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

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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