pptx

```Transmitter/Receiver Vs. Geophones
Seismic Reflection
• One source
• Lots of geophones
• Stacking and Velocity
performed in one step
• Moveout
• Peak Power
GPR
• One source (Transmitter)
• Both are antennas
• Stacking performed by
repeating the EM pulse
• Moveout
• Peak Power
GPR Travel Time Equation
+
=
=

+

=   +

−

Two Way Travel Time =
+

• In seismic reflection, velocity is found during stacking from
the moveout patterns
• In GPR, we only have one receiver, so we need a different
method
• Fixed offset survey (i.e. a normal survey)
• Look for point sources (hyperbolas)
• Common Midpoint Survey (CMP)
• Get reflections off of a horizontal reflector
• Increase antenna spacing, repeat
• Produces a hyperbola
Common Midpoint Surveying
GPR Travel Time Equation
+
=
=

+

=   +

−

Two Way Travel Time =
+

Common Midpoint Surveying
Velocity in Fixed Offset Surveys
• In a regular, fixed offset survey, radar velocity can be
determined only if diffraction hyperbolas are encountered
• Typically point sources
Velocity in Fixed Offset Surveys
1 =
2 =
ℎ2 +
ℎ2 +

+
2

−
2
2
ℎ2
2
=
+

2 +
2
+
1
ℎ2

+ − 2
2
Velocity in Fixed Offset Surveys
Source Wave Properties
(GPR)
Electromagnetic Waves
Seismic Surveys
(Refraction/Reflection)
Seismic Waves
• Velocity (in air)≈ 3x105 km/s
• P, S, R, L waves
• Velocity ≈ 0.4-14 km/s
• I.e. the speed of light (0.3 m/ns)
• Slower in most geologic
materials (0.02 – 0.2 m/ns)
• Depends on electromagnetic
properties of medium
• Frequency ≈ 10-2000 MHz
• Depends on antenna
• Wavelength ≈ 30-1.5x10-8 m
• Depends on mechanical
properties of medium (elastic
moduli and density)
• Frequency = 0.1-100 Hz
• Depends on source
• Wavelength = 140-4,000 m
GPR Frequency Comparison
• Higher Frequency
• Better detail
• Less penetration (penetration also depends on material)
Attenuation Comparison
(GPR)
Electromagnetic Waves
Seismic Surveys
(Refraction/Reflection)
Seismic Waves
• Decay Exponentially with
distance from source
• Decay Exponentially with
distance from source
• Can’t penetrate through
electrically conductive
materials
• Can’t penetrate through
inelastic layers
• Metals/Metallic Ores
• Saltwater
• Clays/Muds
• Fluids
• Fault/Fracture Zones
• Scatter waves
Attenuation
What Causes a Reflection?
(GPR)
Electromagnetic Waves
Seismic Surveys
(Refraction/Reflection)
Seismic Waves
• Change in relative permittivity
• Change in acoustic impedance
=

ℇ
c = speed of light
εr = Relative Permittivity
Acoustic Impedance = ρv
R 
a refl
a incid
Relative Permittivity: a measure of the ability of a
material to store a charge when an electric field is applied

Reflection Coefficient:
=
=

ℇ − ℇ
−
=
ℇ + ℇ  +

 2 v 2   1v1
 2 v 2   1v1
Visualization of Data
• Identical to seismic reflection
• Wiggle traces
• Variable Area
• Variable Density
```