SWANS® demonstrates a 50% time-saving screening for pumping station

The Challenge

As part of the design phase, we were approached by Stantec to carry out a site screening exercise using geophysics and 3D modelling ahead of the construction of a major stormwater pumping station.Time was of the essence and the client wanted to predict ground conditions and avoid unforeseen construction problems and gain early insights into the presence of any potential subsurface risks that could impact the construction and operation of the asset such as:

• Potential cavities – open or partially collapsed

• Zones of fractured or extremely weak rock, to a depth of 100 metres

nsights acquired from rapidly screening the subsurface, would help Stantec to reduce design uncertainty, support earlier and better decision-making and control project costs.

In conventional geotechnical investigations, the shear wave velocity (Vs) of the subsurface is related to the stiffness of the strata. Stiffness is a major design parameter in geotechnical engineering and a diagnostic parameter for geohazards. Our solution was to derive a 3D volume of shear wave velocity and subsequently stiffness through rapid geophysical screening with SWANS® technology.  A significant decrease in velocity/stiffness could indicate the presence of underground cavities or weaker strata.

1. Setting up the grids

Our starting point for the geophysical investigation was to deploy about 400 small, wireless, autonomous and non-intrusive SWANS^® sensors along three nested grids covering the site area: a grid with 6 m spacing directly on top of the target area (150 sq m); and coarser surrounding grids of 12 m and 24 m to capture long wavelength (deeper penetrating) information. Figure 1 depicts the sensors as red dots.

2. Listening to the ground

SWANS® forms part of our initial site screening solution. It is a passive seismic technology that records and analyses ambient noise in the form of surface waves. Surface waves propagate as a function of the ground’s geotechnical properties, especially stiffness. The SWANS® sensors captured ambient seismic noise across the site for six days. The acquired data had a high quality and a good bandwidth meaning good penetration depth and resolution.

3. Processing the data

Acquired SWANS® data were processed using interferometric methods to extract coherent signals from the recorded seismic noise and to generate correlation Green’s functions that provided the input for both phase and group velocity analysis.

4. Generating the 3D ground model

Our proprietary software combines tomography and inversion to construct a robust and dense 3D Vs model of the subsurface, enabling its stiffness properties to be determined. Our 3D passive seismic results were in alignment with measurements derived from borehole PS logging and multi-channel analysis of surface waves (MASW). This correlation confirmed that our innovative SWANS® technology delivered high levels of consistency with more conventional data sources.

The shear wave velocity distribution on the 3D block directly underneath the target area had an impressive range typical of rock: from 800 m/s to 2,000 m/s, down to a depth of 120 m (Figure 2).

Innovative highlight

The site is located in a relatively quiet area of Qatar, so we supplemented the four-day passive phase with a further two days of active acquisition using a weight drop source. This ensured the retrieval of higher-frequency surface waves, needed to resolve the near-surface in higher velocity (stiffer) scenarios.

SWANS® technology is unique because it:

• Provides significantly greater depth penetration than traditional engineering geophysics methods such as MASW

• Generates a 3D stiffness image that cannot be obtained from in-situ geotechnical measurements such as PS logging

• Performs a tomography and inversion on a 3D grid, using all available azimuths and frequencies to maximise spatial resolution

• Can deliver 3D volumetric estimates in geotechnical properties in less than one quarter of the time needed to derive equivalent conventional data.

• Advanced analysis can be used to derive initial characteristic values (small strain stiffness) for geotechnical design with statistical uncertainties ahead of conventional investigation to accelerate design

We applied data analytics to combine SWANS® and borehole data, to create a detailed geological 3D ground model showing the different geological units and their variation in the subsurface.

Impact

By using SWANS^® we delivered:

• Quick and easy set-up with small field crew: minimal environmental and community impact when compared to a fully conventional approach – 75% reduction in people and vehicles

• Deeper investigation (>100 m) than conventional geophysics to meet specific project needs of a deep excavation

• A 3D ground model that significantly reduced uncertainty of ground conditions resulting in an acceptable level of residual risk

• Time saving of at least 50% compared to a fully conventional geotechnical approach