GB1316497A - method and apparatus for visual representation in a model of a surveyed medium - Google Patents

Abstract

1316497 Exploration by wave-energy; modulating light INSTITUT FRANCAIS DU PETROLE DES CARBURANTS ET LUBRIFIANTS 18 May 1970 [19 May 1969 11 Dec 1969] 24029/70 Headings H4D and H4F Recordings of waves received at several spaced reception points from one or more transmission points (or at one or more reception points from several spaced transmission points) and representing reflections or diffractions of the waves within a surveyed zone are, composited to give stacked amplitude values associated with respective notional points arranged in an arbitrary array in the zone, and those notional points having the highest amplitude values are identified as the diffracting points or as reflection images of the transmission and reception points and are represented in a reduced scale model of the zone. The waves may be acoustic or radio waves and the invention is applicable to, e.g., seismic surveying or biological inspections. To locate reflector M (Fig. 4), reflected waves from source S are recorded at receivers C 1 ... C n as separate traces. An array of notional points p<SP>1</SP> is assumed and the travel times t<SP>1</SP> 1 = P<SP>1</SP>C 1 /V m , ... t<SP>1</SP> n = P<SP>1</SP>C n /V m are determined, V m being the average wave velocity through the zone and the law of any velocity variation with distance being known. Trace samples from the C 1 trace at time t<SP>1</SP> 1 , the C 2 trace at time t<SP>1</SP> 2 and so on are composited to give a stacked amplitude value assigned to a point P<SP>1</SP>. Similarly, values are assigned to others of the points P<SP>1</SP>. The point with the maximum assigned value is identified as being coincident with the image S<SP>1</SP> of source S in reflector M; hence reflection point s is determined. Other reflection points on M are determined by repeating the above procedure with other source positions. The procedure can be adapted to locate diffraction points, multiple reflectors and inclined reflectors (Figs. 1, 3, 7, 8, 9, none shown) and may involve two-dimensional arrays of sources and receivers (array P<SP>1</SP> then being three-dimensional). The assigned values are obtained and represented by means of optical or hydraulic models. The optical model of Fig. 10 comprises a liquid-crystal layer 2 sandwiched between transparent (glass or plastics) plates 3 in a regulated-temperature jacket 5. Signals from the wave receivers C 1 ... C n have their timescale compressed and reversed in unit 1 (e.g. by transferring a digital recording of them to magnetic disc storage) and then drive respective piezo-electric transducers e 1 ... e n to produce ultrasonic waves of propagation velocity V<SP>1</SP> in the model. By making the model dimensions represent a vertical section through the surveyed zone with a scale reduction factor α, and by making the time-scale compression factor V<SP>1</SP>α/V m , the ultrasonic waves will produce energy concentrations, and hence colour changes, at points in the liquid crystal representing the diffraction points and reflection images within the surveyed zone. These changes are recorded by camera 4. If the liquid crystal and transparent plate thicknesses are less than the ultrasonic wavelength, V<SP>1</SP> is a function of the relative thicknesses, which thicknesses can be tailored to compensate for changes of V m with depth in the surveyed zone. When two-dimensional source/ receiver and transducer arrays are used, layer 2 is replaced by a volume of liquid crystal. The Fig. 11 optical model also uses a liquid-crystal sandwich 2, 3 which in this case is movable vertically in a water-filled tank 11 to produce representations of the diffraction points and reflection images in a selected horizontal section through the surveyed zone. The hydraulic model of Fig. 12 comprises the transducers e 1 -e n mounted on a vertically movable plate 12 in a water-filled tank 13 and producing a ripple pattern on the water-surface, the highest ripples representing the diffraction points and reflection images in a selected section through the surveyed zone. The ripple pattern is photographed by means of an optical system comprising a collimated light beam 15, a mirror 16 and a lens 17. In other hydraulic models the transducers e 1 ... e n are arranged on a water-tank wall near the water surface (Fig. 13, not shown) to produce a ripple pattern on the surface, or the transducers vibrate rods dipping down into a water tank to produce a ripple pattern (Figs. 14 and 15, not shown).