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.
Correlation of internal salt structures with GPR amplitudes
In salt deposits with low electric conductivity ground penetrating radar (GPR) is an efficient non destructive tool for the exploration of internal structures. Faulted salt layers, potassium, clay, and anhydrite can be mapped by measuring distance and direction of reflecting objects. Various antenna types help to enhance resolution or penetration depth. Additional information about the conductivity of reflecting structures is included in the attenuation of the direct and reflected signals. Within the last two decades unique results from a salt dome in Northern Germany with various GPR systems were achieved. With this information a classification of various salt structures can be made by studying the signal amplitudes of reflections and of the direct wave between transmitting and receiving antenna, especially with borehole measurements.
Velocity moments in alongshore bottom stress parameterizations
The time-averaged alongshore bottom stress is an important component of nearshore circulation models. In a widely accepted formulation the bottom stress is proportional to , the time average of the product of the instantaneous velocity magnitude |u→| and the instantaneous alongshore velocity component υ. Both mean and fluctuating (owing to random, directionally spread waves) velocities contribute to . Direct estimation of requires a more detailed specification of the velocity field than is usually available, so the term is parameterized. Here direct estimates of based on time series of near-bottom currents observed between the shoreline and 8-m water depth are used to test the accuracy of parameterizations. Common parameterizations that are linear in the mean alongshore current significantly underestimate for moderately strong alongshore currents, resulting in overestimation of a drag coefficient determined by fitting modeled (with a linearized bottom stress) to observed alongshore currents. A parameterization based on a joint-Gaussian velocity field with the observed velocity statistics gives excellent overall agreement with the directly estimated and allows analytic investigation of the statistical properties of the velocity field that govern . Except for the weakest flows, depends strongly on the mean alongshore current and the total velocity variance but depends only weakly on the mean wave angle, wave directional spread, and mean cross-shore current. Several other nonlinear parameterizations of are shown to be more accurate than the linear parameterizations and are adequate for many modeling purposes.