If you are running one of these versions of Earthworm, then to set i9 = MinCodaLen = 0 sec you need to make the following change in the subroutine pick_ra.c and then recompile: Change the line const int MinCodaLen = 3; /* Must be an odd number */ to const int MinCodaLen = 0; /* Must be an odd number */ (3) FOR BROADBAND STATIONS ONLY Change c1 = RawDataFilt from 0.985 to 0.939 The first thing that the picker algorithm does is to apply a recursive highpass filter to the data to remove the DC offset. If c1 = 0.985 (the standard NCSN value) and if the sampling rate is 100 Hz, then the corner frequency of the filter is 0.24 Hz. On broadband records, which for this purpose I define as records from seismometers with free periods greater than one second, small amplitude P-wave arrivals are often obscured by microseisms. The microseisms can be filtered out more effectively by setting c1 = 0.939, which sets the corner frequency of the highpass filter to 1.0 Hz. This change ought to improve the performance of the picker on broadband records. I haven't actually verified that it does improve the picking, but I have verified that the change in c1 changes the filtering as predicted. For stations with short-period (1 Hz) seismometers, the standard NCSN value of c1= 0.985 works just fine and should probably be left alone. (4) Change c3 = StaFilt from 0.6 to 0.4 (5) Change c4 = LtaFilt from 0.03 to 0.015 The need for these changes is less obvious than for the other two, but nonethless they are very important. c3 and c4 control the time constants for the short-term and long-term averages, respectively, of the characteristic function used in the identification of P-wave first arrivals (see Allen, 1978, BSSA 68, 1521-1532). The changes above roughly double the time constants for these two averages, whose ratio serves as the primary detector of P-wave arrivals in the picker program. Suppose we define NSTA and NLTA as the number of samples it takes for the weight in the (recursively calculated) weighted short-term and long-term averages, respectively, to drop below 10%. Then the above changes increase NSTA from 3 to 5 samples and NLTA from 75 to 152 samples. The motivation for these changes is that the dominant period of the background noise is 0.05 sec in the primary UUSS data source: analog telemetry data digitized at 100 Hz. It seemed likely that the picker performance would improve if the short-term average was computed over a longer time window to better average out this noise. But how much longer? If the short-term average is too long, then the picks will be late relative to the onset of the arrival. The value for c3 = 0.4 (NSTA = 5 samples) was based on the observation that the shortest zero-to-peak times for first arrivals recorded on the UUSS analog telemetry network are ~ 0.04 sec. I increased the time constant for the long term average proportionately to that of the short-term average on the assumption that the ratio of these time constants had previously been optimized. Experiments using different values of c4 while holding c3 fixed at 0.4 seemed to support this assumption. However, I intend to do a grid search on a set of test events to find the optimum values of c3 and c4. The optimum values for c3 and c4 depend on some complex tradeoffs, the description of which is beyond the scope of this short document.
c6 = DeadSta . For 24-bit digital telemetry stations, we set this parameter to 1000000 instead of 1200--essentially turning off the dead station check. That's because this check is primarily useful for analog telemetry stations, for which telemetry dropouts can cause large amplitude noise. We do not tend to have this problem with the digital telemetry stations we record. Therefore, we set c6 = DeadSta to a higher value for digital telemetry stations in order to help keep this check from causing the rejection of arrival time picks in the codas of prior seismic events. c7 = CodaTerm . We use gain-dependent values so that the coda termination values all correspond to approximately the same ground velocity values. c1 = RawDatafilt . In my 1998 report on suggested picker parameter changes, I recommended that for broadband stations this parameter be set to a value that sets the corner frequency of the highpass filter to 1.0 Hz. One point that I did not make very clear in this report is that the correct value of c1 for a given highpass filter corner frequency depends on the sampling rate. If we define the corner frequency of the filter, fc, as the frequency at which the amplitude response is 1/sqrt(2) times it's maximum value, then it can be shown that fc = (1 / (pi T) arcsin [ (1 - c1) / sqrt(2(1 + c1**2)) ] c1 = [sin(2 pi fc T) -1] / [(2 ( sin(pi fc T) )**2) -1] where T = the sampling interval and pi = 3.14159. For example, if fc = 1.0 Hz, then c1 = 0.939 for T = 0.01 sec and c1 = 0.854 for T = 0.025 sec.