#
SUGGESTED PICKER PARAMETER CHANGES

### James C. Pechmann

### University of Utah, Dept. of Geology and Geophysics

### July 7, 1998

### Updated August 29, 1998

         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.