Geophysics Lecture 7

Question 1

Signal Amplitude

Answer 1

Reduces with depth

Question 2

Skin depth

Answer 2

Depth at which signal strength reduces to 1/e of its surface value (i.e. starts to become difficult to distinguish from noise)

Question 3

Major control on skin depth

Answer 3

Frequency of radiowave transmission

Question 4

Lower frequency signals

Answer 4

Can penetrate deeper

Question 5

Lower frequency signals have high wavelength

Answer 5

Poorer vertical resolution

Question 6

Change in dielectric properties

Answer 6

Energy is reflected and transmitted

Question 7

Reflection measured back at radar reciever also a function of energy losses resulting from:

Answer 7

• Scattering
• Absorption/Atttenution
• Geometrical spreading

Question 8

Attenuation

Answer 8

Depends on electrical conductivity σ,
(and dielectric constant Ɛ, and
magnetic permeability μ)

Question 9

Skin depth vs. Attenuation

Answer 9

Skin depth= inversely proportional to attenuation

Question 9

Vertical resolution

Answer 9

Function of frequency f

Question 10

Water= highly conductive

Answer 10

Poor radiowave penetration

Question 11

Vertical resolution

Answer 11

Characterised as the quarter wavelength
* in the example above, = ¼ λ = 0.15 m = 15 cm.

Question 12

Horizontal resolution

Answer 12

α 1/attenuation – hence better for a high-
loss medium

Question 13

Data displayed in radargrams

Answer 13

– two-way travel time versus
distance along profile (c.f.
seismic reflection)

Question 14

Acquisition geometries:

Answer 14

• 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

Question 15

Diffraction

Answer 15

Occurs off top and sides
of ‘point sources’ such as buried
objects

Question 16

How to see diffraction on graphs

Answer 16

Characteristic upside-down “smile,”
or hyperbola, on radargram
* Causes can be pipes, anomalous wet
zones

Question 17

Time-depth conversion

Answer 17

Use velocity of materials to change two-way travel time into depth on radargram (complicated – details beyond scope of this course)

Question 18

GPR radiowaves

Answer 18

50-150MHz