Blockfilter - 1D Forward Modeling WebApp

Introduction:

During interpretation of layered sequences using band limited seismic data, it is useful to have some idea regarding the resolution of thin beds. This simple program shows the effects of filtering sequences of differing acoustic impedance blocks. The blocks themselves are defined by the times to reflecting horizons with the acoustic impedance corresponding to the layer above, similar to the way simple velocity models are used for time to depth conversion.

In addtion, the analytical amplitude spectra, with and without, filtering of the acoustic impedance and reflection sequences are computed. These can be compared with each other to show the information lost by the filtering process as well as demonstrating the accuracy of amplitude spectra obtained by calculating the discrete Fourier transform (DFT) over a window from corresponding, sampled, filtered traces.

Parameter Input:

The layering model and filter parameters are defined in the form that is displayed when Blockfilter, below is selected. Initially, this form is populated with an example set of values demonstrating the format for describing the model.

The user has a choice of filtering with either a trapezoidal Ormsby filter, defined by its four corner frequencies, or a Ricker wavelet, defined by its peak frequency. The filter and trace parameters are entered in the appropriate boxes. The layering model is defined as a set of horizon time / impedance of the layer above pairs in the lower text box.

The trapezoidal Ormsby filter is defined by its corner points such that 0 ≤ f1 ≤ f2 ≤ f3 ≤ f4. Frequencies below f1 and above f4 are not passed. Between frequencies f2 and f3 there is no attenuation. The filter strength between f1 to f2 and f3 to f4 varies linearly.

The Ricker wavelet based filter is defined by its peak frequency.

The output sample interval is used to define the filtered traces. The trace samples can be optionally displayed.

The DFT window defines the range of samples that are used to calculate the discrete Fourier Transform of the filtered traces. The amplitude spectra, of sampled traces, computed in this way can be compared with the analytical spectra of the model.

The cosine taper is applied during computation of the DFT in order to help suppress artifacts caused by the finite window of samples.

The impedance model is defined in the text area below the parameter input boxes. The symbol '#' is used to indicate that the rest of the line, in which it occurs, is a comment.

The model comprises a series of horizon time / impedance of the layer above pairs. The times are in milliseconds and represent the end of the block with the associated impedance. Thus the first pair represents the background impedance and the time of the start of the first layer. The last time pair represents the end time and impedance of the last block. Times later than this in the model have the background impedance value assigned. The background can be considered to extend to infinity in both directions, thereby eliminating artifacts created by a finite length model. Thus a typical input would be:

200 1 # first horizon at 200 ms with a background impedance of 1 unit extending infinitely upwards

204 2 # Second horizon at 204ms, with an impedance of 2 units in the layer above.

210 1 # Third horizon at 210ms, with an impedance of 1 unit in the layer above.

230 2 # Time of the last horizon, which has an impedance of 2 units in the layer above. Below this, extending to infinity, the model reverts to the background value of 1 unit as defined in the first pair.

The 'submit' button selects calculation and display of the filtered traces and the associated spectra.