TY - ABST

T1 - Characterisation of carbonate rocks from near-surface cross-hole and reflection GPR investigations - A case study from southeast Zealand, Denmark

AU - Nielsen, Lars

AU - Looms, Majken Caroline

AU - Hansen, Thomas Mejer

AU - Cordua, Knud S.

AU - Stemmerik, Lars

N1 - Geophysical Research Abstracts, Vol. 12

PY - 2010

Y1 - 2010

N2 - Carbonates found in the near-surface of southeast Zealand, eastern Denmark, are analogous to deposits serving asgroundwater and hydrocarbon reservoirs in the Danish region.The study site is located in the Boesdal limestone quarry. A 20 by 20 m area of the bottom of the quarry waslevelled using a bulldozer, and a grid of 100 MHz and 250 Mhz reflection profiles were collected to facilitategeological interpretation of structures in the uppermost part of the subsurface. Secondly, four 15 m deep boreholeswere drilled in a square geometry with side lengths of 5 m. Core material was recovered from the boreholes forlithological control and to facilitate laboratory measurements of porosity and permeability. Cross-hole GPR datawere collected between boreholes with 100 MHz Sensors&Software antennae. The distance between source andreceiver antenna positions in the boreholes was set to 0.25 m.Mounded features observed in the upper ca. 7 m of the subsurface imaged by the reflection GPR data areinterpreted to represent bryozoan mounds similar to mounds mapped by others along cliff and quarry profilesclose to our study site. Below the base of the mounds, the reflection signals become too weak to facilitate deeperimaging of the carbonates.The section studied with the cross-hole data is water-saturated. Simple 1D modelling of the cross-hole dataindicates a strong drop in GPR velocity at 7 to 8 m depth. Different 2D inversion strategies are tested for fine scaleresolution of the inter-borehole heterogeneity. Sequential simulation strategies seem to be successful with respectto extracting well-defined correlation lengths and variance estimates of the velocity fluctuations. A strategy inwhich the intervals above and below 8 m depth are treated as separate heterogeneous media appears to be moresuccessful in generating well-defined statistical parameters for the GPR velocity field of the subsurface than thetypical strategy in which the total rock section covered by the cross-hole data is regarded as the same type ofmedium. Modelling strategies in which porosity data from the boreholes are included in the inversion algorithmare tested and compared to the results obtained using the more traditional approaches.The GPR investigations may contribute to setting the framework for future fine-grained models designed tosimulate fluid and gas flow in groundwater and hydrocarbon reservoirs.

AB - Carbonates found in the near-surface of southeast Zealand, eastern Denmark, are analogous to deposits serving asgroundwater and hydrocarbon reservoirs in the Danish region.The study site is located in the Boesdal limestone quarry. A 20 by 20 m area of the bottom of the quarry waslevelled using a bulldozer, and a grid of 100 MHz and 250 Mhz reflection profiles were collected to facilitategeological interpretation of structures in the uppermost part of the subsurface. Secondly, four 15 m deep boreholeswere drilled in a square geometry with side lengths of 5 m. Core material was recovered from the boreholes forlithological control and to facilitate laboratory measurements of porosity and permeability. Cross-hole GPR datawere collected between boreholes with 100 MHz Sensors&Software antennae. The distance between source andreceiver antenna positions in the boreholes was set to 0.25 m.Mounded features observed in the upper ca. 7 m of the subsurface imaged by the reflection GPR data areinterpreted to represent bryozoan mounds similar to mounds mapped by others along cliff and quarry profilesclose to our study site. Below the base of the mounds, the reflection signals become too weak to facilitate deeperimaging of the carbonates.The section studied with the cross-hole data is water-saturated. Simple 1D modelling of the cross-hole dataindicates a strong drop in GPR velocity at 7 to 8 m depth. Different 2D inversion strategies are tested for fine scaleresolution of the inter-borehole heterogeneity. Sequential simulation strategies seem to be successful with respectto extracting well-defined correlation lengths and variance estimates of the velocity fluctuations. A strategy inwhich the intervals above and below 8 m depth are treated as separate heterogeneous media appears to be moresuccessful in generating well-defined statistical parameters for the GPR velocity field of the subsurface than thetypical strategy in which the total rock section covered by the cross-hole data is regarded as the same type ofmedium. Modelling strategies in which porosity data from the boreholes are included in the inversion algorithmare tested and compared to the results obtained using the more traditional approaches.The GPR investigations may contribute to setting the framework for future fine-grained models designed tosimulate fluid and gas flow in groundwater and hydrocarbon reservoirs.

M3 - Conference abstract for conference

Y2 - 2 May 2010 through 7 May 2010

ER -