## 3 Field Investigations
### Geophysical Investigations  
In the country, the most popular method among geotechnical engineers for assessing the strength of soil at a site is through standard penetration testing. Cross- or downhole profiling is also common to obtain in situ measurements of shear wave velocity with depth. These techniques may be precise and can provide measurement at, typically, one-meter resolutions. However, these methods can also rather be difficult and expensive to carry out in urban areas, as they require borehole drilling. Hence, non-invasive geophysical techniques are also utilized to overcome the problems posed by the practicality of borehole drilling. Seismic exploration techniques, e.g., multichannel analysis of surface waves (MASW), spectral analysis of surface waves (SASW), and refraction microtemors (hereafter \"ReMi\") are relatively more economical, faster, and can be performed in a variety of sites, including urban areas. This study utilized ReMi surveys to derive one-dimensional shear wave velocity profiles of the surveyed sites in Metro Manila. The ReMi technique was first developed by [PERSON] (2001) ([PERSON], 2001), to demonstrate the application of ambient noise in determining the shallow shear wave velocity structure of a site. The idea behind this technique is that the vertical component of ambient noise, primarily dominated by Rayleigh waves that are caused by human activities, can be picked up by a geophone array. In this study, ReMi surveys were performed at the twenty sites in Metro Manila where the SPT investigations were performed. The surveys are done by coupling a straight linear array of twelve vertical geophones into the ground. These geophones have a frequency of 4.5 Hz and were arranged to have a spacing that ranged from four to eight meters. The array was then connected to a DAQLink II multichannel recorder to gather thirty iterations of thirty seconds unfiltered noise. The sampling rate of the recording was set at \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Site No.** & **Name** & **City** & **Long. (\({}^{\circ}\))** & **Lat. (\({}^{\circ}\))** & **Water Table** & **Depth (m)** & **Soil Type (USCS)** \\ \hline
18 & & & & & & 0–1.5 & GM \\  & & & & & 1.5–3 & GP \\  & & & & & 3–4.5 & SP-SM \\  & & & & & 4.5–6 & SM \\  & & & & & 6–7.5 & MH \\  & & & & & 7.5–12 & SM \\  & & & & & 12–13.5 & CL \\  & & & & & & 13.5–15 & MH \\ \hline
19 & & & & & & 0–1.5 & SC \\  & & & & & & 1.5–3 & SP-SM \\  & & & & & & 3–4.5 & SM \\  & & & & & 4.5–6 & ML \\  & & & & & 6–7.5 & MH \\  & & & & & & 7.5–9 & GC \\  & & & & & & 9–10.5 & ML \\  & & & & & & 10.5–12 & MH \\  & & & & & & 12–13.5 & CL \\  & & & & & & 13.5–15 & CH \\ \hline
20 & & & & & & 0–1.5 & SP-SM \\  & & & & & & 1.5–4.5 & SW-SM \\  & & & & & & 4.5–6 & SP \\  & & & & & & 6–7.5 & SM \\  & & & & & & 7.5–9 & SP-SM \\  & & & & & & 9–10.5 & CL \\  & & & & & & 10.5–12 & MH \\  & & & & & & 12–13.5 & CL \\  & & & & & & 13.5–15 & SC \\ \hline \end{tabular}
\end{table} 
Table 1: Cont. 2 ms (500 Hz). In the surveyed sites, typical sources of noise include pedestrians, vehicle traffic, construction, and wind. To provide high frequency noise for the recordings, a 10 lbs. sledgehammer was introduced, to hit a 6 inches \(\times\) 6 inches \(\times\) 1 inch steel plate placed at each end of the array, and which was utilized as an active noise source. Data processing was done using ReMi Vspect and ReMi Disper modules. The Rayleigh waves were identified from other wave arrivals using slowness-frequency (p-f) transforms of the noise recording, using the ReMi Vspect module. The fundamental mode phase velocity was chosen along the minimum velocity envelope of energy within the p-f domain. [PERSON] (2001) [47] demonstrated that selecting along this trend is the best procedure for choosing a dispersion curve to obtain the best estimate of the true phase velocities of shear waves (Figure 2A). The ReMi Disper module allows the modelling of a dispersion curve with multiple layers and shear wave velocities, to match the dispersion curve of the field data.