Patent GB-1150361-A (GB)
Title: Method of producing a Gravitational Map
Inventor: LAWRENCE PHILIP LINWOOD
IPC: G01V7/06
Filed: 1966-05-17
Abstract: 1,150,361. Measuring force of gravity. MOBIL OIL CORP. May 17, 1966, No.21750/66. Heading G1W. A method of producing a gravitational map which will accentuate anomalies in gravitational potentials obtained over selected areas in order to provide information as to the depths of the masses giving rise to them comprising (a) generating from a group of gravity profiles across the area a set of groups of smoothed gravity profiles by smoothing each gravity profile over all the smoothing intervals in a set of differing smoothing intervals extending along the gravity profiles. (b) generating area-gravity values each by combining a plurality of gravity profiles smoothed over the same smoothing intervals (i.e. in the same group) (c) generating difference values by subtracting an area-gravity value in one group from an area-gravity value in another group and (d) developing a map of the difference values. A set of averaging operators is used to describe the method designated 2<SP>n</SP>T/g(xy) (where n is an integer and T a distance over the ground) which operator is to be read as the average value at a point (x, y) of the gravity g measured over a square 2<SP>n</SP> T on each side of and centred about the point (x,y). When T was chosen at 3, 000 feet it was found for example that masses at a depth of between 12 and 24 thousand feet were accentuated on a map developed from difference values obtained from data generated by operators 8T/g(x,y) and 16T/g(x,y). Fig. 1 (not shown) illustrates a gravity map which may have superimposed thereon a grid of points representing stations (separated horizontally and vertically by distances T) at which gravity values have been measured or established by interpolation. Each horizontal row thus provides a gravity profile which may be represented (i) in an analogue system by amplitude signals on any reproducible medium as, for example, magnetic tape or variable area film and (ii) by stored values in a digital system. Analogue computing equipment described for carrying out the method of the invention, comprises a series of magnetic drums A, B, C, D and E which can be driven synchronously by a motor 31. The gravity profile of each row is pre-recorded on a separate track round the drum A and a pick-up head Pn is provided for each track. The heads Pn become operative one at a time in successive revolutions of the drums to apply signals to an integrator 35 for smoothing the profiles and temporarily recording the smoothed profiles on a drum 32 in the same manner as the signals obtained by scanning variable area film in Specification 985, 510 and using the same smoothing operator. Pairs of pick-up heads equispaced about a reference point on drum 32 apply signals representative of the recorded gravity profile in pairs of opposite sign to amplifiers 42, 74, 101, 106 and the algebraic sums they produce are applied, through resistors tapped to divide them respectively by the operators 2T, 4T, 8T, 16T corresponding to the respective spacings of the pairs of heads, to recording heads for recording on the set of drums B, C, D, E, each in a track corresponding to the track of drum A from which the profile was picked up. To develop a desired map from the data now recorded on the drums A to E, gravity profiles stored on any one drum can be combined to generate areagravity values. If seventeen profiles are picked up simultaneously from drum E and averaged in amplifier 107 to obtain an output proportional to 16T/g(x,y) and this is subtracted from an output of amplifier 103 proportional to 8T/g(x,y) by an amplifier 108 to provide difference values 8T/g(x,y) - 16T/g(x,y)  for all the points (x, y) in an area, then when these difference values are plotted e. g. by apparatus like that described with reference to Fig. 6 (not shown) a map can be developed accentuating the anomalies caused by masses in the depth range twelve to twentyfour thousand feet, T being 3, 000 feet. A data processing system is described with a detailed program for controlling a computer to carry out the method of the invention when presented with punched cards storing gravity profiles to develop gravity maps with accentuated anomalies.