Post on 13-Jan-2016
description
Environmental and Exploration Geophysics II
tom.h.wilsontom.wilson@mail.wvu.edu
Department of Geology and GeographyWest Virginia University
Morgantown, WV
Radar Methods – General Radar Methods – General OverviewOverview
Part 2Part 2
GPR resolution issues will be similar to those for seismic. Resolution will be dependant on the wavelet’s spectral content – it’s bandwidth and peak frequencies, phase (shape) and duration.
Thickness =Vp/4
~30 Hz Peak
~40 Hz Peak
Horizontal resolution and the Fresnel zone
Destructive interference
Constructiveinterference
Horizontal Resolution: The Fresnel Zone
2
2 16f
lR
The Fresnel Zone
Radius Rf
4
4fc
V tR
f
Depth = 2000Velocity = 13500dominant frequency = 50two-way time = 0.296296dominant wavelength = 270Fresnel zone radius = 523.9812Approximation = 519.6152
An approximation
Topographic variations must also be compensated for.
Daniels, J., 1989
West Pearl Queen Field Area
Surface along the GPR line shown below was very irregular so that apparent structure in the section below is often the result of relief across features in the surface sand dune complex.
GPR data is often collected by pulling the GPR unit across the surface. Subsurface scans are made at regular intervals, but since the unit is often pulled at varying speeds across the surface, the records are adjusted to portray constant spacing between records. This process s referred to as rubbersheeting.
Daniels, J., 1989
Smith and Jol, 1995, AG
Increased frequency and bandwidth reduce the dominant
period and duration of the wavelet
Smith and Jol, 1995, AG
Comparison of the 25MHz and 100 MHz records
Smith and Jol, 1995, AG
rsr eAA
We also expect to see decreased depth of penetration (i.e. increased attenuation) for higher frequency wavelets and components of the GPR signal.
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GPR unitSensors & Software Inc. –
Smart CartVisit http://www.sensoft.ca/
Sensors & Software Inc. – Salt Water Infiltration
Sequestration occurred in a depleted oil field.
• Location near Hobbes, NM• West Pearl Queen formation, ~4500 feet
deep• About 20 years of oil and gas production• A few years of inactivity
• Injection of ~2100 tons CO2 over 2 months
• Subsequent production
Perry-Castaneda Library Map Collection
Bromhal et al., 2004
MM&V activities cover a lot of ground
“MM&V is defined as the capability to measure the amount of CO2 stored at a specific sequestration site, monitor the site for leaks or other deterioration of storage integrity over time, and to verify that the CO2 is stored in a way that is permanent and not harmful to the host ecosystem…Subsurface MM&V involves tracking the fate of the CO2 within the geologic formations underlying the earth and possible migration to the surface.” - Sequestration Roadmap, 2004
Bromhal et al., 2004
First, geologic characterization of deep layers
• Nine wells in immediate area• Several hundreds in
surrounding area• 3-D seismic survey
Bromhal et al., 2004
Simulation of flow in reservoir
• Porosity and permeability maps generated
• Production history
• 4-D seismic for validation
Bromhal et al., 2004
The pilot site is located approximately 25 miles southwest of Hobbs, NM. Oil and gas fields in the area are highlighted in red. Map taken from Ward (1986) (http://geoinfo.nmt.edu/ staff/scholle/guadalupe.html An introduction and virtual geologic field trip to the Permian reef complex, Guadalupe and Delaware mountains, New Mexico-West Texas).
West Pearl Queen Field Area Topography is a problem
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Local Topography in A rea of G PR Survey W est Pearl Q ueen P ilo t S ite
Fee
t
E levation data provided by Bob Benson C olorado School of M inesE levations were surveyed for seism ic source and receiver locations at the site
Shallow Diffraction
Diffraction with
velocity of 0.19m/nsShallow
Diffraction
Diffraction with
velocity of 0.19m/ns
Diffraction hyperbola having a velocity of 0.19 m/ns is displayed next to a shallow diffraction originating in the near surface sand cover.
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S tructure on Top of the M escalero C alicheD erived from C alcu la ted Velocity o f 0 .2 m /ns
Ele
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eet)
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S tructure on Top of M escalero CalicheCorrection Velocity Set a t the Anom alously Low Value of 0 .1 m /ns
R elative drop of about 4 feet
Elevation minus depth to the caliche
Mescalero caliche
Plio-Pleistocene
Upper MioceneOgallala Group
ray
0 10 20 30 40 50
dep
th (
feet
)
0
10
20
30
40
50
Quaternary
Gamma ray log from the Stivason #5 well 400 meters east of the Stivason #4 injection well. The low gamma ray response between 4 and 9 feet subsurface is associated with the Mescalero caliche.
0.020
0.030
0.040
1.050.0100.0150.0200.0- 2 -
250.0300.0350.0374.0SP:
0.020
0.030
0.040
GPR profile located 275 meters northwest of the injection well illustrates the general character of the reflection events from the Mescalero caliche
observed across the site.
Missing caliche zone
Missing caliche zone Abrupt thinning and
disappearance of caliche reflection events are illustrated in this GPR profile.
Stivason #5Thickness of the Mescalero caliche estimated from interpreted two-way radar reflection travel times through the caliche using an interval velocity of 0.15m/ns. Interpreted faults/fracture zones and areas of missing caliche are highlighted.
Concentrations of PDCH measured during a 54-day exposure period are superimposed on an orthophoto of the pilot site. The locations of the injection well, CATS, interpreted faults, and eroded caliche areas are shown for reference.
PDCB concentrations measured during the 54 day exposure period.
Near-surface flow models can help in locating samplers, examining results.
• Atmospheric transport • Diffusion in near-surface
Bromhal et al., 2004
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