Cross-Correlation

Cross-correlation is used to refine the differential travel time between two phase picks at a common station for an event pair. If the correlation coefficient exceeds a configured threshold, the cross-correlation lag is used to correct the absolute travel-time difference as the observed differential time in the double-difference system (see Relocation Process). Phase pairs with correlation coefficients below the threshold retain the differential time derived from their original pick times.

Because cross-correlation is computationally intensive, it is recommended to enable it only after other relocation parameters have been optimized.

Please note that certain cross-correlation settings require inventory information, for example if the horizontal components have to be used, the inventory is required to know which are the horizontal ones.

Waveform Data

SeisComP applications access waveform data through the RecordStream interface (see the official SeisComP documentation for more details). This can be configured in global.cfg or specified via the command line using the -I URI option. RecordStream parameters define the services through which SeisComP accesses real-time and historical waveform data (e.g., SeedLink, FDSN, SDS archives, etc.).

RecordStream Configuration for Single-Event (Real-Time)

A typical RecordStream configuration might look like this:

recordstream = combined://slink/localhost:18000;sdsarchive//path/to/miniseed

This example combines SeedLink and an SDS archive, allowing scrtdd to retrieve catalog waveforms from the archive and real-time event data via SeedLink.

Depending on the responsiveness of the SeedLink server, real-time relocations may incur delays. If data is unavailable, SeisComP may attempt to reconnect indefinitely. To prevent excessive delays, use the timeout and retries parameters. The following example sets a timeout of 5 seconds (the default is 5 minutes) and disables reconnections for missing data, allowing scrtdd to proceed with available data:

recordstream = combined://slink/localhost:18000?timeout=5&retries=0;sdsarchive//path/to/miniseed

RecordStream Configuration for Multi-Event

Accessing catalog waveforms requires a RecordStream connected to a historical archive, such as an FDSN service or an SDS archive.

Example accessing waveforms from an FDSN service:

scrtdd -I fdsnws://service.iris.edu:80/fdsnws/dataselect/1/query [...options...]

Example accessing waveforms from an SDS archive:

scrtdd -I sdsarchive:///path/to/archive [...options...]

Waveform Data Caching

As downloading waveforms can be time-consuming, scrtdd can cache reference catalog waveforms to disk once they have been fetched. Real-time event waveforms are used immediately and are not cached. Cache behavior and storage locations can be controlled via configuration options.

Waveforms are loaded “lazily,” only when required for cross-correlation (e.g., when a phase pair is processed). You can manually force scrtdd to pre-download all reference catalog waveforms for a profile using the following command:

scrtdd --load-profile-wf --profile myprofile  \
       --verbosity=3 --console=1  [-I RecordStream] [db options]

Reusing Cross-Correlation Results

To save processing time, cross-correlation results can be reused. This is particularly valuable when experimenting with relocation settings while keeping cross-correlation parameters constant. When the --dump-diagnostics option is used, the relocation process generates a xcorr.csv file. This file contains computed cross-correlation lags and can be passed back to scrtdd using the --xcorr-cache option.

When this option is active, scrtdd skips cross-correlation for any event pair already present in the cache file. A common use case is evaluating relocation quality across different correlation coefficient thresholds without recomputing waveform cross-correlations.

Waveform Inspection

The --dump-wf option instructs scrtdd to dump the processed waveforms of the specified catalog to disk. To avoid dumping the entire catalog, you can edit the input event.csv file to include only a small subset of events. Waveforms are saved in miniSEED format and can be inspected using external tools like scrttv or ObsPy. These files reflect the waveforms after filtering and resampling have been applied.

Example:

scrtdd --dump-wf station.csv,event.csv,phase.csv --profile myProfile \
       --verbosity=3 --console=1  [-I RecordStream] [db options]