Method to acquire simultaneously seismic data with source arrays designed for specific targets

Abstract

A method and apparatus for acquiring seismic data. In one embodiment, the method includes: moving a first air gun array in the water at a first depth and a second air gun array in the water at a second depth greater than the first depth, in which the total volume of the first air gun array is less than the total volume of the second air gun array, in which the first air gun array is separated from the second air gun array by a distance substantially equal to a shot point interval, firing seismic energy through the first and second air gun arrays through the water into the earth, and recording seismic signals reflected from strata in the earth beneath the water.

Description

By imposing the flat sea boundary condition in the form U(0)=−D(0), i.e. a surface reflectivity of −1, for a data window below a direct arrival, the following expression may be derived:

$P\left(Z_o\right)\left[e^{\mathrm{j2}\pi f\sqrt{1-k^2{v}^2/f^2}{Z}_u}\left(1-e^{-\mathrm{j2}\pi f\sqrt{1-k^2{v}^2/f^2}{Z}_u}\right)\right]=P\left(Z_u\right)\left[e^{\mathrm{j2}\pi f\sqrt{1-k^2{v}^2/f^2}{Z}_o}\left(1-e^{-\mathrm{j2}\pi f\sqrt{1-k^2{v}^2/f^2}{Z}_o}\right)\right].$
Persons of ordinary skill in the art should appreciate that the expressions in brackets are ghost operators, F_O and F_U, for the over and under seismic data, respectively, in the case of a perfectly calm surface 530. Accordingly, the above expression states that the pressure at the “over” seismic receiver 540 multiplied by the ghost operator F_U of the “under” seismic receiver 550 is equal to the pressure at the “under” seismic receiver 550 multiplied by the ghost operator F_o of the “over” seismic receiver 540. In mathematical terms, the above expression may be written in the simplified form: P(Z_o)F_O=P (Z_u)F_U.

However, as discussed above, the surface 530 is virtually never flat, as assumed above and in conventional practice. Moreover, the above expressions do not account for temporal and spatial variations in the water velocity, reflectivity of the surface 530, streamer positioning errors, and other non-ideal conditions that are frequently encountered in real marine seismic surveys. To account, at least in part, for the effects of the aforementioned non-ideal conditions, one or more calibration filters are determined in a manner that will be discussed in detail below. The calibration filters are then used to form an over/under combination of marine seismic data acquired by the “over” seismic receiver 540 and the “under” seismic receiver 550. For example, the over/under combination may be formed by modifying the surface boundary condition using the one or more calibration filters. The over/under combination formed with the modified boundary condition may result in a combined data set with reduced noise relative to a data set formed by an over/under combination using the flat sea boundary condition.

FIG. 6 illustrates a flow diagram of a method 600 for forming an over/under combination using one or more calibration filters in accordance with one or more embodiments of the invention. First and second data sets are selected (at 610). In one embodiment, the first and second data sets are selected (at 610) to be pre-stack over and under data sets acquired by at least one seismic sensor coupled to an “over” streamer and at least one seismic sensor coupled to an “under” streamer in an over/under streamer combination. However, the present invention is not limited to selecting (at 610) all of the data in the pre-stack data set. In one alternative embodiment, portions of the pre-stacked data set acquired within a selected time window and/or a selected offset may be selected (at 610). In another alternative embodiment, portions of the pre-stacked data set from a selected gather, such as a shot gather and/or a receiver gather, may be selected (at 610).

The first and/or second data sets may be provided via transmission over a wired and/or wireless medium. For instance, the over and under data sets may be selected from the data as it is gathered, or shortly after it is collected, from a seismic survey. Alternatively, the first and/or second data sets may be recorded on and transmitted via recording tape, magnetic disks, compact disks, DVDs, and the like. Thus, the first and second data sets can, in some embodiments, can be selected from data previously collected and archived on some magnetic or optical storage medium.

One or more calibration filters are determined (at 620) using the selected over and under data sets. In one embodiment, the one or more calibration filters are determined (at 620) by initially assuming, as discussed above, the pressure at the “over” seismic receiver 440 multiplied by the ghost operator F_U of the “under” seismic receiver 450 is equal to the pressure at the “under” seismic receiver 450 multiplied by the ghost operator F_O of the “over” seismic receiver 440, i.e. P(Z_o)F_O=P(Z_u)F_U. This technique is often referred to as across-ghosting technique.

However, as discussed above, this relationship generally is not precise for the acquired over/under seismic data. The one or more calibration filters, a(f), may therefore be determined using the expression a(f)P(Z_o)F_O=P(Z_u)F_U. For example, the one or more calibration filters may be determined by evaluating the expression a(f)P(Z_o)F_O=P(Z_u)F_U by a least-squares criterion. However, persons of ordinary skill in the art should appreciate that the present invention is not limited to applying the least-squares criterion to the expression a(f)P(Z_o)F_O=P (Z_u)F_U. Further, any desirable expression may be evaluated with any desirable technique used to determine the calibration filters. Persons of ordinary skill in the art should also appreciate that the one or more calibration filters may be determined such that the expression a(f)P(Z_o)F_O=P(Z_u)F_U holds true in a statistical sense, even though it may not hold precisely for all the acquired seismic data used to determine the calibration filters.

The one more calibration filters are then used to combine (at 630) the first and second data sets to form a third data set, such as an over/under combined data set. The one or more calibration filters may be used to define a perturbed boundary condition using over and under data sets. The perturbed boundary condition is then incorporated into a selected over/under combination technique that is used to combine (at 630) the over and under seismic data. Persons of ordinary skill in the art should appreciate that the present invention is not limited to any particular technique for combining (at 630) the over and under seismic data using the one or more calibration filters. In various alternative embodiments, any desirable technique for combining (at 630) the over and under seismic data using the one or more calibration filters may be used.

While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Claims

1. A method for acquiring seismic data, comprising:

moving a first air gun array in the water at a first depth and a second air gun array in the water at a second depth greater than the first depth, wherein the total volume of the first air gun array is less than the total volume of the second air gun array, wherein the first air gun array is separated from the second air gun array by a distance substantially equal to a shot point interval;

firing seismic energy by the first and second air gun arrays through the water into the earth; and

recording a plurality of seismic signals reflected from strata in the earth beneath the water.

2. The method of claim 1, wherein the first air gun array is tuned to a leading peak and the second air gun array is tuned to a first bubble oscillation.

3. The method of claim 1, wherein the first air gun array is separated from the second air gun array by about 37.5 meters.

4. The method of claim 1, wherein moving the first and second air gun arrays comprises moving the first and second air gun arrays at a speed V in an x direction, wherein the first air gun array is separated from the second air gun array by a distance dx in the x direction.

5. The method of claim 4, wherein firing seismic energy through the first and second air gun arrays comprises:

firing seismic energy through the first air gun array; and

firing seismic energy through the second air gun array at a time dx/V after the first air gun array is fired.

6. The method of claim 4, wherein the first air gun array is disposed closer to a seismic survey vessel than the second air gun array, wherein the first and second air gun arrays are moved by the seismic survey vessel.

7. The method of claim 1, wherein firing seismic energy through the first and second air gun arrays comprises firing the first and second air gun arrays sequentially.

8. The method of claim 1, wherein firing seismic energy through the first and second air gun arrays comprises firing the first and second air gun arrays simultaneously.

9. The method of claim 1, wherein the first air gun array is configured to enhance high frequency seismic energy.

10. The method of claim 1, wherein the second air gun array is configured to enhance low frequency seismic energy.

11. A marine surveying arrangement, comprising:

a first air gun array tuned to a first bubble oscillation, wherein the first air gun array has a first total volume;

a second air gun array tuned to a leading peak, wherein the second air gun array has a second total volume that is greater than the first total volume, wherein the second air gun array is horizontally separated from the first air gun array by a distance substantially equal to the speed at which the air gun arrays are moving multiplied by a delay time between the firing of the first air gun array and the firing of the second air gun array; and

one or more seismic streamers having a plurality of hydrophones disposed therealong.

12. The marine surveying arrangement of claim 11, wherein the first total volume is about 5085 cubic inches and the second total volume is about 6780 cubic inches.

13. The marine surveying arrangement of claim 11, wherein the distance is about 37.5 meters.