Geopsy: H/V and Spectrum Toolboxes: Processing Tab

Processing tab giving access to smoothing, parameters (for windows) and how is processed the H/V (available only for H/V computing)

Here are described the different possibilities offered in this tab. This tab is divided in two areas:

-Parameters area

-Horizontal components

Parameters area

Processing tab giving access to smoothing, parameters (for windows) and how is processed the H/V (available only for H/V computing)

This area, common for H/V and Spectrum computings, is divided in three different parts with three different objectives :

Smoothing

Fixing which smoothing will be applied to the windows (2 first lines). During the computing of H/V or Spectrum, the Fourier spectra can be smoothed (it is strongly recommanded) in the goal to clarify the global aspect of the curves.

For Spectrum, each Fourier spectrum (coming from a window) is smoothed and then the averaged curve is computed.

For H/V, the horizontal Fourier spectra (NS and EW, coming from a synchronous window) are first added and then the smoothing is applied on the merged horizontal Fourier spectrum and on the vertical Fourier spectrum.

There are 4 possibilities for the smoothing:

1. no smoothing. This can be sometime usefull to have a rough idea about Fourier spectra;
2. Konno and Ohmachi smoothing. This smoothing use a constant bandwidth in a logarithmic scale and is strongly recommanded because this smoothing function preserves the different number of points at low and high frequency. This smoothing is controled by a smoothing constant varying in-between 0 and 100. A constant of 0 gives a very strong smoothing, when a constant of 100 a very soft smoothing, more details;
3. Constant smoothing. This smoothing function has a triangular shape centered on the current frequency and its width is equal to "Band width" given by the user. This band width varies from 0 (soft smoothing) to 100 (strong smoothing);
4. Proportional smoothing. This smoothing function has a triangular shape and its width depends upon the current frequency. The half width is defined by percentage*frequency. The value of "percentage" cannot be greater or equal to 100% (strong smoothing).

Whatever the smoothing method used, even if the impact of the smoothing is minimized, the result will be different from a processing without smoothing.

Taper

Fixing the use or not of a cosine taper on the individual window.

In the goal to minimize the border effect due to the extraction of a window and so improve its properties in the frequency domain, it is strongly recommended to use a taper to avoid the creation of parasit frequencies.

Filter

Fixing the use or not of a filter on the whole signal.

This option allows to filter the signal before the extraction of the time windows.

To the contrary of the Waveform menu filter there is only a "High pass" filter allowed, rejecting frequencies below a given value

Sometimes, the taper size used in the pre-processing has a strong influence at low frequency. If the signal contains a strong and very low frequency component, then cutting into short time windows may strongly distort the observed spectra and H/V. Even when using the minimum window length criteria according SESAME reports (i.e. 10 times the corresponding period for the minimum frequency of interest) may be not enough to ensure a reliable H/V estimation.

In order to avoid such “tapering effects”, signals can be high-pass filtered before computing the H/V and spectra curves (Geopsy option). The higher value for the frequency of the high-pass filter is the minimum ‘reliable’ frequency, i.e. 10/window_length, more details.

Be carefull with this option. As the signal is high pass filtered, the results below the choosen frequency will display a lake of data.
If the studied frequency is close to 1 Hz and it is applied this filter at 2 Hz, the results will lead to bad data in the frequency of interest
and then to flaky interpretation.

Horizontal components area (only available for HV toolbox)

This area is devoted to indicate how the H/V has to be computed. There are 3 different methods that can be applied.

Processing tab giving access to smoothing, parameters (for windows) and how is processed the H/V (available only for H/V computing)

Squared average

The H/V is computed as followed:

• The Fourier amplitude spectra is computed for each window

• The horizontal average spectrum is computed by a combination of horizontal spectra for each window, following the formula

• The horizontal (averaged) and vertical spectra of each window are smoothed

• The horizontal to vertical ratio (H/V) is computed for each window

• The H/V is computed by averaging all the windows.

Total horizontal energy

The H/V is computed as followed:

• The Fourier amplitude spectra is computed for each window

• The horizontal average spectrum is computed by a combination of horizontal spectra for each window, following the formula

• The horizontal (averaged) and vertical spectra of each window are smoothed

• The horizontal to vertical ratio (H/V) is computed for each window

• The H/V is computed by averaging all the windows.