7.3.2 Processing parameters

7.3.2.1 Windowing

Window length

Table 1: Parameters to define the windows length

Type	Input	Description
Exactly	time (s)	The time duration of each window has a constant value equal to "time". Indeed, the number of point in each window is also constant and define by: INT(time/deltaT)
At least	min_time and max_time (s)	The time duration of windows ranges between "min-time" and "max_time". If the anti-triggering criteria (STA/LTA) is turn off, then the window length is automaticaly set to the "max_time". If the anti-triggering criteria is turn on, then the window length is ajusted preferably with the maximun value "max_time", if not, with the largest possible value (ranges between "min_time" and "max-time". In this case, the number of point in each window may vary from INT(min_time/deltaT) to INT(max_time/deltaT)
Freq. dep.	number of period	The time duration of windows is frequency dependent; i.e., the window length is not constant and is equal to "number of period" * T, where T is the current period (T=1/f). the number of point in each window is not constant.

In order for a measurement to be reliable, it is strongly recommended to fulfilled the following condition: the window length has to be larger than 20-50 times the period of interest.

Anti-triggering

A detailed description of the anti-triggering algorithm can be found in section 7.1.3

To apply the "anti-trigger" algorithm on the current noise signals, check the box "Anti-triggering on raw signal" and activate the "Raw signal" tab to select the appropriate values. Indicate the values of the short term average "STA" and the long term average "LTA", as well as the values of the minimun and the maximun STA/LTA ratio in the corresponding box. You can apply the anti-trigger algorithm to all signals (vertical component), or only to some stations (check/uncheck the name of the selected station in the station name list on the right side of the tab).

Bad sample tolerance/threshold

These two options allow you to optimise the number of selected windows. When the bad sample option is turn on, the algorithm of windows selection is slighty modified: although several points do not satisfy the anti-triggering criteria, the current window can be selected. The number of bad sample which are keept to define one 'good' window is defined by INT("bad sample tolerance"/deltaT). In the same way, you can defined the total amount of 'bad signal' in each window you want to keep by turning on the "Bad sample threshold" option. In this case, you give the number of 'bad point' in term of percentage of the total number of points by window.

You can turn on these two options in the same time.

Overlapping

If the total time duration of the noise signal is too short and/or if the number of selected windows is too small, you can use the overlapping option. To turn on this option, simply click on the "overlap by" box and give the specified amount of overlapped windows in percentage (Figure 8).

The number of point overlapped at the end of each window is given by INT(time*overlap/deltaT), where "time" is the window length in second, "overlap" the amount of overlapped windows in percentage and deltaT the time step.

7.3.2.2 Time duration (Time limits)

This option allow you to select the portion of signal you want to analyse. Click on "From" and "To" buttons in the "Time Limits" tab to specify the beginning time and the ending time:

Table 2: Parameters to define the time duration of analysed signal

Field name	Option	Description
From	this time	The user specifies the value of the beginning time. Two formats are accepted: "hours:minutes:seconds" or "seconds"
	To	The beginning time is defined by the common "start time" (The delay, in seconds, between the time reference and the first sample of the signal) of all signal. If each signal doesn't have the same start time (t0s), then the differences between t0s must be a multiple of sampling period (detaT) and the largest t0s is considered as the commom "start time"
	pick{1...10}	The user speficies the beginning time using the picking tool. Select the appropriate time pick if several picks are defined
To	this time	The user specifies the value of the ending time. Two formats are accepted: "hours:minutes:seconds" or "seconds"
	End	The ending time is defined by the common "EndTime" (Time elapsed between the first and the last sample of the signal (in seconds). If each signal doesn't have the same EndTime, then their differences between must be a multiple of sampling period (detaT) and the smallest EndTime is considered as the commom "ending time"
	during	The user specifies the value of the time duration of the signal. Two formats are accepted: "hours:minutes:seconds" or "seconds"
	pick{1...10}	The user speficies the ending time using the picking tool. Select the appropriate time pick if several picks are defined

If you click on the "Test" button, then the selected signal is displayed in the graphic viewer and shaded in green. The yellow squares indicate the time window length.

Figure 1: Selection of the time limits of the signal for the 2D array wavenumber computation

7.3.2.3 Computation parameters

The following table summarizes the available keyword, value types and allowed value ranges for the processing parameters.

Table 3: description of the computation parameters

Name	Type	Unit	Description	Recommended value
Frequency range	integers	Hz	Minimum (Fmin) and maximum (Fmax) center frequencies to process	Fmin=~Fhv, where Fhv is the H/V ratio peak frequency; Fmax as large as possible
Step	'Log' or 'Lin'	none	Logarithmic ('Log') or linear ('Lin') scale for the frequency step	'Log'
Number of sample	integer	none	Number of frequencies between Fmin and Fmax	Depending on the values of Fmin and Fmax
Band width	real	none	Fraction of center frequency (F) used as half bandwidth: [F-bandwidth/2;F+bandwidth/2]; Fmin<F>Fmax	Depending on the values of Fmin, Fmax and the number of samples
Grid step	real	rad/m	Define the grid resolution for searching the maximum of beampower in the wavenumber plane	grid step < Kmin/4, where Kmin is the minimum wavenumber
Grid size	real	rad/m	Define the grid size for searching the maximum of beampower	grid size > Kmax/2, where Kmax is the maximum wavenumber
V min	real	m/s	Minimmum velocity for searching the maximum of beampower	none

7.3.2.4 Test

This tool allow you to check the reliabily of the results before the computation. Select the frequency of interest (by moving the frequency navigation bar) and click on "Test" button, then the computed beampower in wavenumber plan (Kx,Ky) appears in the 'Time window browser'. Beampower maxima are displayed in purple. By moving the "time window" navigation bar, you can visualise the beampower for a given time window, the corresponding time window is shaded in red in the graphic viewer. At your convience, you can also change the value of the testing frequency.

Figure 2: Test of the capabilities of the FK or HRFK to estimate the phase velocity for a given frequency and a time window

7.3.2.5 Power spectrum maxima

7.3.2.6 Output files and start the processing

Give the name of the file (.max) where the results will be saved in the dialog box "Output file". Alternatively you can choose an existing file by opening the browser and select the wanted path. To do this, click on the icon .

To start the computation, click on "Start" button. The status of the computation is automatically written at the bottom left side of the geopsy sheet.

If necessary, stop the computation by clicking on the "Stop" button. In this case, only the partial results will be saved.