Key Data:
- Site Class – C
- AVS30 – 569.3 m/s
- Peak H/V – 18.261 Hz
- Max Depth – 67 m
- 120 Spacings, 96 pairs
Survey Details:
- # of Surveys – 1
- # of Channels – 16
- Min Wavelength – 1.5 m
- Max Wavelength – 82.5 m
- Data Record Length – 40 mins
Survey Geometry:
- Fibonacci Spiral
Survey Notes
Browns Valley Elementary was the site of the first ever Fibonacci “golden ratio” passive-seismic investigation. This Fibonacci spiral survey geometry was rough in design compared to later attempts, but even without a double right angle survey prism, data came out excellent from 4-30 Hz.
Fibonacci survey design is hallmark for producing the least amount of high-frequency low-phase-velocity noise. This noise is much more present with more triangular or square survey design. This survey was unable to attain any coherent data from 0-4 Hz, and a more sensors near the outside points would solve that problem.
The H/V data is relatively level from 1-13 Hz, slightly dipping from 2 as the frequency increases, and then after 13 Hz the H/V roars up to nearly 3.5, peaking at 18.3 Hz. The cause for peak H/V at 18 Hz is unknown, but similar H/V numbers are observed at N11 Lakeview Park and N59 Tulocay and throughout Browns Valley.
Vs values are quite high at N34, and beyond the surface alluvium Vs quickly increases to an increasing 600+ m/s from 8 to 36 meters and 700+ m/s thereafter.
N34 Survey Geometry
Fibonacci survey design is hallmark for producing the least amount of high-frequency low-phase-velocity noise. This noise is much more present with more triangular or square survey design. This survey was unable to attain any coherent data from 0-4 Hz, and more sensors near (0, 0) and (35, 14) would solve that problem.
The numbers used for this first Fibonacci survey were: 2, 3, 5, 8.5, 14, 20, 30, 50.
The numbers typically used are 1, 2, 3, 5, 8, 13, 21, 34, 55, 89+
Still, you can still see the advantages of a Fibonacci spiral over conventional passive-seismic survey design. As the spacings between geophones increases, the position of the spacings in space spirals downwards in depth.
This corkscrew pattern allows for high coherency data to be collected at all depths surveyed. Recommendations for future surveys would be to use two 89+ m Fibonacci lines on both fields, or with enough seismographs, two Fibonacci spirals running simultaneously.
Phase Velocity Dispersion Curve
HV Peak
Dispersion Curve
