Egon Nørmark

Dynamic smoothing in crosswell traveltime tomography

Variable-size (dynamic) smoothing operator constraints are applied in crosswell traveltime tomography to reconstruct both the smooth- and fine-scale details of the tomogram. In mixed and underdetermined problems a large number of iterations may be necessary to introduce the slowly varying slowness features into the tomogram. To speed up convergence, the dynamic smoothing operator applies adaptive regularization to the traveltime prediction error function with the help of the model covariance matrix. By so doing, the regularization term has a larger weight at initial iterations and the prediction error term dominates the final iterations with a small regularization term weight. In addition, it is shown that adaptive regularization acts by reweighting the adjoint modeling operator (preconditioning) and by providing additional damping. Comparisons of two dynamic smoothing operators, the low-pass filter smoothing and the multigrid technique, with the fixed-size (static) smoothing operators show that the dynamic smoothing operator yields more accurate velocity distributions with greater stability for larger velocity contrasts. Consequently, it is a preferred choice for regularization.

Detecting human-knapped flint with marine high-resolution reflection seismics: A preliminary study of new possibilities for subsea mapping of submerged Stone Age sites

Seismic high-resolution Chirp profiles from the welldocumented submerged Stone Age settlement Atlit-Yam, located off Israel’s Carmel coast, display systematic disturbances within the water column not related to sea-floor cavitation, vegetation, fish shoals, gas or salinity/temperature differences, where flint debitage from the Stone Age site had been verified archaeologically. A preliminary series of controlled experiments, using identical acquisition parameters, strongly indicate that human-knapped flint debitage lying on the sea floor, or embedded within its sediments, produces similar significant responses in the water column. Flint pieces cracked naturally by thermal or geological processes appear not to do so. Laboratory experiments, finite element modelling and controlled experiments conducted in open water on the response to broad-spectrum acoustic signals point to an excited resonance response within humanknapped flint even for sediment embedded debitage, with acoustic signals within the 2–20 kHz interval. The disturbances observed in the water column on the seismic profiles recorded at Atlit-Yam are, therefore, based on these results, interpreted as resonance from human-knapped flint debitage covered by up to 1.5 m of sand. Such a principle, if substantiated by further research, should facilitate efficient and precise mapping of submerged Stone Age sites.