.. _global_iloc: #### iLoc #### Locator in SeisComP implemented by the plugin lociloc. Description =========== iLoc is a locator developed by István Bondár which has been integrated into |scname| by :cite:t:`gempa`. It is invoked by the wrapper plugin *lociloc* - the interface between |scname| and iLoc. Read the sections :ref:`iloc-setup` and :ref:`iloc-application` for configuring and using iLoc in |scname|. Background ---------- iLoc is a locator tool for locating seismic, hydroacoustic and infrasound sources based on :term:`phase picks `. iLoc is based on the location algorithm developed by :cite:t:`bondár-2009a` and implemented at the International Seismological Center, (:cite:t:`isc`, :cite:t:`bondár-2018`) with numerous new features added (:cite:t:`bondár-2018`). The stand-alone iLoc code can be downloaded from the :cite:t:`iloc-iris` software repository. Among the major advantages of using iLoc is that it can * Use any phases with valid travel-time predictions; * Use seismic, hydroacoustic and infrasound arrival time, slowness and azimuth observations in location; * Use travel-time predictions from a global 3D upper mantle velocity model; * Use a local 1D velocity model; * Account for the correlated travel-time prediction error structure due to unmodeled 3D velocity heterogeneities; * Check if the data has sufficient resolution to determine the hypocenter depth; * Identify ground truth (GT5) candidate events. History ------- * Originally developed for U.S. Air Force Research Laboratory, today the standard at the International Seismological Centre (ISC) replacing previous routines * Open source, download website: :cite:t:`iloc-iris` * Integrated first in SeisComP3 in 2019 * Basis of the EMSC crowd-source locator, CsLoc since 2019 * EMSC standard as of 2022 iLoc in a nutshell ------------------ * Accounts for correlated travel-time prediction errors * Initial hypocenter guess from Neighborhood Algorithm search * Linearised inversion using a priori estimate of the full data covariance matrix Attempts for free-depth solution only if there is depth resolution * Default depth is derived from historical seismicity * Seismic, hydroacoustic and infrasound observations * Arrival time, slowness and azimuth measurements * Uses most ak135 or iasp91 Earth model phases in locating * Integrated RSTT travel-time predictions * RSTT is default for Pn/Sn and Pg/Lg * Local velocity model and local phase TT predictions for Pg/Sg/Lg, Pb/Sb, Pn/Sn. Algorithms ---------- This section describes some of the principles. The full description of the applied algorithms can be found in the iLoc documentation provided along with the package on the :cite:t:`iloc-iris` website. Neighbourhood algorithm ~~~~~~~~~~~~~~~~~~~~~~~ Linearized inversion algorithms are quite sensitive to the initial guess. In order to find an initial hypocenter guess for the linearized inversion the Neigbourhood Algorithm (:cite:t:`sambridge-1999`; :cite:t:`sambridge-2001`) is performed around the starting hypocentre if :confval:`iLoc.profile.$name.DoGridSearch` is active. During the NA search, we identify the phases with respect to each trial hypocenter and calculate the misfit of the trial hypocenter. The misfit is defined as the sum of the :confval:`iLoc.profile.$name.NAlpNorm` residual and a penalty factor that penalizes against freakish local minima provided by just a few phases. In the first iteration :confval:`iLoc.profile.$name.NAinitialSample` hypocenter hypotheses are tested, while the subsequent iterations consider the best :confval:`iLoc.profile.$name.NAcells` solutions and resample the search space around them with :confval:`iLoc.profile.$name.NAnextSample` hypocenter hypotheses. The solution with the lowest misfit after :confval:`iLoc.profile.$name.NAiterMax` iteration is taken as the initial hypocenter for the linearized least squares inversion. A grid search can be performed to obtain a better initial hypocenter guess. The search is performed around the starting hypocenter. For a very exhaustive search one can increase :confval:`iLoc.profile.$name.NAinitialSample`, :confval:`iLoc.profile.$name.NAnextSample` and :confval:`iLoc.profile.$name.NAcells` values. Note that the maximum value for :confval:`iLoc.profile.$name.NAinitialSample` is around 3500 before hitting memory limits. An exhaustive search will considerably slow iLoc down, especially when RSTT predictions are enabled (:confval:`iLoc.profile.$name.UseRSTT`, :confval:`iLoc.profile.$name.UseRSTTPnSn`, :confval:`iLoc.profile.$name.UseRSTTPgLg`). Depth resolution ~~~~~~~~~~~~~~~~ Depth resolution can be provided by a local network, depth phases, core reflections and to a lesser extent near-regional secondary phases. iLoc attempts for a free-depth solution if the set of :term:arrivals meets at least one of the following conditions: * Number of pairs of defining P and depth phases :math:`\le` :confval:`iLoc.profile.$name.MinDepthPhases` * Number of pairs of defining P and core phases :math:`\le` :confval:`iLoc.profile.$name.MinCorePhases` * Number of pairs of defining P and S phases :math:`\le` :confval:`iLoc.profile.$name.MinSPpairs` within a regional distance of :confval:`iLoc.profile.$name.MaxLocalDistDeg` degree * Number of defining P phases :math:`\le` :confval:`iLoc.profile.$name.MinLocalStations` within a local distance of :confval:`iLoc.profile.$name.MinLocalStations` degree. If there is insufficient depth resolution provided by the data, or the depth uncertainty for a free-depth solution exceeds a threshold, the hypocentre depth is set to the depth from the default depth grid if a grid point for the epicentre location exists; otherwise it is set to a depth :cite:t:`bolton-2006` assigned to the corresponding Flinn-Engdahl geographic region (:cite:t:`young-1996`). The default depth grid (:cite:t:`bondár-2011`) is defined on a 0.5º x 0.5º grid as the median of all depths in the cell, provided that there were at least five events in the cell, and the 75–25 percent quartile range was less than 100 km. The latter constraint is imposed to avoid regions with both shallow and deep seismicity. Anthropogenic events are fixed to the surface. Finally, the user can fix the depth to the initial depth. iLoc reports back how the depth was determined in the FixedDepthType parameter: * 0 - free depth solution * 1 - airquake/deepquake, depth fixed to surface/MaxHypocenterDepth * 2 - depth fixed to depth reported by an agency (not used in |scname|) * 3 - depth fixed to depth-phase depth * 4 - anthropogenic event, depth fixed to surface * 5 - depth fixed to default depth grid depth * 6 - no default depth grid point exists, fixed to median reported depth * 7 - no default depth grid point exists, fixed to GRN-dependent depth * 8 - depth fixed by user provided value Linearized inversion ~~~~~~~~~~~~~~~~~~~~ Once the Neighbourhood search get close to the global optimum, iloc switches to an iterative linearized least-squares inversion of travel-time, azimuth and slowness observations (:cite:t:`bondár-2009b`; :cite:t:`bondár-2011`) to obtain the final solution for the hypocenter. The convergence test after (:cite:t:`paige-1982`) is applied after every iteration. Once a convergent solution is obtained, the location uncertainty is defined by the a posteriori model covariance matrix. The model covariance matrix yields the four-dimensional error ellipsoid whose projections provide the two-dimensional error ellipse and one-dimensional errors for depth and origin time. These uncertainties are scaled to the 90% confidence level (:cite:t:`jordan-1981`). The final hypocentre is tested against the ground truth selection criteria (:cite:t:`bondár-2009a`), and it is reported as a GT5candidate if the solution meets the GT5 criteria. Some important parameters are: * :confval:`iLoc.profile.$name.SigmaThreshold`: Residuals that exceed :math:`abs(Sigmathreshold * PriorMeasError)` are made non-defining. * :confval:`iLoc.profile.$name.MinNdefPhases`: Minimum number of observations required to attempt for a solution. If the number of defining arrival times exceed :confval:`iLoc.profile.$name.MinNdefPhases`, then slowness observations will not be used in the location. Integration into |scname| ------------------------- * Integration of iLoc into |scname| is provided by an external library of routines (:cite:t:`iloc-iris`). * |scname| modules call iLoc routines by passing the objects via the plugin *lociloc* installed in :file:`@DATADIR@/plugins/lociloc.so`. * iLoc returns objects to |scname| for integration. * The iLoc implementation in |scname| retains all original iLoc functionalities. Read the section :ref:`iloc-setup` for the installation of the iLoc library and the configuration in |scname|. Velocity models --------------- iLoc ships with the global models *iasp91* and *ak135* as well as with regional seismic travel-time tables, RSTT, which, if activated by configuration, replaces the global models in areas where they are defined. .. _iloc-velocity_global: Global models ~~~~~~~~~~~~~ The global models *iasp91* and *ak135* and RSTT are available by default without further configuration. .. _iloc-velocity_rstt: RSTT ~~~~ RSTT are available in :file:`@DATADIR@/iloc/RSTTmodels/pdu202009Du.geotess`. Custom RSTT can be integrated into iLoc and provided to |scname|. For adding custom RSTT to iLoc read the original iLoc documentation from the :cite:t:`iloc-iris` software repository. The usage of RSTT is controlled per iLoc profile by global configuration parameters * :confval:`iLoc.profile.$name.UseRSTT` * :confval:`iLoc.profile.$name.UseRSTTPnSn` * :confval:`iLoc.profile.$name.UseRSTTPgLg` .. _iloc-velocity_local: Local velocity models ~~~~~~~~~~~~~~~~~~~~~ Custom local velocity models can be provided by a file in :file:`@DATADIR@/iloc/localmodels`. Example file :file:`@DATADIR@/iloc/localmodels/test.localmodel.dat`: .. code-block:: properties # # test # # number of layers 4 0.000 5.8000 3.4600 x 20.000 6.5000 3.8500 CONRAD 45.000 8.0400 4.4800 MOHO 77.500 8.0400 4.4800 x Once added, the velocity can be configured in |scname| as set out in section :ref:`iloc-setup`. Station elevation ----------------- iLoc considers station elevation. It calculates the elevation correction, *elevationCorrection*, for a station as .. math:: elevationCorrection = \frac{\sqrt{1 - (surfVel * p)^2} * elev}{surfVel} where * *elev*: elevation of the station * *p*: the ray parameter (horizontal slowness) * *surfVel*: layer P or S velocity of at the surface depending on the last lag of the phase name. .. note :: iLoc does not allow airquakes or source locations above datum (0 km). If the depth of an origin becomes negative, iLoc fixes the depth to 0 km and the depth type of the origin will be "operator assigned". .. _sec-iloc-references: Resources --------- iLoc has taken advantage of many publications or has been cited therein. Read the section :ref:`sec-references` for a list. .. _iloc-setup: Setup in |scname| ================= #. Add the plugin *lociloc* to the global configuration, e.g. in :file:`@SYSTEMCONFIGDIR@/global.cfg`: .. code-block:: properties plugins = ${plugins}, lociloc #. Download iLoc data files from the :cite:t:`iloc-iris` website. Extract and install them in :file:`@DATADIR@/iloc`. Ensure to download a valid file. Example: .. code-block:: sh mkdir $SEISCOMP_ROOT/share/iloc wget -O /tmp/iLocAuxDir.tgz "http://iloc.geochem.hu/data/iLocAuxDir4.1.tgz" tar xvf /tmp/iLocAuxDir.tgz -C $SEISCOMP_ROOT/share/iloc .. note :: * Check the :cite:t:`iloc-iris` website for updates before downloading the file since the version number, hence the name of the download file may change. * Instead of generating the :file:`SEISCOMP_ROOT/share/iloc/iLocAuxDir` directory, you can also create a symbolic link and maintain always the same iLoc version in |scname| and externally. #. Add and configure iLoc profiles for the velocity models. The global models *iasp91* and *ak135* are considered by default with default configuration parameters even without setting up *iasp91*/*ak135* profiles. You may, however, create these profiles for their customization. Create new profiles or consider existing ones for adjusting their configuration: * :confval:`iLoc.profile.$name.globalModel`: The name of the :ref:`global model `, e.g. *iasp91* or *ak135*. * Consider the :ref:`RSTT parameters `. * :confval:`iLoc.profile.$name.LocalVmodel`, :confval:`iLoc.profile.$name.UseLocalTT` and :confval:`iLoc.profile.$name.MaxLocalTTDelta`: The definition of a :ref:`local velocity model `: model file, default usability, distance range. * :confval:`iLoc.profile.$name.DoNotRenamePhases`: Renaming seismic phases automatically impacts the usability of the origins with other locators and locator profiles. Activate the parameter to avoid phase renaming. * Consider the remaining parameters. .. note :: Creating the profiles allows using the same global velocity model along with different local models or RSTT settings in separate profiles. #. Test the locator using :ref:`scolv` or configure with :ref:`screloc` or other locator modules. .. _iloc-application: Application in |scname| ======================= Once the *lociloc* plugin is configured, the iLoc locator can be applied * Automatically e.g. in :ref:`screloc` or * Interactively in :ref:`scolv`. For using iLoc in :ref:`scolv` select it in the locator menu of the Location tab .. figure:: media/scolv-iloc-locator.png :align: center Select iLoc locator along with a profile: .. figure:: media/scolv-iloc-profile.png :align: center Select iLoc profile The parameters for iLoc can be adjusted by pressing the wrench button next to the locator selection combo box .. figure:: media/scolv-iloc-change.png :align: center Start the settings dialog which opens the iLoc settings dialog: .. figure:: media/scolv-iloc-settings.png :align: center Adjust the settings and click OK to confirm .. warning :: By default, automatic phase renaming by iLoc is active. The renaming may change the phase names, e.g. from P to Pn. Renaming seismic phases automatically will later impact the usability of the new origins with other locators and locator profiles. Deactivate *DoNotRenamePhases* to avoid phase renaming. However, when deactivating, iLoc may not provide results if the initial phases do not exist in the phase table for the given source depth and epicentral distance. Example: For great source depth and small epicentral distance, the first arrival phase is p or Pn and not P but |scname| provides P. After relocating, the iLoc locator and the selected profile are shown in the :ref:`scolv` Location tab as Method and Earth model, respectively: .. figure:: media/scolv-iloc-info.png :align: center Information in scolv Locator tab .. _global_iloc_configuration: Module Configuration ==================== .. note:: **iLoc.\*** *Locator parameters: iLoc* .. confval:: iLoc.auxDir Default: ``@DATADIR@/iloc/iLocAuxDir`` Type: *string* iLoc directory for auxialiary files and directories. Some of them must be provided from the iLoc website. Read the documentation for their installation. .. confval:: iLoc.profiles Default: ``iasp91,ak135`` Type: *list:string* iLoc profile name. Multiples names may be set separated by comma. Each profile can have different velocity or parameters. .. note:: **iLoc.profile.\*** *Profiles containing the specific locator parameters. For* *the global models, iasp91 and ak135, profiles are* *automatically considered with defaults.* *To adjust the profile parameters the corresponding profile* *must be created.* .. note:: **iLoc.profile.$name.\*** $name is a placeholder for the name to be used and needs to be added to :confval:`iLoc.profiles` to become active. .. code-block:: sh iLoc.profiles = a,b iLoc.profile.a.value1 = ... iLoc.profile.b.value1 = ... # c is not active because it has not been added # to the list of iLoc.profiles iLoc.profile.c.value1 = ... .. confval:: iLoc.profile.$name.Verbose Default: ``true`` Type: *boolean* .. confval:: iLoc.profile.$name.globalModel Type: *string* Name of globally applied velocity model into which RSTT or the local model is integrated. If unset, the name of the profile is considered instead. .. confval:: iLoc.profile.$name.UseRSTT Default: ``false`` Type: *boolean* Use regional seismic travel\-time tables .. confval:: iLoc.profile.$name.UseRSTTPnSn Default: ``true`` Type: *boolean* Use regional seismic travel\-time tables for Pn and Sn .. confval:: iLoc.profile.$name.UseRSTTPgLg Default: ``true`` Type: *boolean* Use regional seismic travel\-time tables for Pg and Lg .. confval:: iLoc.profile.$name.UseLocalTT Default: ``false`` Type: *boolean* Use local velocity model if defined in LocalVmodel. .. confval:: iLoc.profile.$name.LocalVmodel Type: *string* Full path to a file containing the local velocity model. Requires: UseLocalTT \= true. Empty string or unset or UseLocalTT \= false disables using a local model in this profile. Example: \@DATADIR\@\/iloc\/iLocAuxDir\/localmodels\/model.localmodel.dat. .. confval:: iLoc.profile.$name.MaxLocalTTDelta Default: ``3`` Type: *float* Unit: *deg* Maximum epicentral distance for applying the local velocity model. .. confval:: iLoc.profile.$name.DoGridSearch Default: ``true`` Type: *boolean* Perform neighbourhood algorithm .. confval:: iLoc.profile.$name.NAsearchRadius Default: ``5`` Type: *float* Unit: *deg* Neighbourhood Algorithm: Search radius around initial epicentre .. confval:: iLoc.profile.$name.NAsearchDepth Default: ``300`` Type: *float* Unit: *km* Neighbourhood Algorithm: Search radius around initial depth .. confval:: iLoc.profile.$name.NAsearchOT Default: ``30`` Type: *float* Unit: *s* Neighbourhood Algorithm: Search radius around initial origin time .. confval:: iLoc.profile.$name.NAlpNorm Default: ``1`` Type: *float* Neighbourhood Algorithm: p\-value for norm to compute misfit [1,2] .. confval:: iLoc.profile.$name.NAiterMax Default: ``5`` Type: *integer* Neighbourhood Algorithm: Maximum number of iterations .. confval:: iLoc.profile.$name.NAcells Default: ``25`` Type: *integer* Neighbourhood Algorithm: Number of cells to be resampled at each iteration .. confval:: iLoc.profile.$name.NAinitialSample Default: ``1000`` Type: *integer* Neighbourhood Algorithm: Size of initial sample .. confval:: iLoc.profile.$name.NAnextSample Default: ``100`` Type: *integer* Neighbourhood Algorithm: Size of subsequent samples .. confval:: iLoc.profile.$name.MinDepthPhases Default: ``3`` Type: *integer* Depth resolution: Minimum number of depth phases for depdp .. confval:: iLoc.profile.$name.MaxLocalDistDeg Default: ``0.2`` Type: *float* Unit: *deg* Depth resolution: Maximum local distance .. confval:: iLoc.profile.$name.MinLocalStations Default: ``1`` Type: *integer* Depth resolution: Minimum number of local defining stations .. confval:: iLoc.profile.$name.MaxSPDistDeg Default: ``2.0`` Type: *float* Unit: *deg* Depth resolution: Maximum distance for using S\-P travel\-time differences. .. confval:: iLoc.profile.$name.MinSPpairs Default: ``3`` Type: *integer* Depth resolution: Minimum number of defining S\-P phase pairs .. confval:: iLoc.profile.$name.MinCorePhases Default: ``3`` Type: *integer* Depth resolution: Minimum number of defining core reflection phases .. confval:: iLoc.profile.$name.MaxShallowDepthError Default: ``30.0`` Type: *float* Unit: *km* Depth resolution: Maximum depth error for crustal free\-depth .. confval:: iLoc.profile.$name.MaxDeepDepthError Default: ``60.0`` Type: *float* Unit: *km* Depth resolution: Maximum depth error for deep free\-depth .. confval:: iLoc.profile.$name.DoCorrelatedErrors Default: ``true`` Type: *boolean* Linearized inversion: Account for correlated errors .. confval:: iLoc.profile.$name.SigmaThreshold Default: ``6.0`` Type: *float* Unit: *s* Linearized inversion: Used to exclude big residuals from solution .. confval:: iLoc.profile.$name.AllowDamping Default: ``true`` Type: *boolean* Linearized inversion: Allow damping of model vector .. confval:: iLoc.profile.$name.MinIterations Default: ``4`` Type: *integer* Linearized inversion: Minimum number of iterations .. confval:: iLoc.profile.$name.MaxIterations Default: ``20`` Type: *integer* Linearized inversion: Maximum number of iterations .. confval:: iLoc.profile.$name.MinNdefPhases Default: ``4`` Type: *integer* Linearized inversion: Minimum number of defining phases .. confval:: iLoc.profile.$name.DoNotRenamePhases Default: ``false`` Type: *boolean* Linearized inversion: Do not rename phases. Deactivating allows to rename the phases automatically for this profile.