An experimental approach on calcarenite reservoir dilation factor

The time of flight of a plane wavefront generated from an acoustic pulse is expected to decrease when the medium length between the wave emitter and receiver is shortened. This simple idea is extrapolated to the case of reservoir compaction in order to obtain a geophysical parameter R (dilation factor) that relates the rock deformation to the variation of time of flight (also called time-lapse time shift in 4D seismics) or acoustic velocity of a plane wave propagating in the same direction of deformation. Interpretation of a few laboratory compressive tests with simultaneous ultrasonic acquisition, performed on oil-saturated calcarenite samples, are presented and discussed. The samples were subjected to several stress regimes and simultaneous ultrasonic acquisitions. Despite the formerly ultrasonic acquisition rate limitations, it was possible to obtain R values for various lateral-vertical stress ratios for each sample's linear and nonlinear stress-strain trends.

Wavefronts, light rays and caustic associated with the refraction of a plane wavefront by a conospherical lens

The aim of the present work is to introduce a lens whose faces are a conical surface and a spherical surface. We illuminate this lens by a plane wavefront and its associated refracted wavefronts, light rays and caustic are computed. We find that the caustic region has two branches and can be virtual, real or one part virtual and the other real, depending on the values of the parameters characterizing the lens. Furthermore, we present a particular example where one of the branches of the caustic region is constituted by two segments of a line, one part is real and the other one virtual. The second branch is a two-dimensional surface with a singularity of the cusp ridge type such that its Gaussian curvature is different from zero. It is important to remark that for this example, the two branches of the caustic are disconnected. Because of this property and the result obtained by Berry and Balazs on the relationship between the acceleration of an Airy beam and the curvature of its corresponding caustic, we believe that using this optical element one could generate a scalar optical accelerating beam in the region where the caustic is a two-dimensional surface of revolution, and at the same time a scalar optical beam with similar properties to the Bessel beam of zero order in the region were the real caustic is a segment of a line along the optical axis.

Comparative analysis of methods for calibration of aShack-Hartmann wavefront sensor

This paper is devoted to a comparative analysis of the methods for calibrating the Shack-Hartmann wavefront sensor: on the plane wavefront (calibration by turning the sensor) and spherical wavefront (absolute calibration). For comparison, a gen-eral analysis of the advantages and disadvantages of calibration methods was carried out, including the creation of a reference wavefront and the determination of the ex-act geometric parameters of the sensor. The geometric parameters of the Shack-Hartmann wavefront sensor are used in the reconstruction of the measured wave-front and determine such parameters of the Shack-Hartmann sensor as the dynamic range and sensitivity. Also, for numerical comparison, an analysis of the errors of the dynamic range, determined by aberrations such as tilt and defocus, was carried out, and the dependences of these errors on the error of the geometric parameters of the sensor were constructed.