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APPLICATION OF THE CONTINUOUS VELOCITY LOG TO ANISOTROPY MEASUREMENTS IN NORTHERN SAHARA; RESULTS AND CONSEQUENCES
Author(s) -
SEGONZAC PH. DUNOYER,
LAHERRERE J.
Publication year - 1959
Publication title -
geophysical prospecting
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.735
H-Index - 79
eISSN - 1365-2478
pISSN - 0016-8025
DOI - 10.1111/j.1365-2478.1959.tb01463.x
Subject(s) - anisotropy , geology , refraction , borehole , lithology , anhydrite , reflection (computer programming) , gemology , computation , mineralogy , economic geology , geophysics , seismology , petrology , optics , engineering geology , tectonics , geotechnical engineering , physics , metamorphic petrology , volcanism , paleontology , gypsum , algorithm , computer science , programming language
A bstract Anisotropy measurements were carried out in two wells 300 km apart in the Northern Sahara, in order to improve the interpretation of seismic refraction surveys. These measurements were based on; the shortening of experimental oblique travel‐times with respect to theoretical, travel‐times computed by disregarding anisotropy. A perfect knowledge of the velocity distribution is required in order to eliminate all influence of refraction; therefore a continuous velocity log is indispensable. The results in the two wells are in agreement, and show that anisotropy is essentially a function of lithology. The following values were obtained:Consequences pertaining to seismic refraction are as follows: — Depths computed without anisotropy are too small by 8 % to 9 %, i.e. about 300 meters. — The error on offsets reaches 70 %. — Depth computations carried out by assigning anisotropy factors at the sight of lithology gave very satisfactory results. — High velocity, highly anisotropic formations such as anhydrite play a very important part in the propagation of refracted waves and in interpretation. Consequences pertaining to seismic reflection are as follows: — Velocity profiles yield velocities closer to horizontal velocities than to vertical velocities. — Depth and migration computations are significantly influenced by high velocity, highly anisotropic formations such as anhydrite.