How to Acquire Data for Vs30m Evaluation
Most typical MASW data acquisition for seismic site characterization (SSC) requires a 24-channel seismic acquisition system consisting of 24-channel seismograph, seismic cable with 24 hookups (takeouts), 24 low-frequency geophones (e.g., 4.5-Hz ones), and a heavy sledge hammer (e.g., 10 lb or heavier). The figure below illustrates a typical configuration of all necessary components.
Proper setting of acquisition parameters is critical for a successful survey. They consist of recording and geometry parameters. The former relates to setting those to operate seismograph properly, whereas the latter deals with proper spatial configuration of seismic source and receivers. Sampling interval (dt) and recording time (T) are the most important recording parameters, and receiver spacing (dx) and source offset (X1) are the most critical geometry parameters. Tables 1 and 2 below summarize the most optimum values of these two types of parameters when investigating the top 30 m to produce Vs30m values.
The acquisition software of most seismographs allows the user to setup the surface coordinates of both source and receiver locations in the file saved. If this is correctly executed during the survey, it will significantly simplify the post-acquisition data-analysis procedure by skipping the step of the "acquisition geometry setup", which is usually one of the most time-consuming and error-prone steps in data analysis.
The geometry parameters (dx and X1) influence the maximum investigation depth (Zmax). In general, a longer receiver spread associated with a longer receiver spacing ensures a deeper Zmax. Also, a longer source offset (X1) for a given receiver spread is necessary to ensure the high quality of surface waves for relatively longer wavelengths by minimizing as much as possible some harmful effects, such as near-field effects. Longer wavelengths are necessary for the analysis of deeper depths.
*The receiver spacing must be chosen in such a way that the total array length becomes about twice as long as the investigation depth (i.e., >= 60 m). **Source offsets (X1's) are denoted by number of receiver spacings (dx), and can be applied on both sides of a given receiver spread (i.e., forward and reverse shots, see figure below).