1. Processes altering seismic amplitudes

2. Seismic amplitudes Affected by
Reflection and transmission at an interface
Geometrical spreading
Absorption
Receiver response
Measurement system

3. Geometrical spreading
Energy proportional to: r 1
Plane wave:
constant r 2
Cylindrical wave:
~ 1/r
Spherical wave:
~ 1/r 2
E/4\pi R^2
Energy is proportional to (Amplitude)2
Attenuation due to geometrical spreading:

4. Energy is proportional with A2
Absorption
Transformation of Energy into Heat
Amplitude:
a =
Absorption coefficient
Energy is proportional with A2
Quality factor 2 p
Part of energy, that is lost in a cycle
Relation between Q and a

5. Absorption is frequency dependent

6. Common Earth materials
0.25 < a < 0.75 (dB/l)
300 > Q > 50
Note: exp(-ax)

7. Question 1: 20 Hz seismic wave Travels with 5 km/s
Propagates for 1000 m. through
Medium: absorption coefficient 0.25 dB/l
What is the wave attenuation in dB due solely to absorption?????
Answer: l=v/f= 250 m: absorption: 4*0.25=1 dB

8. Question 2: Wave with l=100 m propagates through homogeneous medium
Between two detectors at radial distances of 1 km and 2 km the wave amplitude is attenuated by 10 dB.
Calculate contribution of geometrical spreading to this value of attenuation and, thus, determine the absorption coefficient of the medium in dB/ l.
10 dB in 1000 m: 1 dB/ l
Geometrical spreading: 20 10log (A0/A)= 20 10log (2)=6dB/ 1km
= 0.6 dB/ l absorption coefficient: 1-0.6=0.4 dB/ l!