Seismic Acquisition
Transcript of Seismic Acquisition
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Hole: GEOS 4174 2.2-1 Data Acquisition: Survey Design
DATA ACQUISITIONSurvey Design
Sheriff & Geldart, Chapter 8reflection method
gather: a set of seismic traces with a common acquisition geometry
common source gather common receiver gather
Ikelle & Amundsen 2005reciprocity: reversal of sources and receivers produces identical signal[for amplitudes, direction of motion (e.g., vertical geophone) must be considered]
common midpoint (CMP) gather common offset gather
Ikelle & Amundsen 2005
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Hole: GEOS 4174 2.2-2 Data Acquisition: Survey Design
common-offset method
produces a low-S/N map of the reflector (usual profiling method with GPR)optimum offset is chosen for a particular target reflector
CMP method
use CMP gather and normal-movout (NMO) correction to improve signal-to-noise ratio (S/N)
stack: sum of NMO-corrected seismic traces for a CMP simulates a zero-offset tracefold: number of traces in a CMP stackfor traces with random noise of similar S/N, a stack with fold N improves the S/N by about
€
N
Reynolds 1997 Yilmaz 2001
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Hole: GEOS 4174 2.2-3 Data Acquisition: Survey Design
CMP method
Yilmaz 2001
dipping structure:CMP collects data from different reflection points;
midpoint is smeareddipping structure does not align properly with NMO
correctionSharma 1997
CMP is also known as “common depth point (CDP)”… but only true for horizontal layers
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Hole: GEOS 4174 2.2-4 Data Acquisition: Survey Design
2D (linear) source and receiver layouts
live recording spread geometry: source is a dot, receivers are x’s
Sheriff & Geldart 1995
split spread: gives higher fold at near offsetend-on spread: gives longer offsets (for a fixed station spacing)gap: near-source gap eliminates near-source stations (that may be dominated by ground roll)
and provides longer offsets
roll-along: the live recording spread moves with the shot along the linemany shots and receivers at overlapping positions gives foldroll-on, roll-off: when the spread hits the ends of the survey line, the shots will move through a
fixed spread to the last possible position
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Hole: GEOS 4174 2.2-5 Data Acquisition: Survey Design
stacking chart
plot traces at shot & receiver positions
Yilmaz 2001
€
xmidpoint = xsource + xreceiver( ) /2 xoffset = xreceiver − xsource
in real life, physical obstacles (e.g., road, creek, building) require gaps in shots and/or receiversundershooting: to maintain fold on a subsurface reflector, the missed sources & receivers are
replaced by placing them on either side of the gapdetailed survey notes are required to connect recorded data to source and receiver stations, and then
to ground positions
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Hole: GEOS 4174 2.2-6 Data Acquisition: Survey Design
survey design considerations
Sheriff & Geldart 1995
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Hole: GEOS 4174 2.2-7 Data Acquisition: Survey Design
2D crooked line
obstacles or access sometimes limit the line to be crooked
a smooth line (or series of straight lines) is drawn through the mapped midpointsmidpoint bins are chosen with shapes perpendicular to the line (or along strike)
Sheriff & Geldart 1995
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ymidpoint = ysource + yreceiver( ) /2 roffset = xr − xs( )2 + yr − ys( )2
the across-line information can be used to infer across-line dip
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Hole: GEOS 4174 2.2-8 Data Acquisition: Survey Design
marine surveying
cost of seismic surveying:most important factor: time, which is roughly proportional to number of sources firednext factor: crew/ship size, which roughly depends upon number of recording channels
marine operations are very time-efficient: real-time surveying, few obstacles, continuous shooting order of magnitude more cost-effective per km (for similar acquisition specs)
marine surveying always uses end-on recording
recording streamers extend km’s behind the ship and are pushed by ocean currents: feathering
Sheriff & Geldart 1995requires a lot of position survey data (compasses and GPS on the cables)CMPs get smeared in cross-line direction
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Hole: GEOS 4174 2.2-9 Data Acquisition: Survey Design
3D seismic
marine: grid of ship lines, multiple streamersreceivers are always close to in-line, so line direction matters for a dipping geologic target
land: grid of shots, multiple geophone lines record each shotvery flexible 3-dimensional survey design possible
marine land
Yilmaz 2001 Reynolds 1997 Yilmaz 2001
4D seismic= time-lapse seismicrepeat a survey to monitor changes: e.g., due to fluid flow, deformation
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Hole: GEOS 4174 2.2-10 Data Acquisition: Survey Design
refraction
to resolve dipping structure, need a reversed refraction line: shots at both endsmany refractors, or continuous increase in velocity with depth, gives turning raysto resolve 2D structure, need many shots recorded on same receivers => fixed spread
Lester MS thesis 2006
refraction shot-receiver offset is usually 5-20 times the depth of imaginglonger rays means lower frequency (for a given depth of imaging) => larger shotsS/N usually good because there is no reflection coefficient to partition energy
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Hole: GEOS 4174 2.2-11 Data Acquisition: Survey Design
Vertical Seismic Profiling (VSP)
1D is most common “VSP walkaway” for 2D image 3D VSP is rare
Reynolds 1997 Ikelle & Amundsen 2005 Paullson et al. 2004 First Break
1D gives very good velocity as a function of depth1D gives absolute depth of reflectors, tie to surface reflection section
2D, 3D gives high-resolution velocity and reflection sectionhigher resolution (higher frequency) than surface data
receivers closer to targettravels through weathering layer only once
VSP image volume is relatively small, close to well
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Hole: GEOS 4174 2.2-12 Data Acquisition: Survey Design
cross-borehole imaging
distance <200 m high resolution due to proximity to target and high frequency
travel times give seismic velocity between wells
Reynolds 1997 Sheriff & Geldart 1995reflection imaging can be performed both above and below the source
Reynolds 1997