Underground GPR measurements for spatial investigations in a salt dome
Ground penetrating radar (GPR) is an efficient non destructive tool for the exploration of underground salt deposits. The complex geological structures (bent, folded, sometimes faulted layering of salt, potassium, clay, and anhydrite) in low-conductive salt can be mapped if distance and direction of reflecting objects are known. In boreholes the information is given by direction sensitive borehole radar. In shafts rotation symmetry helps to detect the direction. In drifts direction finding antennae provide the reflection angle. Various antenna types help to enhance the resolution or the penetration depth. Measurements with different polarizations offer additional information about the reflecting structures. Within the last two decades unique results from a salt dome in Northern Germany with various GPR systems were achieved. With this information and some geological knowledge a three dimensional model of the structures has been created.
Borehole radar measurements in complex geological structures
GPR borehole surveys provide important spatial data on the geological structures within mines. In this paper we want to show how the spatial relationships of complex geological structures are determined in a repository for radioactive waste in salt. Whether a discontinuity within a stratigraphic sequence can be detected depends on differences in the conductivity and permittivity of the geological units. In contrast to data treated in half-space, it is important to record and interpret the data of borehole radar measurements not only as a function of distance but also as a function of direction. The authors have developed GPR instruments and methods for interpreting directional data. Because the wave propagation velocity is known, the data can be a migrated in space by reconstructing the wave front. This allows the construction of reflection horizons that can be precisely oriented spatially. Case studies of GPR borehole measurements to locate anhydrite structures in salt mines will be used to illustrate the value of this method for efficient reconnaissance. The methods for interpreting the reflectors in terms of their geological meaning and for representing them in three-dimensions are discussed. Three-dimensional interpretation allows geologists and miners to better target their activities with greater safety.
Predicting the gas resource potential in reservoir C-sand interval of Lower Goru Formation, Middle Indus Basin, Pakistan
Abstract The integrated study of seismic attributes and inversion analysis can provide a better understanding for predicting the hydrocarbon-bearing zones even in extreme heterogeneous reservoirs. This study aims to delineate and characterize the gas saturated zone within the reservoir (Cretaceous C-sand) interval of Sawan gas field, Middle Indus Basin, Pakistan. The hydrocarbon bearing zone is well identified through the seismic attribute analysis along a sand channel. The sparse-spike inversion analysis has efficiently captured the variations in reservoir parameter (P-impedance) for gas prospect. Inversion results indicated that the relatively lower P-impedance values are encountered along the predicted sand channel. To further characterize the reservoir, geostatistical techniques comprising multiattribute regression and probabilistic neural network (PNN) analysis are applied to predict the effective porosity of reservoir. Comparatively, the PNN analysis predicted the targeted property more efficiently and applied its estimations on entire seismic volume. Furthermore, the geostatistical estimations of PNN analysis significantly predicted the gas-bearing zones and confirmed the sand channel as a major contributor of gas accumulation in the area. These estimates are in appropriate agreement with each other, and the workflow adopted here can be applied to various South Asian regions and in other parts of the world for improved characterization of gas reservoirs.