geophysics chapter 2, Lecture notes of Geophysics

Download geophysics chapter 2 and more Lecture notes Geophysics in PDF only on Docsity!

SEISMIC FIELD SURVEY

AND EQUIPMENT

Reflection Field Survey

• Seismic Crew Organization
  • Usually conducted by seismic contractor

surveyor driller

Party chief

observer camp incharge

Party manager seismologist

interpreters computers

Organization of a land seismic crew

Reflection Field Survey

• Supervisor – responsibles in determining

line location & shotpoints along the lines.

• Seismologist – supervises office work &

interpretation.

• Interpreters – make decision on seismic

cross-section.

Reflection Field Survey

• Crew organization at sea:
  • headed by party manager.
  • observers & technician: operate

navigation equipments and handling the

streamer.

• captain & some personnel: to navigate or

manouver seismic the vessel/ship.

Instruments & Field Procedure

1. Seismic sources on land:

i) explosive dynamite – nitroglycerine + reactive material

• can be used on land & at sea
• • Safety and expense can be an issue

ii) non-explosive

• (a) Sledge hammer
• • Cheap
• • Repeatable once plate is stable
• • Targets 15-50m

Sledge hammer

Instruments & Field Procedure

ii) non-explosive

• (d) Dinosies – an explosion of gas (propane and oxygen).

Detonated inside a closed chamber in contact with the ground. The chamber is expandable so that the bottom plate, which rests on the ground during detonation. (e) Vibrosies – The energy signal is generated by a servo-controlled hydraulic vibrator or shaker unit mounted on a mobile base unit.

• No pulse, frequency sweep
• Significant signal with stacking/deconvolution

Weight dropper

QuickTime™ and a TIFF (LZW) d ecompressor are needed to see this picture.

Instruments & Field Procedure

2. Seismic sources at sea

i) air gun - The most commonly used marine seismic source.

• It consists of one or more pneumatic chambers that are pressurized with compressed air at pressures from 2,000 to 3,000 psi (14 to 21 MPa). The air gun array is submerged below the water surface, and is towed behind a ship. When the air gun is fired, a solenoid is triggered, which releases air into a fire chamber which in turn causes a piston to move and thereby allowing the air to escape the main chamber and to produce a pulse of acoustic energy.
• Very repeatable
• large array for big signal

Instruments & Field Procedure

Instruments & Field Procedure

iii) sparker or plasma sound

• Sparkers send an electric spark between two

electrodes, vaporizing the surrounding water and

simulating an explosion.

• Mainly used in engineering survey
• eg: has been used for the tunnel construction at

English Channel.

Instruments & Field Procedure

3) Geophone

• detector/seismometer - device capable of measuring ground motion (up to 10-^10 m)
• Cylindrical coil suspended in a magnetic field
• The magnetic field of this moving magnet produces an electrical voltage in the wire. This voltage can be amplified and recorded by a simple voltmeter.
• The voltage recorded by the voltmeter is proportional to the velocity (speed) at which the ground is moving.

Seismic acquisition offshore

Instruments & Field Procedure

3) Hydrophone

• pressure geophone used at sea
• Use piezoelectric minerals or transducer that

generates electricity when subjected to a pressure

change.

• A hydrophone converts acoustic energy into

electrical energy.

• OBS (ocean bottom seismometer) – combination

of hydrophones and seismometers.