Environmental and Exploration Geophysics II tom.h.wilson Department of Geology and Geography West Virginia University Morgantown, WV Radar Methods – General Overview Part 2

Thinning layer response and resolution considerations.

~30 Hz Peak ~40 Hz Peak

Horizontal resolution and the Fresnel zone Destructive interference Constructive interference

Horizontal Resolution: The Fresnel Zone

The Fresnel Zone Radius R f 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 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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In the acquisition of GPR data one must worry about overhead reflections. Daniels, J., 1989

…. and tree branches! Daniels, J., 1989

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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 CO 2 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 CO 2 stored at a specific sequestration site, monitor the site for leaks or other deterioration of storage integrity over time, and to verify that the CO 2 is stored in a way that is permanent and not harmful to the host ecosystem…Subsurface MM&V involves tracking the fate of the CO 2 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) ( staff/scholle/guadalupe.html An introduction and virtual geologic field trip to the Permian reef complex, Guadalupe and Delaware mountains, New Mexico-West Texas).

Short photo lineaments (yellow) are shown on an orthophoto of the test site. The locations of injection well (red) and capillary absorption tube samplers (black) are spotted on the photo.

Rose diagram of photolineaments picked from a contrast enhanced digital copy of a 1949 black and white aerial photo of the area (left) surrounding the West Pearl Queen pilot injection well and orthophoto on the right

Diffraction hyperbola having a velocity of 0.19 m/ns is displayed next to a shallow diffraction originating in the near surface sand cover.

West Pearl Queen Field Area

Structure of the Mescalero caliche is superimposed on an orthophoto of the injection site. GPR survey lines are shown in yellow. Locations of the injection well, CATS, and interpreted faults are also shown.

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.

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.

Thickness 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.

Abrupt thinning and disappearance of caliche reflection events are illustrated in this GPR profile.

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.

PTCH concentrations measured during the 54-day exposure period.

PDCH 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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