Rays and Distances

For many of the tools within the TauP Toolkit, a large part of the work is determining the ray through the model that answers the question. But depending on the question there are many ways to specify the ray. For example, what time does P arrive at 35 degrees? Or where does a ray that leaves the source at a 25 degree takeoff angle arrive?

Traditionally, we are interested in arrivals at a known distance. So Using the --deg: option gives that distance in degrees while the --km gives it in kilometers. Multiple of either of these can be given, like --deg 10,30,60. But if we want a regular sampling, we can give --degreerange 5 to get every 5 degrees or --degreerange 30,60 to get every 10 degrees from 30 to 60, or even --degreerange 30,60,5 to get every 5 degrees from 30 to 60. The --kilometerrange argument provides the same functionality, except for kilometers along the surface, with defaul step of 100 and default maximum of 1000.

Some times it is helpful to generate only arrivals at an exact distance and to disregard those that arrive the other way around, or that lap around the earth multiple times. For example a phase like SKKKS that can travel a path of more than once around, we may want to get the arrival at 200 degrees but not the one at 160 degrees. The argument --exactdegree 200 does this. Similar functionality exists for kilometers with --exactkilometer.

If we know the event and station locations, we can use the --evt and --sta to give the latitude and longitudes and TauP can calculate the distance. The --geodetic argument does the distance calculation using an ellipticity, the default is purely spherical. The internal time calculations are always spherical. We can also read station locations from a StationXML file with --staxml and event locations from a QuakeML file with --qml. Even more fun is to give a station id, like CO_HAW or a USGS event id, like us7000pn9s, and TauP will use the IRIS FedCat and USGS FDSN Event web services to get the station and event locations. TauP will do the calculations for all event station pairs.

There are also times when we do not know the final distance, but are interested in rays that start or end in a particular way. For example we can shoot rays of specific ray parameters with --rayparamdeg, --rayparamkm and --rayparamrad for ray parameters of seconds per degree, kilometer or radian. Or we can give a takeoff angle with --takeoff and a range of takeoff angles with --takeoffrange similar to the distance range arguments. And we can give an incident angle with --incident and a range of incident angles with --incidentrange.

As long as we are using spherical calulcations, not --geodetic, then if we give event location via --evt and azimuth via --az, then the resulting station location will be calculated. The reverse, giving station location via --sta and back azimuth via --baz will calculate the event location.

For debugging purposes, the --allindex and --rayparamidx will show the calculation at the model sampling, for all or one ray by index.