H/V spectral ratio

From GeopsyWiki
Jump to: navigation, search


This a short tutorial presenting a step-by-step H/V method procedure, using some of the main parameters.

For a more detailed description of the Toolboxes parameters see H/V and Spectrum Toolboxes


Introduction

H/V curve example

The technique originally proposed by Nogoshi and Igarashi [1], and wide-spread by Nakamura[2], consists in estimating the ratio between the Fourier amplitude spectra of the horizontal (H) to vertical (V) components of ambient noise vibrations recorded at one single station.


This tool is used to obtain Horizontal to Vertical (H/V) spectral ratios from any type of vibration signals (ambient vibrations, earthquake…). The example used for explanations is an ambient vibration recording.


All parameters necessary to process signal data are set up in the Toolbox.


For H/V processing, the data should have:

  • 3 signals: North-South, East-West and Vertical;
  • a common name (see here to modify the name of the component);
  • enought common samples in time to be processed.

So, the user has to be careful when the 3 signals of a same recording are loaded from 3 different files. In some early versions (before 2010-06-01), failing to fulfill those conditions may lead to results based on silently stacked signals, for instance, when signal Name is no set in file header (e.g. column text files without header).

As for the other tools, the data may be processed:

  • from a graphic viewer, to verify the windowing and to display the results;
  • from a table. In this case, Geopsy will display only the resulting curves.

The reference document for the H/V processing is the SESAME user guidelines, which is strongly recommended for further reading [3].



Getting ready

Window selection

General sub-tab in Time tab of the H/V toolbox


Raw signal sub-tab in Time tab of the toolbox with the anti-trigger parameters
  • Click on the Raw signal sub-tab in Time tab, to check that the anti-trigger parameters, used to get rid of saturated signal, are as in the example window.


Select pop-up menu
  • Click on the Select button and select Auto.


Number of selected windows
  • The number of selected windows appears at the bottom right corner of the Time tab.
    The number of windows should be as in the example.


Selected windows
  • Selected windows should appear as green rectangles on the Signal viewer.
    Notice that the selected windows are common for the three components.



Processing parameters

Processing tab in toolbox
  • Click on the Processing tab, and make sure that parameters are as in the example window.


Output parameters

Output tab in toolbox
  • Make sure that you are in the Output tab, and that parameters are as in the example window.


Data processing

  • Click on the Start button at the bottom of the toolbox, whatever tab is displayed. HV toolbox star.png


No selected windows pop-up window
  • If no window selection has been performed, a pop-up window appears.

Simply click on the Yes button and processing will follow its way.

Results

Signal display

Signal display with colored windows used for computing H/V matching with colours of individual H/V curves presented below
  • Selected windows are colored, matching colours in he H/V graphics, i.e. colour of individual H/V curves have the same colour as the signal windows.


H/V curve

H/V curve
  • The black curve represents H/V geometrically averaged over all colored individual H/V curves. The two dashed lines represent the H/V standard deviation.
  • The grey area represent the averaged peak frequency and its standard deviation. The frequency value is at the limit between the dark grey and light grey areas.


Peak frequency and standard deviation values
  • Use the cursor to get information on the H/V peak. By sliding the cursor onto the H/V graph or title, the following information appear in a yellow frame: data file name, exact value of the H/V peak frequency and its standard deviation, exact value of the H/V peak amplitude and its standard deviation, and interpretation category.


Changing H/V graphics appearance

Pop-up menu to restore a make-up
  • It is possible to change the appearance of the H/V graphics at the user's will, using a make-up.
  • As an example, first download the compressed make up file.
  • Then right click in the left hand margin of the H/V graphics.
  • Select Restore make-up in the pop-up menu and then select HV_Makeup.mkup file.


New H/V graphics appearance change after applying a make-up
  • The appearance of the H/V graphics is now at the user's taste.


Saving results

Pop-up menu to save H/V graphics

The H/V graphics may be saved in various image formats.

Image formats

  • Right click in the left hand margin of the H/V graphics.
  • In the pop-up menu choose Export image.


Directory were to save H/V graphics
  • Choose a destination folder, the image format, and a file name.


Numerical values

Numerical values can be saved in .hv text files.

  • Select all stations to save
  • Go to menu Tools/Save results

It works the same way as the Output section of the toolbox.

References

  1. Nogoshi M. and Igarashi T. (1971) On the amplitude characteristics of microtremor (part 2) (in Japanese with english abstract). Journal of seismological Society of Japan, 24, 26-40.
  2. Nakamura, Y. (1989) A method for dynamic characteristics estimation of subsurface using microtremor on the ground surface. Quaterly Report of the Railway Technical Research Institue 30 (1), 25-30.
  3. Guidelines for the implementation of the H/V spectral ratio technique on ambient vibrations measurements, processing and interpretation, [1], 62 pages, April 2005