In seismic exploration, an index set of travel time curves is specified. This index set of travel curves is used to determine velocity by a signal detection technique. At different values of zero offset time, different velocities are associated with each of the curves. Since the index set of travel time curves is computed only once, for a prescribed velocity time range, considerable computer time is saved. BACKGROUND OF THE INVENTION This invention relates to the continuous estimation of acoustic velocity from a set of seismic traces and more particularly to producing an index set of travel time curves for use in the estimation of acoustic velocity. In seismic exploration, the measurement of velocity is generally considered the major parameter in the processing and interpretation of seismograms. The determination of the acoustic velocity characteristics of seismograms is described in "Seismic Velocities From Subsurface Measurements," C. H. Dix, Geophysics, Vol. 20, pages 868-886, 1955. In general, acoustic velocity is determined from seismograms by the following relationship: .DELTA.T.sup.2 = X.sup.2 /V.sup.2 - 2 T.sub.0 .DELTA.T One type of field technique which produces sets of traces in which the reflection times follow the above relationship is commonly referred to as common depth point, or common reflection point, seismic exploration. The acoustic velocity characteristic of the earth is an important parameter used in geophysical interpretation. It is also necessary to know acoustic velocity as accurately as possible in order to make normal moveout corrections of the seismic traces. U.S. Pat. No. 3,417,370, Brey, shows a typical system in which signal detection techniques are used to estimate acoustic velocity from seismic traces. In techniques such as this velocity is computed for each trace for each time of interest, typically for equally spaced zero offset times. These computations of velocity consume a great amount of computer time. SUMMARY OF THE INVENTION In accordance with an important aspect of the present invention, an index array of travel time curves specifies .DELTA.Tversus X for a set of velocities passing through sample points on the outer trace of a set. This array is computed only once and it could be used to obtain velocity at every CDP set of traces with a minimum of computation time. Initially, an a priori estimate of velocity is made to determine the range of velocities over which arrival time curves will be computed. Then the array of travel time curves is computed in this bracketed range. At the first sampling time on the first trace in the set of traces, a set of travel time curves is generated relating the horizontal distance X to each source, the time shift from trace to trace .DELTA.T, the acoustic velocity V, and the zero offset time T.sub.0. For succeeding zero offset times, the velocity associated with each curve in the set is recomputed. Seismic reflections in the traces are identified by detecting the signal across the traces along the travel time curves. In one particular signal detection technique, the traces are summed along each of the travel time curves in the set. The maximum signal power identifies the proper velocity. In accordance with a further aspect of the present invention, the selected acoustic velocity is plotted at the zero offset time and at the horizontal position for which it was selected. A determination is made as to whether selected reflections have continuity between sets of seismic traces. If continuity is detected, a continuous line segment is plotted with the associated acoustic velocity.
An ultrasonic testing method and apparatus stores the reflected ultrasonic signals from each of a plurality of locations along a scan path over a piece of material to be tested. For each individual location, the reflected signal at that location is added to contributing signals from each other location to produce a composite signal which has a good signal-to-noise ratio. The contributing signals from each other location is formed by multiplying the signal of those other locations by a corrective phase shift factor which is a characteristic of a time delay for a reflected signal from the other locations to the individual location of interest. The contributing signal from each other location thus represents a rotated reflected signal from the other locations.
The specification discloses a method and system wherein an operator interacts with an automatic data processing and display system to classify and sort subsurface reflective surfaces or segments in a multidimensional context. The technique utilizes a properly programmed general purpose digital computer in combination with a plurality of storage tube display screens. Seismic reflection data in the form of computer picked reflection segments, hereinafter referred to as seismic segment data, produced from previous computer processing steps is entered into the computer. Displays of the velocity, amplitude, dip and length of the segments in a space gate are then provided on the display screens. A data responsive surface having a network of conductive wires and a detecting stylus are utilized by the operator to operate upon the displayed segment data to classify primary segments and to define boundaries relative to each of the parameter displays. The computer sorts the data according to the defined boundaries and segments are displayed which meet the defined boundary conditions. Provision is made to enable easy alteration of the displayed segment parameters and designated boundaries.
Field seismograms are gathered into sets having reflections from asymmetrical common reflection points where there is a conversion between longitudinal waves and shear waves. The gathered seismograms are stacked to produce a seismic section which emphasizes the reflections from particular types of formations. In particular, reflections from formations having a significant difference in longitudinal to shear wave velocity, such as oil and gas saturated formations, are emphasized in the stacked section.
A multiple coverage seismic exploration technique provides for a plurality of seismic trace recordings along a line of exploration. From these recordings, sets of common-offset-distance traces are gathered. Initial estimates are made of the apparent dips associated with the seismic reflection signals across each set of common-offset-distance traces. These initial dip estimates are smoothed and the sets of common-offset-distance traces filtered along the apparent dips associated with the smoothed dip estimates to enhance the signal-to-noise ratio of the primary reflection signals.
In seismic exploration a method for improving seismogram displays is disclosed wherein seismograms having an offset which produces poor signal power are muted and seismograms having offsets with high signal power are stacked. The signal power in a CDP set is determined as a function of different values of assumed acoustic velocity of the earth. This signal detection is repeated for successive values of travel time and offset. The maximum signal power is plotted for values of velocity as a function of travel time and for different ranges of offset to form contours of the maximum signal power as a function of time and offset.
