Geophysics and Subsurface infrastructure

Question 1

Recall a general overview of the geophysical methods for subsurface inspection

Answer 1

Geophysical techniques are based on source type and quantity.
Source types:
Active methods where wave source is generated then bounce back from earth and is collected by a receiver. Such as GPR, Seismic and Electromagnetic methods.
Passive methods where source is natural and is only received using receivers from earth. Examples – gravity, magnetism.
Quantity:
Discontinuities – sudden change in physical property, like sharp transition from clay to sand
Bulk properties – physical property that is not dependent on the amount of it, like density

Question 2

Theory of GPR and Seismic geophysical methods

Answer 2

• GPR – Ground Penetrating Radar – uses energy waves in the microwave band, raging in frequency from 1 to 1000 MHz. Transmitter and receiving antenna is required. Transmitter sends electromagnetic energy into the soil and other material. GPR works by emitting a pulse into the ground and recording the echoes that result from subsurface objects. GPR imaging devices also detect variation in the composition of the ground material.
If the electromagnetic impulse this and object, the density of the object reflects, refracts and scatters the signal. The receiver detects the returning signals and records variations within them. GPR system has a software that translates these signals into images of the objects in the subsurface. This is how it is used to map structures and utilities buried in the ground or in mad-made structures.
Using higher frequency we get higher resolution, but penetration depth is lower.
Mostly for shallow detections
• Seismic – seismic waves are generated by a force or impact and the travel through earth. Travel time of the waves from the source to the receiver is used to derive information about the geological structures in the subsurface. This method can be used for shallow and deep detection.
 Sources in shallow surveys: sledge hammer with metal plate, weight drops, explosives, shotgun, earth compactor.
 The reflected signals are recorded with geophones (on land) and hydrophones (on water)

Question 3

Describe how a GPR project is conducted in the industry

Answer 3

• Data collection
Data is collected by walking with a GPR unit that emits electromagnetic waves using transmitter, then captures reflected waves with a receiver and using a GPR system converts it to visible data for operator. Hyperbolic signatures are created as 2D image that indicate various subsurface phenomena.
Different materials have different permittivity properties
Basically gathering different traces of the area. Traces are received signals.
Putting all traces together to create radargram (2D image of the subsurface)
• Data Processing
Goal is to convert images from raw data to processed data where hyperbolas are highly visible. Different software’s can be used for data processing.
Processes: visualization / removing bad or poor data / direct wave removal / gain / frequency filters / background removal / deconvolution and migration
There are four freq. filters:
• Low pass filter (filter out high frequencies)
• High pass filter (filter out low frequencies)
• Background removal filters (filter out low frequencies)
• Stacking (filter out high frequencies)
Gain is used to enhance the signal to better distinguish objects at high depths.
• Data Interpretation

Question 4

Visualize, process and interpret the simple GPR data

Answer 4

Parabolas in image are caused by fact that signal is already reflected before antenna is directly above it. The real depth of a object is therefore at the top of the parabola.

Steep parabola = high travel velocity of signal through medium
Flat parabola = lower travel velocity of signal through medium
In order to derive exact location of object from parabola we use migration with analysis software

Question 5

Familiarized with the new technologies that can be used for geophysics

Question 6

Explain what characterizes street-works as being different from conventional civil engineering project

Answer 6

Most of the utilities underground are are privatized, only water and sewerage aren’t.
Five isolated islands on a single construction site
Network operator
Engineering firm
Contractor
Sub-contractor
Excavator operator and onsite laborers

Question 7

Illustrate the main challenges and needs within the subsurface infrastructure sector

Answer 7

Challenges:
• Uncertainty and distributednes of maps and data completeness, accuracy, redundancies.
• Coordination challenges: long-term planning and short-term construction schedule alignment
• Construction efficiency: optimizing existing processes and schedules
• Utility strikes, sub-optimal maintenance and network failures, knock-on effects
Needs:
• Detection of underground infrastructure and structures
• Mapping underground in semantically-rich 3D digital twins
• Registering condition assessment data of subsurface infrastructure
• Design and planning activities (3D/4D, BIM, AR)
• Excavation safety
• Facilitate coordination of street interventions

Question 8

Explain how subsurface infrastructures become smart and resilient

Answer 8

• Internet of Things (IoT) – sensors on infrastructure
• Data-driven engineering – more data of different quality levels and variety
• New types of infrastructure – wireless and physical networks
• Energy transition – towards more sustainable sources of energy
• Livable and green cities – integration of greenery and trees
• Climate change adaptation – separate sewer systems and sustainable drainage

Question 9

Know the predominant utility surveying methods and their applications

Answer 9

Utility surveying - identification of subsurface cables and pipes using a combination of invasive and non-invasive means

(i) desktop utility records search
(ii) site reconnaissance
(iii) detection
(iv) verification