Land and Marine Seismic Acquisition from 2D to 3D

Land and Marine Seismic Acquisition from 2D to 3D From chapters 7-12 “Elements of 3D Seismology” by Chris Liner

Outline-1 • CMP METHOD (Harry Mayne) • Seismic sensors • geophones • hydrophones • gimballed geophones and hydrophones • accelerometers • Sources • Explosives • Vibroseis • SEGY data

Outline-2 • Acquisition Parameters • Time Sample Rate • Offset Range • Listen Time • Sample Rate and Temporal Aliasing • Geophone Spacing and Spatial Aliasing • Shooting geometry • inline • cross-line

Common Midpoint Method (CMP Method) Please take a look at the powerpoint presentation for the radio-telemetry field trip at the following link: http://www.geol.lsu.edu/Faculty/Juan/ReflectSeismol05/labs/ppt/Radio-telemetry.ppt This link has information to complement the explanation on the CMP method.

Common Midpoint Method (CMP Method) Hydrophone groups Shotpoint # 1 #6 #5 #4 #3 #2 #1 Midpoints Separation between midpoints is 1/2 separation between hydrophone groups

Common Midpoint Method (CMP Method) Hydrophone groups Shotpoint # 2 #6 #5 #4 #3 #2 #1 Midpoints

#6 #5 #4 #3 #2 #1 Common Midpoint Method (CMP Method) Hydrophone groups Shotpoint # 3 Midpoints

Common Midpoint Method (CMP Method) Hydrophone groups #6 #5 #4 #3 #2 #1 Shotpoint # 1 Midpoints

Common Midpoint Method (CMP Method) Hydrophone groups #6 #5 #4 #3 #2 #1 Shotpoint # 1 Shotpoint # 2 Shotpoint # 1 Shotpoint # 2 Midpoints

Common Midpoint Method (CMP Method) Hydrophone groups #6 #5 #4 #3 #2 #1 Shotpoint # 1 Shotpoint # 2 Shotpoint # 3 Shotpoint # 1 Shotpoint # 2 Shotpoint # 3 Midpoints

Common Midpoint Method (CMP Method) Hydrophone groups #6 #5 #4 #3 #2 #1 Shotpoint # 1 Shotpoint # 2 Shotpoint # 3 Shotpoint # 4 Shotpoint # 1 Shotpoint # 2 Shotpoint # 3 Shotpoint # 4 Midpoints

Common Midpoint Method (CMP Method) Hydrophone groups #6 #5 #4 #3 #2 #1 Shotpoints # 1-8 1 2 Midpoints 3 4 6 7 8 8 13 5

Common Midpoint Method (CMP Method) Fold or Multiplicity is the number of times that the same midpoint is sampled by different shots and different receivers Signal-to-Noise increases as the square root of the fold Fold 1 2 Midpoints 3 4 6 7 8 8 13 5

Common Midpoint Method (CMP Method) Maximum Fold is achieved after the 6th shot Fold 1 2 Midpoints 3 4 6 7 8 8 13 5

Common Midpoint Method (CMP Method) When shotpoint spacing and group spacing are equal then Maximum fold = number of geophones or hydrophones Midpoint separation = 1/2 distance between geophones In a more general case: Maximum Fold = #recording groups * distance between groups 2 * distance between shots Midpoint separation = 1/2 smaller of the two: receiver group spacing or shot spacing

Gather Types A gather i.e. “a subset of the traces from the entire data set” can be of different types: • Shotpoint gather • Common source-receiver offset gather (COS) • Common midpoint gather

Shotpoint Gather e.g. Shotpoint gather #3 #6 #5 #4 #3 #2 #1

#6 #5 #4 #3 #2 #1 Shotpoint Gather Shotpoint #3 #6 #5 #4 #3 #2 #1 Hydrophone groups A shotpoint gather samples various midpoints and a variety of angles

#6 #5 #4 #3 #2 #1 What happens to the reflecting points in a shotpoint gather when the reflecting interrface dips? Shotpoint #3 #6 #5 #4 #3 #2 #1 Hydrophone groups A shotpoint gather samples various midpoints and a variety of angles

#6 #5 #4 #3 #2 #1 What happens to the reflecting points in a shotpoint gather when the reflecting interface dips? Shotpoint #3 #6 #5 #4 #3 #2 #1 Hydrophone groups Reflecting points Midpoints A shotpoint gather samples different reflecting points at a variety of angles

Common Midpoint Method (CMP Method) Hydrophone group #4 Common source-receiver offset and common receiver, shotpoints 1-8 #6 #5 #4 #3 #2 #1

Hydrophone group #4 Common source-receiver offset and common receiver, shotpoints 1-8 #6 #5 #4 #3 #2 #1 COS means equal reflection angle Midpoints

In the case of a COS gather where are the true midpoints when the reflecting, geological interface has a dip? #6 #5 #4 #3 #2 #1 COS means equal reflection angle Midpoints

COS NO LONGER implies equal reflection angles #6 #5 #4 #3 #2 #1 Actual reflecting points Midpoints

Common Midpoint Method (CMP Method) Hydrophone group #4 Common mid-points and shotpoints 1-8 #6 #5 #4 #3 #2 #1 Midpoints

Hydrophone group #4 Common mid-point and shotpoints 1-8 #6 #5 #4 #3 #2 #1 group 8 7 6 5 3 2 1 4 Midpoint #6 CMP gathers sample varying angles but a common geological midpoint

What happens to a common midpoint gather when the reflecting interface has a dip? #6 #5 #4 #3 #2 #1 group 8 7 6 5 3 2 1 4 Midpoint #6 CMP gathers sample varying angles but a common geological midpoint

CMP gathers SAMPLE varying angles but with a relatively smaller spread of reflecting points than the shotpoint and common-offset gathers #6 #5 #4 #3 #2 #1 group 8 7 6 5 3 2 1 4 Midpoint #6 True Reflecting Points

A common midpoint gather minimizes the effect of dip while it helps increase the signal-to-noise ratio

Geophones Convert ground motion into electricity at a rate of about 1 Volt/inch/sec Natural Resonance Frequency 100 Hz GS-100 from Geospace

Geophone layout

Seismic Sensors • Hydrophones convert changing pressure into Volts • (Volts/bar) e.g. Preseis 2517 from I/O 1V/microPascal

Gimballed Geophone-hydrophone combinations for sea-bottom work Sea-Array 4 from Geospace

Streamer layout

Accelerometers Convert ground acceleration into Volts d(dx/dt) dt E.g. VectorSeis from I/O 3-component digital accelerometer (requires battery) full-scale at 3.3 m/s2; noise level 0.44 microm/s2 140db = 20 log (3.3/4*10^-7)

Vibroseis Method (Liner, 2004; p.157, para. 4, ) An output sweep (e.g., 10-80 Hz) enters the earth …..and undergoes various reflections

+ + = ...something too complicated to draw Field correlation “unravels” the raw data into ….

Vibroseis images from the Lithoprobe Project, Canada www.lithoprobe.ca A vibrator truck “12 elephants dancing in unison” (LITHOPROBE, CANADA)

Explosives Noble Explochem Limited

GI Watergun Array NSF R/VIB NBPalmer- February/March 2003

Land and Marine Seismic Acquisition from 2D to 3D