Transcript
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
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
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N
Reynolds 1997 Yilmaz 2001
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
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
Hole: GEOS 4174 2.2-5 Data Acquisition: Survey Design
stacking chart
plot traces at shot & receiver positions
Yilmaz 2001
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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
Hole: GEOS 4174 2.2-6 Data Acquisition: Survey Design
survey design considerations
Sheriff & Geldart 1995
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
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
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
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
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
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
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