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 sections:
- the global time range section
- the time windows section
A third section gives information on the number of selected windows obtained after applying to the signal the parameters of the two precedent sections.
At the very bottom two buttons indicate wether to load parameters from previouly processed data or to start signal processing signal processing.
Global time range section
Used to select the portion of the signal that will be processed (more informations).
Time windows section
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 [1]. Windows are then processed indidualy to get individual window H/V or Spectrum, which are then averaged.
Time window section in the Time tab of he H/V and Spectrum tollboxes
This section 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.
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 possibilities
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.
- Anti-triggering 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-triggering 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.
All combinatins of checking the boxes are possible, including both boxes unchecked.
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 sub-tab
This sub-tab is divided into two sections.
- STA/LTA parameters section (to the left), includes STA length, LTA length, as well as minimum and maximum STA/LTA thresholds (more information).
- Apply to section (to the right), indicates to which tracks the anti-trigger is applied (top, above the … : e.g. Vertical, North, East) and to which file (bottom underneath the … : e.g."Strong Peak").
Simply check the boxes of the tracks and files that are to be processed.
Filter sub-tab
Contains the filtering parameters used in the Filtered Signal sub-tab (more details).
Filtered Signal sub-tab
This sub-tab is divided into two sections.
- STA/LTA parameters section (to the left), includes STA length, LTA length, as well as minimum and maximum STA/LTA thresholds (more information).
- Apply to section (to the right), indicates to which tracks the anti-trigger is applied (top, above the … : Vertical, North, East) and to which file (bottom underneath the … : "Strong Peak").
Simply check the boxes of the tracks and files that are to be processed.
Widowing and data flow sub-section
Two boxes and one pop-up menu located at the bottom of the Time windows section are used to control windowing and real time data acquistion flow.
- Update box: should be checked when using geopsy for real time data acquisition. Checking the box when no data flow is entering as no effect on the processing.
- Common box: check this box to force selecting the same windows on all signal tracks, i.e. STA/LTA threshold must be verified simultaneously on all tracks in order to keep a window. When the box is unchecked, STA/LTA threshold must be verified on each track taken separately. There is no need to check this box in H/V processing, as in this case it is automatically applied.
