N81 – Napa Plums

Key Data:

  • Site Class – D
  • AVS30 – 228.2 m/s
  • Peak H/V – 0.95 Hz
  • Max Depth – 62 m

Survey Details:

  • # of Surveys – 1
  • # of Channels – 18
  • Min Wavelength – 1.0 m
  • Max Wavelength – 68.7 m
  • Data Record Length – 40 mins

Survey Geometry:

  • Fibonacci line & corner points

Survey Notes

This passive-seismic survey was done on a 1/2 acre rectangular lot in a quiet residential neighborhood situated between HWY 29 and West Park Elementary School. The name comes from the plum tree on-site, which was loaded with a prodigious number of plums.

A capped well is on site, drilled 10m down in 1939. The flow rate is unknown, and the well is capped because of the convenience of city water.

At nine meters, the passive seismic coherencies drop remarkably to under 0.4 for 0-30 Hz. Coherencies at 7 and 8 meters are subdued down to under 0.8 for 0-10 Hz. At 10-12 Hz coherencies rise close to 1 again, staying in the 0.6-1 range. Looking at the velocity model, we see a large jump in shear wave velocity at 33.6 meters, from 321 to 385 m/s. A dispersion curve inflection points sits right at 30 meters.

All of this data provides strong evidence for bedrock existing at approximately 30 meters. At this soil rock interface, water pools up and is able to be pumped. Other zones of low coherencies are observed below 30 meters as well, these could also be zones of higher water content. The West Napa Fault runs 2.5 km to the west of the fault.

Phase Velocity Dispersion Curve

N81 Phase Velocity Dispersion Curve

H/V Peak

Dispersion Curve