Seismic imaging: Data acquisition
We acquired 9-½ miles (15.2 km) of seismic data in less than 8 working days, see Figure 1.With the assistance of IDOT District personnel for traffic control, data acquisition proceeded at a rate of more than 1 mile/day (1.6 km/day) with a shot spacing of 20 ft (6 m) and a receiver spacing of 10 ft (3 m) (see table 1 for the recording parameters). The energy for the system was derived from the impact of an accelerated 100-lb weight dropped onto the pavement. The energy pulse is transmitted to the ground beneath the road through an aluminum plate. Initial testing indicated that the weight bounced on concrete pavement, producing a second pulse about 100 ms after the initial impact. The second pulse caused considerable noise in the data, often obliterating the true signal. We found that we could dampen the bouncing of the weight by inserting a lead plate beneath the aluminum impact plate. This adaptation did not affect the initial energy pulse, but the deformation of the lead consistently dampened the bouncing of the weight and eliminated the second pulse.
Figure 2 shows three raw shot records chosen from the survey to illustrate some of the processing issues that had to be addressed in this study. The first record was acquired above the old 80 m deep clay mine. Deep reflections are almost missing and major static shifts are present in this record. This time shift is corrected by alignment of the head waves (Fig. 2.C). The other two records illustrate the extreme variation in signal frequency obtained in this survey. High frequencies present in Record 2 are missing in Record 3. Surprisingly, these two shots are separated by only 20 ft (6 m). For Record 2, the thumper was located on a concrete bridge and for Record 3 the shot was on the artificial fill next to the bridge. Apparently, at the location of Record 2, the sound propagated through the foundation of the bridge which was set on the bedrock itself. The lower frequencies on Record 3 are caused by a natural high-cut filter effect induced by the shallow unconsolidated artificial fill. A band-pass filter of 110 Hz-165 Hz applied to both records removes these differences (Fig. 2.B). Records 2 and 3 illustrate another common problem with this data set: a low-frequency, high-velocity guided wave. For these two records a band-pass filter adequately removes both the guided waves and the ground-roll. However, when the bedrock was shallower than 5 ft (2 m) we were not able to remove the high-frequency guided wave phase from the data set. This coherent noise has slightly decreased the quality of the section in those areas.