Novel Marine Electromagnetics: from Deep into Shallow Water · 3 A Brief History of CSEM • Deep...
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KMS Technologies – KJT Enterprises, Inc.
Novel Marine Electromagnetics: from Deep into Shallow Water
Thomsen, L., Meaux, D., Li, S., Weiss, C., Sharma, A., Allegar,
N., and Strack, K.-M.
Recent Advances and the Road Ahead SEG, San Antonio
2007
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Leon Thomsen1, David Meaux1, Shenghui Li1, Chet
Weiss1,2, Arvind Sharma1, Norm Allegar1,3, Kurt Strack3
Recent Advances and the Road Ahead Society of Exploration Geophysicists, San Antonio, Sept. 24, 2007
1 BP 2 Virginia Tech 3 KMS
DRAFT 5, 9/19/07
Novel Marine Electromagnetics: from Deep into Shallow Water
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Outline
• Context • Seismics and EM: the deep connection • CSEM, seismic-style • The Egyptian Field Trial • Conclusions
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A Brief History of CSEM
• Deep roots in whole-earth, academic geophysics: − e.g. Parker,1977… − e.g. Constable et al, 1987…
• Grew out of natural-source EM (“magnetotellurics”), inherently a continuous source (“always on”); moved to controlled-source, as technology developed.
• Note analogies with seismics: − Seismic refraction, using techniques adapted from
whole-earth, academic seismologists ( with natural (earthquake) sources).
− Ocean Bottom Seismics evolved late.
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CSEM Acquisition
Base figure courtesy of:
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CSEM in practice
• Marine CSEM practice evolved with a set of expectations:
− Source same as natural source = “always on”.
− (Uses variations on a square wave, with frequency < 1 Hz.)
− Data averaged over many cycles.
− Expressed as amplitude (MVO) and phase (PVO) as functions of source/receiver positions.
− Data formally inverted for resistivity distribution in subsurface.
− (Requires accurate knowledge of source & orientation; navigation uncertainty is currently accuracy limitation .)
− Offset larger than depth of investigation
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As a consequence?
• The technique ;imited to deep water (> ~2x target depth)
− Water shields from interference by the “air wave”
− Many proprietary solutions to the “deep-water limitation” exist;
none satisfactory.
• Could there be a fundamentally different solution?
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Outline
• Context • Seismics and EM: the deep connection • CSEM, seismic-style • The Egyptian Field Trial • Conclusions
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There is a deep similarity, between seismic and EM data, despite the difference in the fundamental equations
• Previous work highlights similarity between seismic and EM, by e.g. − Jackson (1962), Feynman et al (1964) − Ursin (1983), Lee et al (1987) − Land CSEM: Rueter & Strack, 1991 − Marine CSEM: Amundsen, Hokstad, Rosten; Ellingsrud; Carcione,
(2004-07)
• Further, EM energy propagates at speeds similar to seismic velocities! − e.g. at ω=.25 Hz, ρ=1 Ω-m: Vphs = 1.6 km/sec; Vgrp = 3.2 km/sec
• As for land CSEM, seismic-style acquisition may be used for marine EM acquisition.
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Outline
• Context • Seismics and EM: the deep connection • CSEM, seismic-style • The Egyptian Field Trial • Conclusions
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Acquisition considerations
• In EM, as in seismics, the subsurface signal is very weak; − best to detect it in the absence of source-generated noise.
50 sec
• Data should be acquired unaliased. (We had sources @ 50 m; receivers @ 400 m) • Modeling is important, prior to acquisition.
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Transient-source CSEM acquisition
sediment wave
target wave
We detect the (weak) target signal while the source is off
Base figure courtesy of:
Air wave
water waves
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1-D transient-source modeling
0 - 100 m .3 Ωm
1 Ωm
+ 1 km
100m @100 Ωm Reservoir present
Reservoir absent
scal
e
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Outline
• Context • Seismics and EM: the deep connection • CSEM, seismic-style • The Egyptian Field Trial − Field context − Acquisition − Results
• Conclusions
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Taurus 700 BCF Taurus 700 BCF
30 km
Exploration Context
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15 10 Kms.
Fayoum
= Pliocene Discoveries = Residual Gas
Abu Sir
Several 2-D lines were acquired, with both continuous and transient sources
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Outline
• Context • Seismics and EM: the deep connection • CSEM, seismic-style • The Egyptian Field Trial − Field context − Acquisition − Results
• Conclusions
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Survey Operations: 11/29/06 – 1/2/07
>> 200 5C receiver deployments
Duration: 33 days
BP records world’s first successful at-scale marine transient CSEM data:
December 5th, 2006
The BP Acquisition Team
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Conventional receiver deployment
Photos courtesy of emgs
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Tail buoy
Streamer
Conventional antenna deployment
Photos courtesy of emgs
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Deploying modified source
transformer
Photos courtesy of emgs
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Outline
• Context • Seismics and EM: the deep connection • CSEM, seismic-style • The Egyptian Field Trial − Field context − Acquisition − Results
• Conclusions
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Continuous-source results
Analysis by emgs
Abu Sir
10 Kms. Fayoum
Abu Sir
Abu Sir is clearly detected by such analysis
Line 3 Normalized ratio
Fayoum channel
But conventional techniques fail to detect Fayoum
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These differences are probably due to differing subsurface geology
Narrow channels at Fayoum
Abu Sir reservoir more oblate
Abu Sir
6km
4km
1000m
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Computed pathways of EM energy transport
resistor
sediment
water
air
Propagation is faster, and amplitude is stronger
when the current intersects a resistor
1-D modeling of EM energy transport
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10 km
Subsurface Response
0 Impulse response
1-D transient-source modeling compared to real data
Reservoir present
Reservoir absent
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Common receiver gather, off the reservoir
• trace-normalization
• Only
• air-wave,
• water-wave
• sediment-wave visible
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Common receiver gather, on the reservoir
• Additional energy visible
• Arriving late
• Moving out at seismic-style velocities
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Common receiver gather, on the reservoir
• Additional energy visible
• Arriving late
• Moving out at seismic-style velocities
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Common receiver gather, at edge of reservoir
• Additional energy visible
• Arriving late
• Moving out at seismic-style velocities
West end deep water only
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Outline
• Context • Seismics and EM: the deep connection • CSEM, seismic-style • The Egyptian Field Trial • Conclusions
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Conclusions
• Similarities between EM & seismic allows seismic style
acquisition and processing for EM
• It is viable for marine EM data:
− acquisition with a transient source
− seismic-style acquisition design
− moveout – based noise removal
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Conclusions
• Results:
− Pilot demonstration of marine deep transient measurements
− Case history linked known gas reservoir
− Acquisition completed with industrial CSEM system (with special modifications)
− Frequency & time domain CSEM acquired with one receiver deployment
− Data proves the “seismic-style” understanding
− Time domain CSEM reveals known reservoir where conventional CSEM is not successful.
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Acknowledgements
We thank: • BP for permission to present, and especially − Duncan Attoe, Gary Nicol, Brian Barley, Lisa Rebora and Dave
Cowper (BP Egypt) for material assistance − Eric Green, Michelle Judson, Tim Lane,
Mark Truxillo, and Tim Summers for organizational support • Charles Stoyer (KMS) for custom programming • Tilman Hanstein (KMS) for field operations & processing and
Yardenia Martinez (KMS) for processing assistance • emgs, inc. for field operations
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Comments
• After BP agreed before the presentation, we contributed for free, and after, they finally gave us a paper copy version of the following slides.
• Please do NOT publish or handout or publically present this material. It should be only used for KMS internal use.
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