Active source experiment (MASW)

General overview

First introduced by Al-Husseini et al. (1981), Mari (1984), Gabriels et al. (1987), the Multichannel analysis of surface waves was popularized by Park et al. (1999). This technique relies on the recording along a 1D linear profile of seismic signals produced by a controlled source (hammer, vibrator, explosion, etc.) and analysis of surface wave dispersion properties after applying slant-stack or FK transform to the recorded seismic section. In the following tutorial, only the FK transform is used.

Getting ready

Exemple of a seismic section.

• Download the compressed signal file.
• Launch Geopsy and set Rxxx as name in the Geopsy Loading Preferences.
• Load and view these downloaded signals. A seismic section as the one displayed in the figure on the right should then appeared.
• Check in the Table signal viewer that sensors location is ranging from 0 to 92 meters with a 4 meters spacing.
• Check that source location is X=-4 meters by editing Set sources in Geopsy Edit menu. Alternatively, you can also add the Source X field in the Table signal viewer.

Description of the MASW toolbox

Overview of Masw toolbox

Open the MASW toolbox by pushing the following plugin icon or, alternatively, by selecting Linear F-K for active experiments in Tools menu. Two windows should then open as displayed in the figure on the right: the Linear FK toolbox to set up processing parameters and a Linear fk results to display MASW results. The Linear FK toolbox is composed of four tabs:

• Pre-processing tab is used to define the time window, the tapering, the amplitude normalization, etc.
• Processing tab allows to define the processing scheme (FK or HRFK)
• Output tab is used to set up the frequency and slowness band to be processed and the output filename
• Time tab allows to select the time limits and part of the signals to be processed and the time window length;
• Curves tab is used to save, remove or load dispersion curves that are picked

Parameters setting

In Pre-processing tab:

• Select the minimum and maximum source-to-receiver distance which will be processed. This allows to exclude signals close to the source shot (this distance should be at least one wavelength in order to reduce near-field effects) or at large distances for which the signal-to-noise ratio may be low.
• Select the time limitsto be processed. A taper is then applied to the selected time window.
• Finally, by checking the Normalization check box, one can specify the amplitude normalization of the seismic traces. This normalization can be set to 1/sqrt(distance along the profile) to account for geometrical spreading of surface waves or to 1/distance. maximum amplitude and spectrum energy allow normalizing by the maximum amplitude and energy in the spectral domain, respectively.

• HRFK (cross-link avec HRFK de Mathias)
• normalize energy

FK Computing

• computation for one single file and picking of the curve
• alternatives : 1) create a stack file and then compute the FK; 2)compute FK for all shots and then stack the FK maps

FK Computing

• computation for one single file and picking of the curve
• alternatives : 1) create a stack file and then compute the FK; 2)compute FK for all shots and then stack the FK maps