N85 – 3rd Ave

N85 Velocity Model

Key Data:

  • Site Class – D
  • AVS30 – 330.5 m/s
  • Peak H/V – 2.079 Hz
  • Max Depth – 113 m
  • 151 Spacings, 171 pairs

Survey Details:

  • # of Surveys – 1
  • # of Channels – 19
  • Min Wavelength – 1.0 m
  • Max Wavelength – 159.8 m
  • Data Record Length – 97 mins

Survey Geometry:

  • Fibonacci line with GPS scatter

Survey Notes

The geology of east Napa can be quite varied. East of the Napa River the landscape is defined by rolling hills, underneath which lie volcanic rocks such as tufts, pumices, andesite, and more. Inherently there is a lot of variability to this area, and this is evident with the N84 – North Ave and N85 – Third Ave surveys. Both surveys were done in large relatively flat fields, only 1 kilometer away from each other. Yet N85 has a AVS30 of 330 m/s while N84 has a AVS30 of 810 m/s. Meanwhile at 100 meters depth they converge to similar shear-wave velocity values, with N84 having a Vs of 1300 m/s and N85 with a Vs of 1250 m/s. N85 was situated near the bottom of a hill, with a small creek running through the back of the property, and N84 was situated more on a hill. N85 has active alluvium deposition, N84 does not.

A 8″ steel well was installed on-site prior to 1985 to a depth of 55 meters, and in 1988 the flow rate was 15 GPM. Below are three coherency plots for SPAC pairs 43.0, 43.6, and 44.9 meters. Following the black line, notice the large dip in coherencies centered near 1 Hz. Other depths show this trend too. This 1 Hz dip has been common throughout Napa, and might be related to hydrological processes.

Underneath the SPAC pairs is the full data snapshot. Even though the site was very quiet during the survey, with only a light breeze, it appears plenty of fast traveling noise still appears in the data. We know the noise was fast traveling as the sensors were spread out over 3.5 acres, but the arrivals are nearly simultaneous for all geophones. The source is this noise is unknown, possibly traffic or construction related. Since passive-seismic methods work by averaging data over time, the effect of this noise was minimal, and as can be seen, a couple sections were removed to enhance data quality.

Phase Velocity Dispersion Curve

HV Peak

Dispersion Curve

Notice the Vs inflection point at 4.5 Hz, 700 m/s. This is a

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