Lecture 1 - intro Flashcards
Active methods
The operator generates the signal and records its propogation/ return
2 types of geophysical methods
Active and passive
Passive Methods
The operator records signals that have been generated naturally or by other processes
Gravity, magnetics, seismic and SP
Types of passive methods
Seismic, EM,GPR, IP and resistivity
Types of active methods
GPR
Ground Penetrating Radar (method)
Velocity of P waves
Can help determine rock type
Key measurables in geophysics
Electric, gravity, seismic and magnetic
Examples of reasons for different property values
Porosity, weathering, age, fracturing, temperature, pore fluid
If void covered/ full of water..
Gravity anomaly much smaller
Geophyical surveys
Non-invasive probe of subsurface
Contrast between target and surrounding material very
important
Allows identification of anomalies
Initial survey considerations
Feasibility, Finances, Logistics, Geology/observations
Forward modelling
Have a model and can calculate data at certain points in time/space
Inverse modelling
Collect data and from them try to generate a model
The only way you can be 100% sure of a model
If you have infinite and perfect data
How to choose best method (things to consider)
- Target and subsurface properties
- Time
- Max depth of penetration
Method useful for subglacial features
Seismic reflection array
Aliasing
The failure of sampling to identify the original signal
What is aliasing the result of ?
Insufficiently frequent sampling
Noise
Can lead to irregular fluctuations that accompany a
transmitted signal but are not part of it and tend to obscure it
Signal- Noise ratio desired in data acquisition
High ratio
Coherent noise
Regular/predictable = effect can
be removed from data during processing (e.g.
power line signal)
Incoherent noise
Unpredictable/random = may
average out (e.g. waves breaking) or may cause
bigger problems (e.g. a car driving past)
What is the usual maximum depth of penetration for VLF?
40 meters
Applications of VLF?
Locates vertical fracture zones and dykes within basement rocks or major aquifers.
What does Ground Penetrating Radar (GPR) measure?
Reflections from boundaries with different dielectric constants.
Max depth of penetration for GPR?
10m
Applications of GPR?
Determines sand/gravel thickness and detects buried pipes but doesn’t penetrate clay.
What does seismic refraction measure?
P-wave velocity through the ground
Max depth of penetration for seismic refraction
30 m
Applications of seismic refraction?
Locates fracture zones and drift thickness but is slow and hard to interpret
Max penetration depth of magnetics?
100m
Issues with magnetics technique?
Prone to noise from metallic objects
What does resistivity measure?
Apparent resistivity of the ground
Max penetration resistivity
100m
Applications using resistivity?
Used for aquifers and water quality studies, slow but detailed
What does Frequency Domain EM (FEM) measure?
Apparent terrain electrical conductivity
Max penetration depth FEM
50m
Applications of FEM?
Quick for identifying weathered zones/alluvium but needs careful interpretation.
What does Transient EM (TEM) measure?
Apparent electrical resistivity from decay of induced EM fields.
Max depth of penetration of TEM
150m
Applications of TEM
Effective through conductive overburden with better penetration than FEM; expensive and complex.
Desired signal to noise ratio in data acquisition
High
