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Calculation Of Relative Arrival Times Displaying Array Channel Outputs |
Setting Array Processing Parameters
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This is a Windows application for processing data from medium aperture seismic arrays of the U.K.A.E.A. type. It can be used for the EKA,GBA,YKA and WRA. The application has been built using visual C++5.0. It can be built and executed either using the NMAKE facility or in the interactive development environment (IDE). The data files must be made available in the same folder as the program.
(1) Calculation Of Relative Arrival Times : The application calculates the relative arrival times of a seismic signal travel across the array with a specified apparent phase velocity, and from a specified direcion.
(2). Displaying Array Channel Outputs : The application reads the data from the binary data file (which is the standard at the time of writing the application), and displays it channelwise on the monitor screen. The statistical paramerters of the data channels can be examined by selecting that item from the Applications Menu.
(3). Synthetic Data : A provision has been made to synthesize array data to allow testing the applications functionality.
(4). Array-Beam-Forming: The application enhances signal-to-noise ratio in the ground motion wave forms recorded by the array. In the process of, what is called Array-Beam-Forming, outputs from the array sensors are added in to groups with 'appropriate' time shifts, so that the signal is added in phase and the noise is added out of phase to form two sets of Array-Phased-Sums corresponding to the two arms - the Blue and the Red Arms-of the array. The two sums are then multiplied to form a "Time-Averaged-Product (TAP).
(5). Estimating Signal Directions : Array beams are formed with combinations of different apparent velocity and azimuths. Apparent velocity is used in the application in its reverse form (1.0/apparent velocity), which is called slowness, and has the units of seconds per degree. The process of estimating directions is based on a 'slowness-azimuth' search in which TAP values are determined for combinations of azimuth and slowness, and that combination which corresponds to the hihest value is taken as the estimate of the signal direction (the horizontal direction determines the slowness whereas the vertical direction determines the azimuth. The search is carried out in two steps:
(a) Coarse Search: To begin the search for the signal directions, the slowness is varied between 14.0 and 7.5 seconds per degree in steps of -0.3 seconds per degree. For each value of the chosen slowness the azimuth is varied between 0 and 360 degrees in steps of 10 degrees. This process provides a course estimate of the slowness and azimuth of the signal.
(b) Fine Search: After the coarse search estimates are obtained the search parameters are modified. The slowness is varied in steps of 0.1 seconds per degree and the azimuth in steps of 1.0 degree around the values obtained in the coarse search.
(6) TAP Matrix Display: The TAP values calculated during the two types of searches can be displayed using the Applications menu. These values are also stored in ASCII files.
(7) Location of The Epicenter: Values of the apparent velocities for seismic signals arriving from different epicentral distances, as given in the travel time tables of Jefferey and Bullen, are stored in tabular form in the applications code. The epicentral distance of the source of the seismic signal is obtained by determining the distance corresponding to the results of the search, in the coarse as well as the fine searches. The epicentral distance and the azimuth are then converted into latitude and longitude of the seismic source.
(8) Seismic Region: Seismic region numbers and their names for different latitude and longitude combinations, as fixed by Engdhal and Flinn (1968) are stored in the applications code along with the seismic region retrieval codes. A seimic region is assigned to the seismic source located by the search processes.
(9). Search For Multiple Phases: Several phases follow the first arrival in a seismic signal at different times. A slowness search is carried out by fixing the azimuth on the basis of the known epicenter. The slowness in the seismic wave train is then plkotted along with the enhanced array sum as a function of time.
(10). Fourier Transforms : Fourier Transforms of any of the array channels can be computed by selecting the chanannel from the Applications menu. 256 data points are used for computing the Fourier Transforms.
(11). Excluding Low Quality Data Channels : The application allows exclusion of data from the channels which show a poor signal-to-noise ratio.
(12). Computation Tools : Programs used frequently in different types of computations in seismology and earthquake engineering, which a user needs from time to time, are made available in the 'Tools' Menu.
Setting Array Processing Parameters
