Seismic Field Survey And Equipment

SEISMIC FIELD SURVEY AND EQUIPMENT Reflection Field Survey • Seismic Crew Organization – Usually conducted by seismic contractor Party chief Party manager surveyor driller observer camp incharge seismologist interpreters computers Organization of a land seismic crew Reflection Field Survey • P...

SEISMIC FIELD SURVEY AND EQUIPMENT Reflection Field Survey • Seismic Crew Organization – Usually conducted by seismic contractor Party chief Party manager surveyor driller observer camp incharge seismologist interpreters computers Organization of a land seismic crew Reflection Field Survey • Party chief – supervise field crew & office. – Responsible in quality control, data interpretation, final report & recommendation • Party manager – supervise fieldwork, to get maximum quality data production at apropriate cost. • Observer – responsible in operating equipment & field layout & data accuracy. 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. Seismic acquisition onshore Sub-horizontal beds Unconformity Dipping beds 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 (b) Rifles and guns • Cheap • Repeatable – fire into water filled hole • Shallow targets 0-50m (c) Weight dropper – 3 ton of plate • Cheap • Repeatable – automated • Targets > 50m– 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. Vibroseis 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 2. Seismic sources at sea ii) aquapulse (gas-gun) -The principle is based on the rapid combustion of a mixture of propane and oxygen in a cylindrical rubber sleeve. -The sleeve expands, sending out an initial pulse of compression; it then contracts and overshoots, starting the bubble oscillation. 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. Geophone 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. Hydrophone Instruments & Field Procedure 4) Cable - Cable used at sea called streamers -For petroleum exploration used multiplechannel streamer cable -For engineering survey used single-channel streamer - Consists of plastic tube with diameter of 2½ - 3”, and can be floated neutrally. - Cable can be controlled by a device called bird/depth controller to control depth. A seismic ship shooting a 3D marine survey in 1991. The four streamers under tow create the wake patterns seen at the edges of the photo. Immediately to either side of the ship's wake is an air-gun array. Each array contains four strings of air guns. With the two sources firing in an alternating pattern, eight lines of seismic data were acquired at once (Western Geophysical). The Leading Edge 2005; v. 24; no. Supplement; p. S46-S71; Instruments & Field Procedure - Magnetic compass has a remote control – can determine coordinate of the cable -Recent surveys have streamer length up to 5000 to 6000 m and more than 240 recording segments (channel). Seismic ship QuickTime™ and a TIFF (LZW) d ecompressor are needed to see this picture. Recording Equipment • - Store the ground motion detected by a number of geophones. - signal from geophone selectively amplified in order to enhance weak reception, filtered in order to remove noise (unwanted frequency or ground motion due to factors other than the seismic wave energy). - For old (analog) recording system and shallow refraction works, seismic camera was used to produce records on photographic film/paper as vertical time lines called wiggly line. - Each line represents from one or a group of geophones. Recording systems QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture. Recording systems QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Recording Equipment - recently, digital recording system is used by using digital magnetic tape and then automatic processing is carried out by digital computer. -in digital recording, signal from geophone can be read at an interval of millisecond, and recorded as binary digits along a tape width. Recording Equipment