Month: July 2014

Program of the month: sfltft

July 13, 2014 Programs No comments

sfltft (Local Time-Frequency Transform) decomposes input data into frequency components.

The algorithm is described in the paper Seismic data analysis using local time-frequency decomposition and is based on regularized non-stationary regression. The following example from tccs/ltft/timefreq shows 1-D synthetic data composed of two chirp signals and the magnitude of coefficients in its time-frequency decomposition:

The frequency sampling in the output of sfltft is controled by nw=, w0=, and dw=. By default, these parameters correspond to the sampling of the discrete Fourier transform. The critical parameters for regularized regression are rect= (smoothing radius in time, in samples) and niter= (number of iterations). To output the details of iterative regularization, use verb=y. The frequency sampling and the rect= parameter provide explicit controls on time-frequency resolution. In the example above, rect=7. It is possible to change smoothing radius with frequency by using alpha= parameter. The iterative inversion can be controlled additionally by specifying a data weight (with mask=) or a model weight (with weight=). Optionally, the Fourier basis used in the decomposition can be extracted from the program by specifying basis= file.

To perform the inverse transform from time-frequency back to time domain, use inv=y. sfltft takes real-valued input and produces complex-valued output.

An analogous program for transforming complex-valued data is sfcltft. A related program is sftimefreq described in Time-frequency analysis of seismic data using local attributes.

10 previous programs of the month:

Madagascar in the cloud

July 10, 2014 Systems No comments

SageMathCloud is a free cloud computing platform for computational mathematics created by William Stein, the leader of the Sage project.

SageMathCloud provides a rich environment, which allows one, for example, to easily install Madagascar and to access it interactively through its Python interface. The example above shows Madagascar running interactively in the cloud using an IPython notebook hosted by SageMathCloud. Support for interactive widgets is a new feature in IPython version 2 released earlier this year.

See also:

Making a wedge

July 1, 2014 Examples No comments

The example in rsf/tutorials/wedge reproduces the example from Evan Bianco of simple convolution modeling with a wedge model.

See also: