The data analysis procedure is demonstrated from source/receiver (SR) setup to dispersion imaging steps, while the remaining steps will be identical to those demonstrated in the active data sets. Because the most common way of utilizing passive data is to provide useful dispersion information at such low frequencies where active data usually does not provide objective dispersion trends, it is also demonstrated to combine the passive and active dispersion images. All passive data sets are synthetic (model) data created by using the reflectivity modeling module included in the main menu [see the user guide "Modeling (Seismic Data)"]. A 48-channel acquisition of relatively short recording time (T≈8 sec) with a 4-ms sampling interval (dt) was used during the modeling. Although the actual recording time for a passive survey will be usually much longer than this (for example, T = 30 sec), the short recording time was due to the limitation in the modeling module. Generation of surface waves (from 5 to 100 Hz) was simulated during the modeling by placing multiple (4) active source points at a certain distance (i.e., approximately the same distance as the dimension of the receiver array) away from the 2-D RA distributed along the full 360-degree azimuth range with an equal interval of 90 degrees as shown in Figures 1-4. Excitation time of each source point was modeled with a 2-sec interval between the two successive points (for example, 1- sec, 3-sec, 5-sec, and 7-sec for source points at 0 degrees, 90 degrees, 180 degrees, and 270 degrees, respectively). This azimuth and source excitation time information is obtained as by-products during the dispersion imaging process by Park (2010) (see the user guide "Dispersion Image Generation" for more details). How to display this information after generation of the dispersion image from the passive data set is also demonstrated.
Active/Passive Combined Survey Data Set
A real data set of 24-channel acquisition with 120-sec recording time (4-ms sampling interval) is provided in "...\Combined\RDMASW.dat." This data was acquired along the roadside (RD) of a local highway. There are a total of eighteen (18) field records included in the data set. It was acquired during a roll-along active MASW survey using a land streamer of 4.5-Hz geophones with 4-ft interval spacing (i.e., dx=4 ft) that moved over a relatively short surface distance of 18 successive shot points separated by 8-ft (i.e., dSR=2dx) for the purpose of experimentation (Figure 1). A sledge hammer (10-lb) was used as the source to deliver an impact at 24-ft (i.e., X1=6dx) ahead of the first (1st) channel. All acquisition geometry parameters (X1, dx, and dSR) were identical to those used during the active survey. However, recording parameters of T=120 sec with dt=4 ms were used, which are significantly different from those used during the active survey (i.e., T=1 sec with dt=0.5 ms). The objective with this combined-survey was to demonstrate the advantage of this longer recording time adopted during an active survey (making it an active/passive combined survey) so the chance of capturing lower frequencies (longer wavelengths) of surface waves from ambient vibrations of cultural (e.g., traffic) and/or natural (e.g., ocean surf activities) origins are increased. This advantage will eventually lead to the generation of a velocity (Vs) profile (1-D or 2-D) with the deepest investigation depth ever possible with a given acquisition configuration and/or a velocity profile with the most accurate bedrock velocity (or velocity at depths in general) as a result of including lower frequencies in the analyzed dispersion curve. In theory, two conditions have to be met to increase the investigation depth (or velocity accuracy at depths)—generation of low frequency (long wavelength) components of surface waves and the use of a long receiver array to capture such low frequencies (long wavelengths). The former is the condition to generate such surface waves responding to subsurface velocity (Vs) at deep depths, whereas the latter is the condition to analyze propagation properties (phase velocities) of such low frequency components as accurately as possible. Increasing the recording time will make the receiver array "listen" to the ambient vibration right after it finishes "listening" to the active surface waves coming from the active source point. In this way, the chance of recording lower-frequency components will be increased. However, in theory, accurate analysis of phase velocity for these components requires the use of a "wide measurement aperture", which is the long receiver array. This indicates that recording low frequency by itself will be limited in increasing the investigation depth (or accuracy of velocity at depths) unless accompanied by the use of an accordingly long receiver array (RA). On the other hand, the use of a long RA decreases lateral resolution in the final output of a 2-D Vs cross section. In reality, therefore, the advantage of the combined survey will be maximized only when a moderately long receiver array (e.g., > 100-ft) ─ a trade-off between investigation depth and lateral resolution ─ is used at the place where relatively strong ambient vibration prevails with frequencies (e.g., ≤ 15 Hz) lower than those expected in active surveys (e. g., ≥ 15 Hz).
The sample data set "RDMASW.dat" was obtained along the roadside of a local highway using a 10-lb sledge hammer source to trigger a 120-sec recording at each place of measurement. To illustrate the advantage of this survey in comparison to the active survey with a short recording time (T=1 sec), a set of active data was prepared by selecting only the first 1-sec portion of all 18 field records and then the normal active-data analysis procedure was applied to it. Figure 6a shows the average dispersion image obtained by sacking all (18) individual dispersion images generated from this active data set, and Figure 6b shows the average dispersion image obtained from the combined-survey data set of full 120-sec recording time. The latter dispersion image clearly shows more energy at lower frequencies (e.g., ≤ 15 Hz) than the former dispersion image does. Figures 7a and 7b show the 2-D Vs cross sections obtained by processing individual dispersion images included in each set of dispersion-image data. The same investigation depth of 50 ft, which is considered the optimal depth for the active data set, was used for construction of a combined-survey Vs cross section. Both sections show relatively shallow bedrock (≤ 10 ft) with a mild lateral topographic variation. Overburden velocities are shown less than about 800 ft/sec in both sections. However, bedrock velocities are in a range of 2000-3000 ft/sec in the active section, whereas they are 3000-4000 ft/sec in the combined-survey section. Bedrock velocities in the latter section would be more reliable.
Figure 1.Configuration of circle receiver array and incoming direction of surface waves.
Figure 2. Configuration of cross receiver array and incoming direction of surface waves.
Figure 3. Configuration of L-shape receiver array and incoming direction of surface waves.
Figure 4. Configuration of random receiver array and incoming direction of surface waves.
Figure 5. Source/receiver (SR) configuration used during the combined survey.
Figure 6. Average dispersion image for the first 1-sec portion (a) and the entire 120-sec portion (b) of the combined survey data set.
Figure 7. 2-D shear-wave velocity (Vs) cross sections obtained from the first 1-sec portion (a) and the entire 120-sec portion (b) of the combined survey data.
Four (4) passive survey data sets are provided that were recorded using four (4) different types of 2-D receiver arrays (RA's)—circle ["...\PAS(Circle-RA).dat"], cross ["... \PAS(Cross-RA).dat"], L shape ["...\PAS(L-RA).dat"], and random ["...\PAS(Random-RA).dat"]. A passive data set recorded using a 1-D (linear) receiver array is considered identical to the data set from the active/passive combined survey described below.