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

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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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