LOCSAT

Locator in SeisComP for computing source time and hypocenter coordinates from phase picks.

Description

LOCSAT is a locator with a travel-time interface in SeisComP for computing source time and hypocenter coordinates from phase picks considering:

  • Pick time and pick uncertainty,

  • Backazimuth and backazimuth uncertainty,

  • Slowness and slowness uncertainty,

  • Phase-specfic travel-time tables.

The LOCSAT locator interface implements a wrapper for the LocSAT locator by Bratt and Nagy [43] (according to the README file shipped with the LocSAT distribution) referred to as LOCSAT in SeisComP. The LOCSAT travel-time interface provides travel time for specfic phases, epicentral distance, soure depth and station elevation.

Locator Interface

LOCSAT provides the hypocenter parameters through the locator interface.

Travel-Time Interface

LOCSAT provides an interface for computing travel times based on coordinates and depth. The times are plotted on waveforms, e.g., blue marks in scolv picker window.

Use “LOCSAT” as a value for the travel-time interface when configurable, e.g., by FixedHypocenter.

Travel-Time Tables

SeisComP ships with two sets of predefined travel-time tables which are made available as the profiles tab and iasp91.

The default profile is iasp91.

LOCSAT travel time tables are located as plain ascii files under @DATADIR@/locsat/tables/. The tables provide the travel times for particular seismic phases at given depth and epicentral distance in one file per Earth model and seismic phase. E.g. P-wave arrival times in the iasp91 model are found in @DATADIR@/locsat/tables/iasp91.P. You may easily add your own tables for any available Earth model and seismic phase by adopting existing ones in new files which are added by configuration to your SeisComP modules.

Limitations

  1. Only phases for which a travel-time table exists can be considered.

  2. LOCSAT currently considers travel-time tables for phases which are hard-coded

    • seismic body waves: P, Pg, Pb, Pn, Rg, pP, sP, PKP, PP, PKPab, PKPbc, PKPdf, SKPdf, PcP, S, Sg, Sb, Sn, Lg, SKS, SS, ScS,

      where P and S are the direct P and S phases, respectively, at all distances no matter the take-off angle at the source.

    • seismic surface waves: LQ, LR.

    • infrasound: Is, It, Iw.

  3. The maximum number of distance and depth intervals per table file is currently 210 and 50, respectively.

    Warning

    • Travel-time tables with larger numbers of distance or depth samples are reported along with command-line error output (stderr). The travel-time tables should therefore be tested, e.g., with scolv before unsupervised application.

    • Travel times at distance and depth samples exceeding the limits are ignored. This may lead to undesired behavior during location.

    • Phase picks observed outside the distance and depth ranges defined by travel-time tables may lead to undesired behavior during location.

  4. The considered minimum depth is 0 km. Elevations and depths above datum are not natively considered. The effects of station elevation can be corrected for empirically.

Station elevations

LOCSAT does not natively support corrections of travel-time tables for station elevations. At least checking the code:

sta_cor[i]  = 0.0;    /* FIX !!!!!!*/

However, the SeisComP wrapper adds this feature. It allows to define a .stacor file which defines emperic corrections of observed travel times. The corrections are provided in seconds and subtracted (not added) from the observation time to be compatible with the NonLinLoc [53] station correction definitions.

Each LOCSAT profile (travel time table) can have one associated station correction file. E.g. for adding station corrections to the iasp91 tables, the file $SEISCOMP_ROOT/share/locsat/tables/iasp91.stacor needs to be created.

A station correction table takes the form:

# LOCDELAY code phase numReadings delay
LOCDELAY GE.MORC P 1 -0.1

with

  • code (string) station code (after all alias evaluations)

  • phase (string) phase type (any of the available travel time tables)

  • numReadings (integer) number of residuals used to calculate mean residual/delay (not used by NLLoc, included for compatibility with the format of a summary, phase statistics file)

  • delay (float) delay in seconds, subtracted from observed time

Note

The fourth column (numReadings) is ignored and just provided for compatibility reasons with NonLinLoc.

Application and Setup

LOCSAT is the default and only locator for scautoloc with iasp91 as the default profile. However, LOCSAT can be used optionally in other modules such as scolv or screloc.

Custom travel-time tables

  1. Generate your travel-time tables from a custom Earth model, depth and distance intervals. Use the same format as the defaults as the iasp91 tables. Tools such as TauP [25] allow the generation.

  2. Add your custom travel-time tables along with station corrections to @DATADIR@/locsat/tables/

  3. Add your available custom LOCSAT travel-time tables in global configuration, e.g., to the list of tables of travel-time interfaces

    ttt.LOCSAT.tables = iasp91, tab, custom

    and to the list of locator profiles

    LOCSAT.profiles = iasp91, tab, custom

    and optionally to locators which make use of LOCSAT tables, e.g., FixedHypocenter.

Application with modules

Additional parameters of LOCSAT may be configured in global module configuration (LOCSAT.*).

  • The profiles for locating may be extended or limited by LOCSAT.profiles.

  • When using picks with time uncertainties, consider LOCSAT.usePickUncertainties and LOCSAT.defaultTimeError.

  • Measurements of backazimuth and slowness may be deactivated by LOCSAT.useBackazimuth and LOCSAT.useSlownewss, respectively. Such measurements may be obtained from array processing or from feature extraction using scautopick.

You may also configure some SeisComP modules with LOCSAT and a profile.

  • scautoloc: Configure a profile for automatic locations,

  • screloc: Configure LOCSAT along with a profile for automatically relocating.

  • scolv: Configure LOCSAT along with a profile as defaults for interactive locations.

When using LOCSAT in scolv you may interactively some settings. The changes only apply during runtime.

../_images/scolv-locsat-settings.png

scolv Location tab with LOCSAT selected and the settings menu.

Module Configuration

Note

LOCSAT.* Locator parameters: LOCSAT

LOCSAT.profiles

Default: iasp91, tab

Type: list:string

Defines a list of available LOCSAT travel-time tables.

LOCSAT.depthInit

Default: 20.0

Type: double

Unit: km

The initial depth estimate for LOCSAT.

LOCSAT.usePickUncertainties

Default: false

Type: boolean

Whether to use pick time untertainties for arrival deltim rather than a fixed time error. If true then the uncertainties are retrieved from each individual pick object. If they are not defined then the default pick time uncertainty will be used as fallback.

LOCSAT.defaultTimeError

Default: 1.0

Type: double

Unit: s

The default pick time uncertainty assigned to LOCSAT’s arrival deltim attribute if pick uncertainties are not going to be used or if they are absent. A time uncertainty of 0 s may result in errors of the SVD decomposition in LOCSAT.

LOCSAT.usePickBackazimuth

Default: true

Type: boolean

Whether to forward pick backazimuth to LOCSAT or not. In an automatic mode backazimuth measurements might be inaccurrate and disabling their usage in LOCSAT can be controlled with this parameter.

LOCSAT.usePickSlowness

Default: true

Type: boolean

Whether to forward pick horizontal slowness to LOCSAT or not. In an automatic mode slowness measurements might be inaccurrate and disabling their usage in LOCSAT can be controlled with this parameter.

LOCSAT.degreesOfFreedom

Default: 9999

Type: int

Number of degrees of freedom.

LOCSAT.confLevel

Default: 0.9

Type: double

Confidence level between 0.5 and 1.0.

LOCSAT.enableConfidenceEllipsoid

Default: false

Type: boolean

Compute the confidence ellipsoid from covariance matrix in 3D.