Method and system for detecting hydrocarbon reservoirs using amplitude-versus-offset analysis with improved measurement of background statistics

A computer-readable memory having a storage medium configured so that, when read and used by a computer, the computer is directed to correct for offset-dependent effects in seismic survey signals, the seismic survey signals comprising a plurality of series of time-based signals obtained at a plurality of detectors at the earth responsive to seismic energy imparted to the earth, and gathered into gathers of seismic survey signals, each gather representative of energy reflected from depth points below a corresponding one of a plurality of surface locations at a plurality of angles of incidence, said computer directed by a plurality of operations comprising:

• processing the seismic signals in each of the gathers to produce a plurality of stacks, each associated with a location of the earth and each including an AVO intercept trace and an AVO slope trace;
• for each of a plurality of windows, each window associated with a depth point of interest along a selected trace, including a plurality of neighboring traces to the selected trace, and covering a selected portion of the selected and neighboring traces, so that the window includes a plurality of depth points having AVO intercept and an AVO slope values associated therewith, performing the steps of:
• determining a first pair of scale factors, including an AVO intercept scale factor and an AVO slope scale factor, corresponding to the p.sup.th root of a summation of the p.sup.th power of amplitudes of the AVO intercept and AVO slope values, respectively, in the window, using a preselected value of p;
• generating scaled standard deviations of the AVO intercept and an AVO slope traces over the window based upon the first pair of scale factors;
• generating a group of quantities, for each depth point in the window, corresponding to scaled sums and differences of its AVO intercept and AVO slope values;
• determining a second group of scale factors over the window, each scale factor corresponding to the p.sup.th root of a summation of the p.sup.th power of one of the group of quantities over the window;
• generating a scaled correlation coefficient from the ratio of differences of the second group of scale factors to sums of the second group of scale factors;
• and
• modifying the AVO intercept and AVO slope values of the depth point of interest according to a relationship between the scaled correlation coefficient and scaled standard deviations to desired correlation coefficient and standard deviation values for the survey;
• generating AVO indicators from the modified AVO intercept and AVO slope values;
• and
• outputting a survey using the AVO indicators.

A digital computing system for analyzing seismic survey signals to distinguish the presence of sand formations in a region of the earth corresponding to the seismic survey signals, comprising:

• a memory for storing data corresponding to a plurality of series of tie-based signals obtained at a plurality of detectors at the earth responsive to seismic energy imparted to the earth and gathered into gathers of seismic survey signals, each gather representative of energy reflected from depth points below a corresponding one of a plurality of surface locations at a plurality of angles of incidence;
• a computer system output device;
• and
• a programmed computer, coupled to the memory and to the output device, for:
• retrieving, from the memory, digital data corresponding to the plurality of gathers;
• processing the seismic signals in each of the gathers to produce a plurality of stacks, each associated with a location of the earth and each including an AVO intercept trace and an AVO slope trace;
• for each of a plurality of windows, each window associated with a depth point of interest along a selected trace, including a plurality of neighboring traces to the selected trace, and covering a selected portion of the selected and neighboring traces, so that the window includes a plurality of depth points having AVO intercept and an AVO slope values associated therewith, performing the steps of:
• determining a first pair of scale factors, including an AVO intercept scale factor and an AVO slope scale factor, corresponding to the p.sup.th root of a summation of the p.sup.th power of amplitudes of the AVO intercept and AVO slope values, respectively in the window, using a preselected value of p;
• generating scaled standard deviations of the AVO intercept and an AVO slope traces over the window based upon the first pair of scale factors;
• generating a group of quantities, for each depth point in the window, corresponding to scaled sums and differences of its AVO intercept and AVO slope values;
• determining a second group of scale factors over the window, each scale factor corresponding to the p.sup.th root of a summation of the p.sup.th power of one of the group of quantities over the window;
• generating a scaled correlation coefficient from the ratio of differences of the second group of scale factors to sums of the second group of scale factors;
• and
• modifying the AVO intercept and AVO slope values of the depth point of interest according to a relationship between the scaled correlation coefficient and scaled standard deviations to desired correlation coefficient and standard deviation values for the survey;
• generating AVO indicators from the modified AVO intercept and AVO slope values;
• and
• outputting, to the output device, a survey based upon the AVO indicators.