Why Geophysical Methods Provide Superior Vs₃₀ Measurements Compared to Invasive Drilling
- Vs profiling for seismic site classification (Vs30), without a borehole
- Delivers more reliable data compared to Seismic Cone Penetration Testing and Downhole Testing as it covers a larger area
- Downhole and SCPT are accurate but only at a point, being vulnerable to soft zones and anomalies
- Entirely from surface, non-invasive and risk free
- 2D Profile up to 60m in depth
- Ground stiffness evaluations and Slope Stability Analysis
- Liquefaction assessment, particularly in gravels and cobbles where SCPT is not possible
- Site reconnaissance prior to drilling and in-situ testing
- Bedrock or glacial till mapping
- Backfill testing
- Delineation of “soft zones”, potential voids, other anomaly detection
- Engineer’s preferred method for disturbed areas as MASW profiles deliver an average Vs across a site
- Paired with HVSR readings to find competent bedrock depth, maximizing potential Vs
- Comparative studies between Downhole/SCPT and MASW will show a 95% accuracy if ground is homogenous and flat. In heterogenous or disturbed soils, MASW is vastly preferred by engineers
- Accepted by NBC2020
Comparison of Our Non-Invasive Geophysical Methods Against Invasive Drilling
| Criterion | MASW | Invasive Drilling Methods |
|---|---|---|
| Data Continuity & Resolution | High (continuous velocity profile) | Low (discrete point data) |
| Site Disturbance | None | Moderate |
| Spatial Representativeness | Broad, integrated over surface and depth | Limited to single-point measurements |
| Cost | Lower, $2,000 – 2,500 typically, all in | Higher, about $5,000 >$9,000 for Crosshole |
| Safety and Accessibility | High (no heavy equipment required) | Lower (rigs, deep access needed, hydraulics) |
British Columbia adheres to the National Building Code of Canada (NBCC) seismic site classification system, which categorizes sites based on their dynamic response characteristics. The classification scheme, summarized below, directly informs the seismic design forces specified in the BC Building Code:
| Site Class | Type | Vs₃₀ Range (m/s) |
| A | Hard Rock | >1500 |
| B | Rock | 760-1500 |
| C | Very Dense Soil/Soft Rock | 360-760 |
| D | Stiff Soil | 180-360 |
| E | Soft Soil | <180 |
| F | Special Soils | Site-specific (typically organics, peat) |
