Title of Invention

A METHOD FOR SIMULTANEOUSLY VISUALIZING AND COMPARING IMAGES OR VOLUMES OF PHYSICAL QUANTITIES

Abstract

Abstract Method for visualizing and comparing two images or volumes of data of physical quantities or information referable to the same, recorded by means of suitable equipment, comprising the following steps: -defining which of the two quantities will be represented as colour variations, and which as brightness variation -defining a chromatic representation system based on three colour coordinates, in which one coordinate represents the brightness and the other two coordinates represent appropriate colour attributes; -applying a suitable transformation to the values of the physical quantities to be represented, so that the values of the physical quantities to be visualized are transformed from the original coordinates into the preselected chromatic coordinates; -transforming the values of the quantities to be visualized from the system of pre-selected coordinates into the chromatic coordinates typical of the pre-selected visualization system; -visualizing the image/volume containing the combination of the two quantities.

Full Text

METHOD FOR VISUALIZING AND COMPARING IMAGES OR VOLUMES OF DATA OF PHYSICAL QUANTITIES The present invention relates to a method for visual¬izing and comparing images or volumes of data of physical quantities or information referable to the same recorded by means of suitable equipment. The method can be applied for representing several types of quantities: in particular it can be successfully applied for studying the movement of fluids in a hydrocar¬bon deposit by means of the technology called "4D", which includes the acquisition and processing of two or more seismic surveys recorded at a distance of a few years from each other. The first seismic survey is called "base", whereas the subsequent surveys are called "monitor". The movement of fluids in the deposit causes a change in the acoustic impedance of the deposit; if the conditions are favourable, the variation can be identified by compar¬ing the acoustic impedances estimated from the various seismic surveys. In principle, it is therefore possible to optimize the exploitation of hydrocarbon deposits using the 4D tech¬nique, by identifying, for example, the non-produced areas, the permeability barriers, etc.. The 4D technology however cannot be easily applied, as numerous factors limit its efficacy: noise, repeatability of the surveys, mechanical characteristics of the reservoir rocks, etc.. Furthermore, even if the acquisition conditions of the 4D surveys are favourable, the joint interpretation of the 4D data is not easy, as the interpreter has to deal with a multiple amount of data with respect to that typical of a 3D seismic survey: in addition to the initial surveys, in fact, the subsequent monitor surveys and/or their differ¬ences with respect to the base survey, .must be examined, si¬multaneously. Visualization of seismic data is currently made by means of two representation techniques: the "wiggle" pres¬entation, wherein the magnitude of the seismic signal is represented as a graph, and the "raster" presentation, wherein the seismic signal magnitude is represented as a shade of grey or a colour (R. Sheriff: Encyclopaedic Dic¬tionary of Applied Geophysics, SEG, ISBN 1-56080-118-2). The two representation methods can be combined, so as to simultaneously represent two seismic signals, or a seismic ' signal and a quantity associated with it (velocity, imped¬ance, etc. . ) . As previously mentioned, the 4D method requires the si¬multaneous visualization of the acoustic impedances of the base and monitor surveys, as the interpreter must be in a condition to evaluate both the initial impedances, and their variation with time. So far, the method used for the visualization of 4D data has been based on the simultaneous representation of the impedance data; it was observed how¬ever that the interpretation was not easy, as the useful human vision range has a limited angular opening, and com¬paring two noisy images one close to each other, is not al¬ways easy. The two data could be visualized simultaneously by means of a wiggle/raster combined representation, but tests proved that the resulting image is practically use¬less . With the aim of easing the interpretation of 4D data, a new method has been found with the production of innovative visualization equipment. The instrument is based on the original idea of simul¬taneously visualizing the data of the base survey together with the data of the monitor survey, combining them in a single image. The interpreter's task is thus facilitated as the amount of data to be simultaneously visualized is re- duced, and also because the simultaneous vision of the base and monitor surveys allows a rapid identification of par¬ticulars which would have been difficult to identify by ex¬amining the two surveys separately. The same technique can also be applied, after suitable adaptation, outside the 4D seismic field, when the differ¬ence between seismic surveys must be examined, in order to evaluate, for example, the difference between two seismic elaboration sequences, the difference between "near" and "far" traces in AVO analysis, etc., or, more generally, whenever it is necessary to examine the difference between two generic physical or geophysical quantities. The visualization and comparison method dedicated to the representation of 4 D acoustic impedance images, must have various basic requisites: • it must give a graphic representation which is coherent with the standards normally used for the visualization of acoustic impedance: it is normal practice, in fact', to identify low impedance areas by means of a colour (normally red) and high impedance areas by means of a contrasting colour (normally blue); • it must allow the visualization of the two surveys in a single image, so as to allow the simultaneous observa¬ tion of the initial impedance and its variation with time. A Study of the functioning of the human vision has al¬lowed a visualization technique having the prescribed req¬uisites, to be found. The human apparatus of visual percep¬tion, does, in fact, have, among other things, these char¬acteristics (W. K. Pratt: Digital Image Processing, J. Wiley & sons, New .York 1991, ISBN 0-471-85766-1; M. Del-brijck: Mind from Matter? An Essay on Evolutionary Episte-mology, Blackwell Scientific Publications, Palo Alto, 1986): • there are two types of photoreceptors: cones (sensitive to colours) and rods (sensitive to brightness) . There are three different types of cones, whose sensitivity is maximum for different colours • the response of the human sight system to variations in brightness, is such that, assigning brightness I = 0 to black and I = 1 to white, brightness variations of the type AI/I = constant, are uniformly perceived when the brightness I is approximately included in the range [0.25-0.75]: outside this range the response of the system becomes strongly non-linear. The method proposed exploits the idea of encoding the percentage variations in acoustic impedance as brightness variations (maintaining the colour hue and saturation con¬stant) and the impedance values of the base survey as variations in colour hue and saturation (maintaining the brightness constant). This is made possible by using a suitable system of chromatic coordinates, which are then transformed into the chromatic coordinates (normally RGB) used by the graphic visualization systems. The method, object of the present invention, for visu¬alizing and comparing two images or volumes of data of physical quantities or information referable to the same, recorded by means of suitable equipment, comprises the fol¬lowing steps: defining which of the two quantities will be repre¬sented as colour variation, and which as brightness variation; defining a system for chromatic representation based on three colour coordinates, in which one coordinate rep¬resents the brightness and the other two coordinates represent appropriate colour attributes; applying a suitable transformation to the values of the physical quantities to be represented, so that the val¬ues of the physical quantities to be visualized are transformed from the original coordinates into the pre¬selected chromatic coordinates; transforming the values of the quantities to be visual¬ized from the system of pre-selected coordinates into the chromatic coordinates typical of the pre-selected visualization system; visualizing the image/volume containing the combination of the two quantities. The transformation in general includes linear and non¬linear operations, also aimed at improving the representa¬tion of the quantities (filtering, threshold application, etc.), compensating the intrinsic limits of the technology used for the representation of the image, and compensating the limits of the human sight apparatus. The codification method also allows the operators of the image processing normally used, such as limit thresh¬olds, gamma correction, denoising, etc. to be applied sepa¬rately to the two image components (difference and base im¬pedance) . In this way it is possible to emphasize at will the details of the seismic data under examination. The results obtained by applying the visualization technique described herein to a rfeal case, show that the simultaneous codification of 4D surveys in a single image, allows the user to easily identify the interesting regions of the survey, as areas having different properties appear with different colour shades: it is therefore quite simple to identify the areas which require a deep analysis, from those which are associated with false signals (caused by noise, lithological effects, etc.). Furthermore, the grouping of the two surveys into a single image allows the immersion of the image into environments of virtual real- ity, thus improving the understanding of the seismic data. This visualization and comparison method can be prof¬itably adopted for the simultaneous visualization of other data of interest for seismic exploration. For example: • amplitude and phase of a seismic signal • "near offset" and "far offset" amplitudes of an AVO survey • comparison between two seismic signals subjected to different elaborations. It can be affirmed that, in general, the visualization method according to the invention can be used for the visu¬alization and comparison of any physical quantity, in par¬ticular a geophysical quantity. The economical advantages expected from the use of this technology are mainly linked to a reduction in the work time needed for the qualitative analysis of time lapse seismic data (TLS) . This can be quantified as a 50% reduc¬tion in the times necessary for the interpretation of TLS data and relative supporting results (seismic attribute maps). In addition to the undoubted economical advantages, this technology allows the interpreter a better interpreta¬tion of TLS data: it seems reasonable to assume that the application of the technology also to other seismic data (AVO maps, amplitude/phase attributes, etc..) can produce analogous benefits. An embodiment example, in which further technical de¬tails are described for the detailed description of the technology, is provided for a better illustration of the present invention. Example The transformation of two 3D seismic images forming a 4D survey in a single image is made in the following way. Let us assume that B = B{x, y, z) and M = M(x, y, z) are the acoustic impedances of the 3D base and monitor sur¬veys which form the 4D survey. For the representation of the colours of an. image, we will adopt a system of chromatic coordinates which allows brightness to be separated from the colour shade, such as, for example, the system called YCBCR (ITU-601) . As image visualization systems (computer monitor, printers, etc..) normally use RGB chromatic coordinates, the transformation Y = 0.299 R + 0.'587 G + 0.114 B CB = - 0.168736 R - 0.331264 G + 0.5 B + 0.5 (1) CR = 0.5 R - 0.418688 G - 0.081312 B + 0.5 allows the coordinates YCBCR to be associated with each other, and the RGB coordinates normally used for the visu¬alization on a graphic peripheric unit. Transformations similar to (1) also exist for other co¬ordinate systems, the choice of the YCBCR system is there- fore not compulsory. The two 3D base and monitor surveys are combined with each other to obtain the percentage difference: M(x,y,z) - B {x,y,z) D = D (x.y.z) = (2) B (x,y,2) [Dm DM] being the range which includes all the values as¬sumed by D. The variation range is typically included be¬tween -0.1 / 0.1. D represents the percentage variation in the impedance between one survey and the other of 4D: bearing in mind the characteristics of the human sight system previously de¬scribed, it is evident that if D is codified as brightness of an image and if the range [D^ DM] is associated with the brightness range wherein the eye reaction is uniform, the variations in the acoustic impedance of 4D will be cor¬rectly perceived by the user. Assigning the value of zero to the absence of brightness (black) and the value of 1 to the maximum brightness (white), the optimum codification is obtained by transforming the range [Dm DM] into the range [0.25 0.75]. This is easily obtained by establishing: y = 0.5 ((D - Dn,i„) / (Dn>az " D^in) ) +0.25 (3) (The limits 0.25 and 0.75 can be possibly adapted so as better adapt the transformation to possible diversities of the sight apparatus of the user). It may be convenient to process the D and/or Y values by introducing, for example, thresholds on the minimum and maximum values assumed by D, or applying a "gamma correc¬tion" operator (Pratt, [2]) to the Y values found, of the type: Y(Y) = YY (4) so as to enhance/mitigate various characteristics of the 4D survey. Furthermore, (non) linear filters can be applied, if necessary, to the Y values in order to attenuate the noise, etc.. The image created so far, contains a black and white representation of the acoustic impedance variations of the 4D survey: we must now superimpose the image of the base survey, without modifying the image brightness. The use of the chromatic coordinates YIQ allows the above to be easily obtained: it is in fact sufficient to encode the informa¬tion of the impedance of the base survey in the coordinates CB, CR, without modifying the Y value. The function which associates B(x,y,z) with the CB and CR values must be se¬lected so as to respect the representation standard of the acoustic impedance normally used. This can be easily accom¬plished by means of a couple of functions of the type: CB = f(B.) CR = g(B) In our case, we have selected (but other solutions are wherein are the minimum and maximum value assumed by B(x,y,z), respectively. Also in this case, it is obviously useful to apply operators of pre/post image proc¬essing, as for the previous case. At this point, an image is obtained codified in YCRCB which, once transformed in the RGB space, can be visualized on a graphic peripheral unit of the traditional type (moni¬tor, printer, etc.). CLAIMS 1. A method for visualizing and comparing two images or volumes of data of physical quantities or information ref¬ arable to the same, recorded by means of suitable equip¬ mint, comprising the following steps: defining which of the two quantities will be repre¬sented as colour variation, and which as brightness variation defining a system for chromatic representation based on three colour coordinates, in which one coordinate rep¬resents the brightness and the other two coordinates represent appropriate colour attributes; applying a suitable transformation to the values of the physical quantities to be represented, so that the val¬ues of the physical quantities to be visualized are transformed from the original coordinates into the pre¬selected chromatic coordinates; transforming the values of the quantities to be visual¬ized from the system of pre-selected coordinates into the chromatic coordinates typical of the pre-selected visualization system; visualizing the image/volume containing the combination of the two quantities. 2. The method according to claim 1, wherein the physical quantities are seismic quantities. 3. The method according to claim 2, wherein the seismic quantities are acoustic impedance, elastic impedance, mag¬nitude, instantaneous frequency, phase or velocity of seis¬mic waves.

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