Spectral amplitudes

This tool is used to obtain spectral amplitude from any type of vibration signals (ambient vibrations, earthquake…). The example used for explanations is an ambient vibration recording.

Getting ready

• Download compressed signal file. If the file has been already downloaded (as for H/V purposes for example), do not download it again.
• Load and view these downloaded signals.
• Select Spectrum in menu Tools (alternative ways to start a tool)

Window selection

General sub-tab in Time tab of toolbox

• Make sure you are in the Time tab, and that parameters are as in the example window. The Anti-triggering on raw signal button has been checked.

Raw signal sub-tab in Time tab of toolbox with the anti-trigger parameters

• Click on the Raw signal, to check that anti-trigger parameters are as in the example window. As we want the same windowing as in the H/V processing, the Common button should be checked.

Select pop-up menu

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

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. Here, the windows are synchronous on the three components as we required a common windowing.

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 you are in the Output tab, and that parameters are as in the example window.

Data processing

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

No selected windows pop-up window

• In case no window selection has been performed, a pup-up window appears.

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

Results

Signal display

Signal displayed with colored windows used for computing spectrum matching with colours of individual spectrum curves presented below

• Selected windows are colored, matching colours in the spectrum graphics, i.e. colour of individual spectrum curves have the same colour as the signal windows.

Spectrum curves

Timing tab in toolbox

• As the spectrum has been computed on each components, the results are displayed for each component. So the graph is divided in three different curves.
• In each graph, the black curve represents the spectrum averaged curve over all colored individual spectrum curves The two dashed lines represent spectrum standard deviation.
• The greyed areas represent the frequency peak and its standard deviation. The frequency value is at the limit between the dark and clear areas.

Peak frequency and standard deviation values

• Use the cursor to get values on the spectrum curves. The exact value of the spectrum peak frequency and its standard deviation may be obtained by sliding the cursor onto the spectrum curve title (Strong_peak Z, Strong_peak N, Strong_peak E) in the upper right corner above the graphics.

Spectrum results displaying the 3 computed averaged curves

• For the spectrum computation, a synthetic graph displays the averaged spectrum curves coming from each waveform.

Further readings

• Spectral ratios along a profile

• Mapping spectral ratios