Seismic Site Classification Vs30 – 2D MASW/Surface Seismic Soundings

Features of Surface Seismic Soundings

Sometimes referred to as MASW, but Sitka’s performance is significantly more elaborate
Vs and Vp profiling for seismic site classification (Vs30), without a borehole
Entirely from surface, non-invasive, risk free, no permitting required
2D Profile up to 60m in depth, but most reliably to 40m
Bedrock Depth Profiling included
Applicable to Ground Stiffness Evaluations, Slope Stability Analysis, Soil Liquefaction Studies
Delineation of “soft zones”, potential voids, other anomaly detection
Elastic Modulus: Poisson’s Ratio, Shear, Young’s, and Bulk Moduli

The Survey

Often called MASW, but this is a significant understatement to the methodology which Sitka performs
7 geophysical methods used in one survey:
2D MASW – 2D Profile of Vs shear wave velocities for the upper 30-60m
MAM – Passive for collecting deeper features of Vs
2D SRT – 2D Profile of Vp compressional wave velocities for rock and water table features
HVSR – Excellent corroborating data for confirming bedrock and other stiff layers
ARI – Acoustic Resonance Imaging which generates 2D profiles of overburden layers, especially effective
Seismic Reflection – Additional 2D profile of overburden, rock fractures, weathering and bedrock
EFWI – A computer processing algorithm used to clarify data sets and profiling imagery
While a 1D result will always be included, a 2D profile of the site will most completely capture seismic data.

Advantages

SPT and CPT always must use a conservative value for Seismic Site Class – so the lowest value on the span of values which encompass a site class letter. A Site Class D, determined by SPT and CPT will be 180m/s. Using geophysical methods, like Surface Seismic Soundings, the Vs30 of a site will always be higher, even if the site class is a D. From a structural engineers perspective, the difference between a 180m/s Vs30 versus a 240m/s Vs30 is significant. The cost saving for the end client can be incredibly large
Surface Seismic Soundings will never encounter refusal conditions, permitting, unsafe conditions, or risk factors for cost.
Structural Engineers now consider Vs, and not the base letter classification to be the pertinent factor when designing structural elements. These Surface Seismic Soundings satisfy these Vs requirements at a very reasonable rate. Typically $3,000 before taxes if within the Lower Mainland of BC

This was a case in which a sinkhole was appearing near new a development and a road. We found what was seemingly a minor amount of settling from surface, but in reality was a deep large void, with significant weak areas surrounding it.

Here was a fascinating case of an earth dam. Introduced material making up the earth dam was about 4m in depth, as this long sounding was conducted atop the dam itself. Below this 4m depth was the native material, which was significantly weaker. Eventually this weak material strengthened with depth and eventually reached bedrock. This bedrock undulated significantly, being indicated in blue.

Commonly very homogeneous sites are in places like the Lower Mainland of BC. These will still need testing like 2D MASW/Surface Seismic Soundings in order for the Vs30 to be confirmed. In this circumstance, the developer wanted to place their large building on a till layer which was about 6m in depth. Their Vs30 value increased to 459m/s due to this excavation, while a slab on grade would be 375m/s. We recorded 3 HVSR readings to confirm this till layer depth and also found bedrock depths, which were very deep: >100m in depth. Bedrock is easiest to identify with accuracy when it is within the upper 60m of earth.

Comparison of 2D MASW/Surface Seismic Soundings Against SPT, CPT, DCPT & SCPT

Criterion	Surface	Invasive Drilling Methods
Data Continuity & Resolution	High (continuous velocity profile)	Low (discrete point data)
Site Disturbance	None	Moderate, drilling
Spatial Representativeness Depth of Measurement	up to 60m in depth, but typically 30-40m. Across a 69m spread.	Limited to single-point measurements No limit of depth, but point based. SCPT risks refusal conditions which are costly.
Cost	Lower, $3,000 typically, all in	Higher, about $7,000 for SCPT >$9,000 for Downhole
Safety and Accessibility	Very Safe (no heavy equipment required, not one injury or near-miss to date)	Less safe (rigs, deep access needed, hydraulics)