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Oliver Chrisp, Head of Geophysics, Aecom

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Enhancing geophysical investigations: Recent advances in science and technology

2 Enhancing Geophysical Investigations – [email protected]

Geophysics – What Is it?

A Tool Box

Measures Different Properties of

the ground

Reconnaissance

Detailed

Linear profiling

Down Hole

Point collection tools

Mass Data


Why Use It? - Identify & Target Change

Laterally

With Depth

Then Join the dots…


Reconnaissance – EM / Mag / GPR

LIMITATIONS• Depth of resolution• Environmental Noise• (Metallic) Surface

obstructions• Anomaly overprinting


Linear Profiling – Seismic / Electrical / (GPR)

Seismic• Excitation of a shockwave in the ground• Geophones placed on the ground map arrival times of

reflected/refracted/direct shockwaves

Electrical• Injection of current into the ground

• Electrodes placed on the ground map Electrical

Potential

LIMITATIONS• Near Surface resolution• Seismic Noise• Current Channelling• Resolution reduces with depth• Requires long profiles• Requires physical contact with the ground


Linear Profiling - High Speed – Asset Management - GPR

Limitations• Depth of Penetration• Scattering of signal (rebar)• Attenuation of signal (moisture/clay)• Not Measuring the properties you’re

interested in• Multiple Specialisms - one technique

TECHNIQUE:• High frequency EM pulse is transmitted

from the antenna, reflecting on

subsurface objects

• Very high resolution, relatively small

depth of penetration

• Typically surface only (although

Borehole applications are possible)

USES• Mapping of Buried Utilities/ Features• Asset Management Construction & Condition Assessments• Voiding• Structural surveys


Targeted – Microgravity ++LIMITATIONS

• Depth of resolution

• Topography

• Collection Speed


Point Mode – Down Hole

LIMITATIONS• Requires boreholes• Type of casing limits techniques• Presence of borehole fluid limits techniques

TECHNIQUE:

- A wide variety of downhole

geophysical logging techniques

- Borehole only,

- Borehole to surface

- Cross hole

- Monitoring tools

- In Situ Values

Digitisation

Size & Weight

Ease of Use

Accessibility


Geophysics – What’s Changed



Collection Speed

Accuracy

Mass Date Collection Tools


Geophysics – What’s Changed – Processing / Software



Not Physics –

(Predominantly)

• Quantum Gravity

Understanding of Physics:

• Ground Roll/ Noise

oRayleigh Waves

oMASW /CSW = Stiffness

In MASW surface waves travelling along interfaces are analysed (Rayleigh waves). Data is collected in a similar fashion as other seismic surveys, but processing focuses on the frequency spectrum of the waves measured along the whole geophone spread instead of travel time analysis of each individual geophone.

This approach allows determination of S-wave velocities with depth, without the need of an S-wave source for each shot & receiver array position. By moving the shot & receiver arrays along a profile a continuous image can be constructed.

MASW / CSW

F-K

transform

inversion

Contouring/

plotting

Enhancing Geophysical Investigations – [email protected]

MASW – Project Example


New Road

Fill materials

Localised Soft Ground

Intrusive samples not representative

Geophysics Identified changes in

stiffness along the route allowing

effective designs

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HVSR

HVSR (Horizontal-Vertical Spectral Ratio) is a passive seismic method, using background vibrations to map the shear wave velocity with depth.

The system incorporates a 3 component accelerometer, set to measure for a specific time interval. Each (micro-) seismic event recorded forms part of the signal which is analysed and interpreted in the frequency domain to map Vs with depth.

The tool has seen increased use in the past years and provides a good, user-friendly first-pass approximation of ground conditions.Alternatively it can be used for structural analysis



Geophysics - Why We Mis-use it:

Join the Dots between GI…

Poorly scoped

Badly timed

Unreasonable expectations

No feedback


Geophysics – How We Use it Well:

TIMING / Phasing

Scope / Contracts & Scale

Expectations

Feedback

Interpretation


What’s Changing? - YOU

Geophysical Tools

Electrical

Magnetic

Mechanical

Gravity

Light

Geomatics

Processing

Observation

Calibration

Knowledge

Increased Understanding

Trials with Feedback

Development /

Standardisation

Good Practice / Guidance

Increased Usage

Helping understand risk in the sub-surface

Delivering the full spectrum of ground investigations, geophysics and materials testing services, our Asset Investigation team generates insights into the performance of hidden assets.


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