lecture 8- instruments, geophone array and energy source
TRANSCRIPT
Subject 8: Instruments, Geophone Array and energy source Lecturer: Dr. Bakhtiar Q. Aziz Objective: Several equipments have been used in seismic work, in this lecture the students will take sufficient knowledge about the type of seismic field instrument in both land and sea survey as well as they will learn how to arrange a set of geophone around a shoot point. The type of energy used in the field is very important so the students have to be known which energy is suitable for certain survey. Scientific contents
1- Instruments used in seismic work
2- Spread Configuration.
3- Energy source used in seismic work.
References
1. Applied and environmental geophysics, 1999, Sharma,V.,P. 2. Introduction to geophysical prospecting, 1988, Durbin, M. B. 3. www.Geophysics.net 4. www.seismicexploration.net/
Instruments used in seismic work
Geophones (Seismometers): It is used to transform
energy into electrical voltage
Type of Geophones:
A- Land survey;
Two types of geophones are used
1- Moving coil geophone.
2- Moving magnet geophone.
Spike
Magnet
Spring CoilLeaf Spring
Case
N NS
+-+-
1 2
B- Sea survey:
Hydrophone : It is a pressure geophone has piezoelectric property transform pressure to electrical voltage. Usually mounted on a large cable towed to seismic ship at depth 10 to 20 m below sea level.
Spread Configuration: It means the relative position of the shot point to the geophone.
1- Split Spread: (SP) is placed at the center of spread.
G12G1 G2 G3 G4 G5 G6 G7 G8 G9 G10 G11SP
Seismic Record
Seismic Record
G12G1 G2 G3 G4 G5 G6 G7 G8 G9 G10 G11SP
2- Split Spread with a gap: (SP) is placed at the center of spread
With plotting a gap.
3- End of Spread: (SP) is placed at the end of spread.Seismic Record
G12G1 G2 G3 G4 G5 G6 G7 G8 G9 G10 G11SP
4- End of Spread with a gap: (SP) is placed at the end of spread with plotting a gap.
Seismic Record
G12G1 G2 G3 G4 G5 G6 G7 G8 G9 G10 G11SP
5- Unbalanced Spread : (SP) is not placed at the center
G12G1 G2 G3 G4 G5 G6 G7 G8 G9 G10 G11SP
Seismic Record
Energy source used in seismic work:
1- Dynamite: %60 of seismic work used dynamite. It is a mixture of Nitroglycerin + Gelatin + Binding material
Advantage
1- It has sharp peak
2- Decrease surface noise
3- It has wide range of frequency
Disadvantage
1- It require drill hole
2- It is dangerous
3- Stack of charge
2- Dropping weight (Thumper): It is composed of a rectangular steel plate, 2 to 3 tons usually, and dropped 3 to 4 times at height of 3 m, used in desert area.
Advantage
1- Not dangerous
2- Easley used , cheap
3- Not distort the surface
Disadvantage
1- Record is weak, so dropped 3-4
2- Low frequency
3- Not used in mountainous area
3- Dynoseis: It is also explosion, material, made up of a mixture of propane and oxygen.
Advantage
1- It is cheap
2- Easily used
3- Not dangerous
Disadvantage
1- It has broad peak
2- Small energy
3- Low frequency
4- Geoflex: It is used detonating cord buried below 1/2 m.
Advantage
1- it is cheap
2- Easley used
3- Very active
Disadvantage
1- It is dangerous
2- Not used when soil is too thick
3- Not used in mountainous area
5- Vibroseis: It is used to generate seismic wave in the form of a wave train of control energy. Always 4 vibrators are used
Advantage
1- The frequency is controlled
2- The energy is controlled
3- Not distort the surface
Disadvantage
1- Not suitable when soil is too thick
2- Expensive
3- Not used in mountainous area
6- Hammer: It is used to generate seismic wave of small energy, and it is used for shallow investigation.
Advantage
1- Very cheap
2- easily used
3- Not distort the surface
Disadvantage
1- Give small energy
2- used only for shallow
3- Low frequency