Geophysics Lecture 7 Flashcards
Signal Amplitude
Reduces with depth
Skin depth
Depth at which signal strength reduces to 1/e of its surface value (i.e. starts to become difficult to distinguish from noise)
Major control on skin depth
Frequency of radiowave transmission
Lower frequency signals
Can penetrate deeper
Lower frequency signals have high wavelength
Poorer vertical resolution
Change in dielectric properties
Energy is reflected and transmitted
Reflection measured back at radar reciever also a function of energy losses resulting from:
- Scattering
- Absorption/Atttenution
- Geometrical spreading
Attenuation
Depends on electrical conductivity σ,
(and dielectric constant Ɛ, and
magnetic permeability μ)
Skin depth vs. Attenuation
Skin depth= inversely proportional to attenuation
Vertical resolution
Function of frequency f
Water= highly conductive
Poor radiowave penetration
Vertical resolution
Characterised as the quarter wavelength
* in the example above, = ¼ λ = 0.15 m = 15 cm.
Horizontal resolution
α 1/attenuation – hence better for a high-
loss medium
Data displayed in radargrams
– two-way travel time versus
distance along profile (c.f.
seismic reflection)
Acquisition geometries:
- Side-by-side to form reflection
profile - Wide-angle reflection
- Common midpoint (CMP)
- Trans-illumination, either side
of e.g. a borehole, within a
mine, or man-made structures
like concrete pillars for non-
destructive testing
Diffraction
Occurs off top and sides
of ‘point sources’ such as buried
objects
How to see diffraction on graphs
Characteristic upside-down “smile,”
or hyperbola, on radargram
* Causes can be pipes, anomalous wet
zones
Time-depth conversion
Use velocity of materials to change two-way travel time into depth on radargram (complicated – details beyond scope of this course)
GPR radiowaves
50-150MHz
