Geopsy: H/V and Spectrum Toolboxes: Time Tab

Use

This toolbox tab contains the parameters to select the part of the signal used for H/V or Spectrum processing on which window selection is performed. The two series of paramaters are described in two areas:

• the global time range area

• the time windows area

A third area gives information on the number of selected windows obtained after applying to the signal the parameters of the two precedent areas.

At the very bottom two buttons indicate wether to load parameters from previouly processed data or to start signal processing signal processing.

Time tab of the H/V and Spectrum toolboxes

Global time range area

Select the portion of the signal that will be processed (more informations).

Global time range area in the Time tab of he H/V and Spectrum tollboxes

Time windows area

Short-duration disturbances of the signal can be avoided during H/V or Spectrum processing by using an anti-trigger window selection to remove the transients. The objective is to keep the most stationary parts of ambient vibrations, and to avoid the transients often associated with specific sources (footsteps, close traffic). This objective is exactly the opposite of the usual goal of seismologists who want to detect signals, and have developed specific trigger algorithm to track the unusual transients possibly indicating the satrt of an earthquake. Here an anti-trigger algorithm is used, which is exactly the opposite: it detects transients but it tries to avoid them. For more information on this topic it it suggested to consult the SESAME user guidelines . Windows are then processed indidualy to get individual window H/V or Spectrum, which are then averaged.

This area comprises four sub-tabs:

• general

• raw signal

• filter

• filtered signal

General sub-tab

This sub-tab contains the parameters used to select stable signal windows.

General sub-tab in the Time windows area of the Time tab

Window length

There are three ways to define window length (duration, in seconds)

• exactly: all selected windows will have the same exact duration. Only one value needs to be entered.

• at least: selected windows will have a duration comprised between two values.

This may be useful in case of very perturbated signal to get the most possible windows.

Window Overlaping

• When the Overlap box is not checked

contiguous selected signal windows will not overlap, resulting in this type of windowing

• When the Overlap box is checked

contiguous selected signal windows will overlap by a chosen percentage, resulting in this type of windowing

Bad Sample Definition and Use

• It is possible to define a level above which a sample is considered as bad, i.e. that should rejected when defining stable windows.

To do so, check the Bad sample threshold box and enter the desired threshold, which is a percentage of the maximum amplitude over the entire signal.

• A maximum number of bad samples may be accepted inside windows during the selection process.

Check the Bad sample tolerance button and enter the number of tolerated bad samples. This option may only be used when the previous one is checked.

Anti-triggering

The goal of the anti-triggering is to keep the most stationary parts of ambient vibrations, and to avoid the transients. Eventualy, user's defined bad sample are also considered as part of the non-stationary signal.

Two types of anti-trigger may be applied to he signal, both based on STA/LTA detection. Italic text

• Anti-trigger on raw signal

When the Anti-triggering on raw signal box is checked, the anti-trigger is applied sample per sample all along the entire signal, using the STA/LTA parameters entered in the Raw signal sub-tab.

• Anti-trigger on filtered signal

When the Anti-triggering on filtered signal box is checked, the signal is first filtered using the parameters entered in the Filter sub-tab, and then the anti-trigger is applied sample per sample all along the entire signal, using the STA/LTA parameters entered in the Filtered signal sub-tab.

IMPORTANT NOTICE: when using the anti-triggering on the filtered signal, the signal is filtered ONLY to select stable windows.
The processing is then performed on the RAW signal.
In order to process filtered signal the raw signal should first be filtered with the Filter in the Waveform menu.

Raw Signal

This sub-tab is divided into two areas.

[[Image: Time-time_antitriggering.png|300px]

• STA/LTA parameters area (to the left), includes STA length, LTA length, as well as minimum and maximum STA/LTA thresholds.

• Apply to area (to the right), indicates to which tracks the anti-trigger is applied (top, above the … : ) and to which file (bottom underneath the … : "Strong Peak").

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