seismic acquisition

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


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


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


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


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


survey design considerations



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)


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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


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


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


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


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


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

seismic acquisition

Documents

survey line

survey design refraction

survey notes

x receiver x source

common receiver

common offset

receiver offset

d seismic marine