This package uses interpolation and SU library functions (sucore) to sample time traces from gridded spatial data and read/write these traces to SEGY format files. Main features:
A "destructor" and "default constructor" are also provided, to give tracegeom and its associated functions as many of the characteristics of a class as possible.
The central concept underlying the structure of this package is the relation between GLOBAL and LOCAL grids. The GLOBAL grid describes a (virtual) gridding of all of R^n; its characteristics are the dimension, the grid steps along each axis, and the coordinates of the grid origin (point with grid indices = 0). Sampling takes place from a LOCAL grid, which is a (finite) subgrid of the GLOBAL grid. Data corresponding to the LOCAL grid is assumed to reside in the memory space of the process. The LOCAL grid must have the same grid steps along each axis as the GLOBAL grid, and is additionally characterized by the number of samples aong each axis and the coordinates of the first sample. The function init_tracegeom uses the descriptors of GLOBAL and LOCAL grids together with trace headers read from an input file to compute indices and relative in-cell coordinates of each receiver point and of the source point. It is assumed that the input file describes a single shot record, with a unique source location, and this is read from the first trace in the input header file. This sampling information is stored in a tracegeom object, which is subsequently used by other functions in the package to extract samples from a (LOCAL) grid of data, and to construct trace headers for trace output.
The time sample rate for simulation is an independently prescribed input - it is presumed that a sample rate is set somehow in the setup phase of the simulator, and is available to the trace package as an input. Sampling to or from the internal trace buffer of tracegeom occurs at this simulation sample rate. The function writetraces uses cubic spline interpolation (see Cubic Spline Interpolation and Adjoint Interpolation) to resample this data as prescribed by the trace headers used to initialize the tracegeom object. Thus output traces with exactly the same geometry as these input headers. An important exception: only those traces with receiver positions lying inside of the LOCAL grid will be sampled - traces for receiver positions outside of the LOCAL grid are neither sampled, nor read from an input file, nor written to an output file.
Alternatively, the user may specify the number of time steps and first sample time, in which case the external traces have this length and are sampled at the internal simulation rate. This option is use when precise control of simulation time step is essential, for instance in some convergence studies. See the documentation of init_tracegeom for precise details.
Parallel implementation: all i/o takes place on rank 0. Reads: data read trace-by-trace into segy trace buffer from file on rank 0, then broadcast to all other ranks. Each rank computes that part of the data needed locally, then stores it. Writes: roughly the reverse of read, but implemented with point-to-point communication, hence slower.
Source and receiver coordinates may be interpreted as relative to a coordinate offset vector, an argument to the construct function. This feature is useful in using this data structure to implement a towed streamer source, in which the input coordinate vector is interpreted as the coordinate of a reference point on the source array, and the receiver coordinates of the data as relative coordinates to it. Thus for example a superposition of three point sources, at 10 m to left and right of a central source, might be implemented by (1) creating a SEGY data file with three traces, having gx=-10, 0, and 10 respectively, and (2) passing the coordinates of the source reference point via the auxiary offset vector. Alternatively, the same effect can be achieved by passing a zero offset vector (for example an RPNT_0) and adding the reference point coordinates to the trace header gx.
Note above all that, in defining sources using the load option with the traceio package, the source coordinates play no role at all - the receiver coordinates give the source positions, at which data is added into the grid, either relative to a source reference point or absolute in global grid coordinates.
1.4.7