This article is taken from a part of book draft: Rosyidi, S.A., Taha, M.R., Chik, Z. & Nayan, K.A.M., 2007, Seismic Surface Wave Method for Geotechnical Engineering.
The goal of field measurement in the SASW testing is to determine the phase differences between two receivers over a wide range in frequencies. A completed investigation of each soil site using the SASW testing consists of the following phases :
1. field testing,
2. developing of the an experimental dispersion curve, and
3. inversion of the dispersion curve to generate a site profile.
The objective of this paper is to describe the field procedures for conducting the SASW measurement in the soil site for geotechnical investigation. The discussion about the field testing and configuration phase is presented herein.
2. TESTING EQUIPMENT
The testing configuration used in the field is shown in Figure 1. The measurement configuration consists of a source, two receivers and a data acquisition unit. The brief explanation of each equipments of SASW method are discussed in the following section.
Figure 1. SASW testing configuration
A source for SASW measurement should be able to generate energy of surface waves over a wide range in frequencies with adequate amplitude so that they can be detected by the receivers. In this measurement, the type of transient source is used. The transient source corresponds to impact sources like small hand held hammer, sladge hammer and drop weight. These impact sources are the most common for SASW measurements. The reason is portability, ruggedness and ease of use. In general, the heavier impact source will generate the lower frequencies. However, the same source does not always generate the same frequency at all site. Besides, the material and weight of the impact source, there are other factors which control the range of generated frequencies. These factors include the stiffness of the material profile and the impact hammer strikes (Nazarian, 1984; Joh, 1996). Consequently, the selection of the impact source is often made after trying several sources in situ.
For sampling the shallow depth, the maximum frequency excited is of most important. It is not necessary to transfer much energy to the medium because the receivers and the source are placed close to one another. High frequencies translate to short wavelength, which correspond to the shallower depths of sampling. For determining of the soil properties of relatively deep layers, the energy couple into the medium is of greater importance.
Selection of appropriate receivers is necessary in any seismic tests. There are two types of receivers which are usually used in the SASW measurement: velocity transducers (geophones) and acceleration transducers (accelerometers). The geophones are coil-magnet system. A mass is attached to a spring and a coil is connected to the mass. The geophone system can be considered as a one-degree-of-single-freedom system. The limit of lower frequencies which is translate to sampling of deeper layers is limited by the natural frequency of the receivers as well as frequencies being generated by the source.
c. Spectral Analyzer unit
A convenient recording device for performing SASW measurement is a spectral analyzer. Spectral analyzer is a digital oscilloscope that, by means of a microprocessor attached to it, has the ability to perform signal analysis directly in either the time or frequency domain. The spectrum operations should be set up in a spectral analyzer device before starting a measurement. The spectrum operation of transfer function, coherence function, cross power and auto power are needed to display in the analyzer.
3. FIELD TESTING
The general procedure used in performing the SASW testing in the soil site can be summarized as follows:
Determine a series of receiver spacings required to obtain the necessary range of the waveleghts to sample the soil site. The spacings are based on guess or a priori information of the shear wave velocity profile (velocities and depths) of the material at the site. For the shortest wavelength, the receiver spacing is set to one to three times of the minimum wavelength. If a measured phase spectrum has good quality up to only two cycles, the receiver spacing to generate the minimum wavelength for example one meter, has to be shorter than two times or two meters. For largest wavelength, the receiver spacing is set to a half or a third of the maximum wavelength. Once the minimum receiver spacing is determined, the next spacing is conventionally determined by doubling the previous receiver spacing. This calculation is followed until the maximum receiver spacing is reached. Doubling the receiver spacing has been found to given enough overlapping of the wavelengths between the adjacent receiver spacing to determine a robust dispersion curve. The SASW measurements usually start with smallest receiver spacing.
Select a source and receivers appropriate for the frequency range under consideration. The frequency range is strongly influenced by the receiver spacing. The frequency range is calculated using the relationship between shear wave velocity, wavelength and frequency.
Place the receivers at the locations determined by the common receiver midpoint configuration illustrated in Figure 2. Before place the receivers, an imaginary centerline for the receiver array is selected. Two receivers are placed with an equal distance from the centerline. For a better measurement, the distance between the source and near receiver is equal to the distance between two receivers (Figure 2). This configuration can reduce the near field effect in the measurement. The receivers should be well coupled to the material (soil site) so that both receivers monitor the ground motion correctly and no disruptive phase shifts happen because of different receiver responses.
Figure 2. The common receiver midpoint configuration
Use the mechanical source to excite the ground and obtain the resulting vertical responses at two receivers. The signal averaging which is summing multiple measurements in the frequency domain is preferred to eliminate random noise and incoherent signals.
Calculate phase velocities at several frequencies from the phase spectrum, if the phase spectrum can be determined in situ by a spectrum analyzer. Check if the calculated phase velocities are reasonable agreement with initial assumption. It is a good strategy to have two or four cycles in the phase spectrum to have good resolution in the dispersion curve (Joh, 1996).
Reverse the location of the source at each receiver spacing whenever possible. The measurements using the reversed location of the source are called reverse measurement and the measurement with original location of the source are called forward measurement. The result of the forward and the reverse measurement are averaged in the frequency domain to minimize the effects of dipping layers, to reduce effects of lateral inhomogeneity between the source and the receiver, and to compensate for any potential phase differences in the measurement equipment.
Change the measurement set-up to the next receiver spacing while maintaining the common midpoint between the receivers, and repeats steps 2 to 6 until the measurements for all receiver spacing are completed.
The testing procedures of the SASW measurement were briefly reviewed. The field measurements of SASW were summarized to cover the testing equipment, SASW configuration and field testing procedures.
Joh, S.H., 1996, Advances in interpretation and analysis techniques for spectral-analysis-of-surface-waves (SASW) measurements, Ph.D. dissertation, the University of Texas at Austin.
Nazarian, S., 1984, In situ determination of elastic moduli of soil deposites and pavement systems by spectral-analysis-of-surface-waves method, Ph.D. dissertation, the University of Texas at Austin.