Systems and methods of performing velocity surveys using spaced source activation lines

1 . A method for performing a seismic survey of an earthen subterranean formation, comprising:

(a) deploying a node patch comprising a plurality of seismic receivers to an offshore seabed in a survey area whereby the node patch is positioned on the seabed, and wherein a water column extends from the seabed located at a bottom of the water column to a waterline located at a top of the water column;

(b) performing a velocity survey having a velocity activation pattern extending in an inline direction and an orthogonal crossline direction, wherein (b) comprises:

(b1) transporting an array of seismic sources for a first plurality of passes in the inline direction while repeatedly activating the array of seismic sources to generate seismic waves to define a first velocity activation line of the velocity survey containing a first plurality of velocity source points and forming a portion of the velocity activation pattern;

(b2) transporting the array of seismic sources in the crossline direction across a predefined gap without activating the array of seismic sources whereby the gap does not contain any velocity source points;

(b3) transporting, after traversing the gap, the array of seismic sources for a second plurality of passes in the inline direction while repeatedly activating the array of seismic sources to generate seismic waves to define a second velocity activation line of the velocity survey containing a second plurality of velocity source points and which is separated from the first velocity activation line in the crossline direction by the gap, wherein the second velocity activation line forms another portion of the velocity activation pattern; and

(b4) repeating steps (b1), (b2), and (b3) to define a plurality of the first velocity activation lines, a plurality of the second velocity activation lines, and a plurality of the gaps whereby the plurality of second velocity activation lines are separated in the crossline direction from the plurality of first velocity activation lines by the plurality of gaps;

(c) performing an imaging survey having an imaging activation pattern extending in the inline direction and the crossline direction and which overlies the velocity activation pattern of the velocity survey in a plan view, wherein (c) comprises:

(c1) transporting the array of seismic sources through the survey area while repeatedly activating the array of seismic sources to generate seismic waves to form a plurality of imaging source points forming the imaging activation pattern defining an outer periphery within which each of the plurality of imaging source points are located.

2 . The method of claim 1 , wherein both the first velocity activation line and the second velocity activation line has a minimum crossline width of at least 400 meters.

3 . The method of claim 1 , wherein both the first velocity activation line and the second velocity activation line has a minimum crossline width of at least 500 meters.

4 . The method of claim 1 , wherein a ratio of an average crossline width of both the first velocity activation line and the second velocity activation line to an average crossline width of the gap is between 0.10 and 0.40.

5 . The method of claim 1 , wherein each of the seismic sources comprises a seismic airgun.

6 . The method of claim 1 , wherein an activation density of both the first velocity activation line and the second velocity activation line is greater than an activation density of the imaging activation pattern.

7 . The method of claim 1 , further comprising:

(d) correcting a seismic velocity model associated with the subterranean formation based on seismic data collected from the velocity activation pattern; and

(e) creating a seismic image of the subterranean formation based on seismic data collected from the imaging activation pattern and the seismic velocity model corrected at (d).

8 . A method for performing a seismic survey of an earthen subterranean formation, comprising:

(a) deploying a node patch comprising a plurality of seismic receivers to an offshore seabed in a survey area whereby the node patch is positioned on the seabed, and wherein a water column extends from the seabed located at a bottom of the water column to a waterline located at a top of the water column;

(b) performing a velocity survey having a velocity activation pattern extending in an inline direction and an orthogonal crossline direction, wherein (b) comprises:

(b1) transporting an array of seismic sources for a first plurality of passes in the inline direction while repeatedly activating the array of seismic sources to generate seismic waves to define a first velocity activation line of the velocity survey containing a first plurality of velocity source points and forming a portion of the velocity activation pattern, wherein the array of seismic sources, upon activation, is configured to produce a combined average amplitude of less than 200 decibels (dB), integrated over about 10 seconds, at frequencies less than 1.7 Hertz (Hz);

(b2) transporting the array of seismic sources in the crossline direction across a predefined gap without activating the array of seismic sources whereby the gap does not contain any velocity source points;

(b3) transporting, after traversing the gap, the array of seismic sources for a second plurality of passes in the inline direction while repeatedly activating the array of seismic sources to generate seismic waves to define a second velocity activation line of the velocity survey containing a second plurality of velocity source points and which is separated from the first velocity activation line in the crossline direction by the gap, wherein the second velocity activation line forms another portion of the velocity activation pattern; and

(b4) repeating steps (b1), (b2), and (b3) to define a plurality of the first velocity activation lines, a plurality of the second velocity activation lines, and a plurality of the gaps whereby the plurality of second velocity activation lines are separated in the crossline direction from the plurality of first velocity activation lines by the plurality of gaps;

(c) performing an imaging survey having an imaging activation pattern extending in the inline direction and the crossline direction and which overlies the velocity activation pattern of the velocity survey in a plan view, wherein (c) comprises:

(c1) transporting the array of seismic sources through the survey area while repeatedly activating the array of seismic sources to generate seismic waves to form a plurality of imaging source points forming the imaging activation pattern defining an outer periphery within which each of the plurality of imaging source points are located.

9 . The method of claim 8 , wherein both the first velocity activation line and the second velocity activation line has a minimum crossline width of at least 400 meters.

10 . The method of claim 8 , wherein both the first velocity activation line and the second velocity activation line has a minimum crossline width of at least 500 meters.

11 . The method of claim 8 , wherein a ratio of an average crossline width of both the first velocity activation line and the second velocity activation line to an average crossline width of the gap is between 0.10 and 0.40.

12 . The method of claim 8 , wherein each of the seismic sources comprises a seismic airgun.

13 . The method of claim 8 , wherein an activation density of both the first velocity activation line and the second velocity activation line is greater than an activation density of the imaging activation pattern.

14 . The method of claim 8 , further comprising:

(d) correcting a seismic velocity model associated with the subterranean formation based on seismic data collected from the velocity activation pattern; and

(e) creating a seismic image of the subterranean formation based on seismic data collected from the imaging activation pattern and the seismic velocity model corrected at (d).

15 . A method for performing a seismic survey of an earthen subterranean formation, comprising:

(a) deploying a node patch comprising a plurality of seismic receivers to an offshore seabed in a survey area whereby the node patch is positioned on the seabed, and wherein a water column extends from the seabed located at a bottom of the water column to a waterline located at a top of the water column;

(b) performing a velocity survey having a velocity activation pattern extending in an inline direction and an orthogonal crossline direction, wherein (b) comprises:

(b1) transporting an array of seismic sources for a first plurality of passes in the inline direction while repeatedly activating the array of seismic sources to generate seismic waves to define a first velocity activation line of the velocity survey containing a first plurality of velocity source points and forming a portion of the velocity activation pattern;

(b2) transporting the array of seismic sources in the crossline direction across a predefined gap without activating the array of seismic sources whereby the gap does not contain any velocity source points;

(b3) transporting, after traversing the gap, the array of seismic sources for a second plurality of passes in the inline direction while repeatedly activating the array of seismic sources to generate seismic waves to define a second velocity activation line of the velocity survey containing a second plurality of velocity source points and which is separated from the first velocity activation line in the crossline direction by the gap, wherein the second velocity activation line forms another portion of the velocity activation pattern; and

(b4) repeating steps (b1), (b2), and (b3) to define a plurality of the first velocity activation lines, a plurality of the second velocity activation lines, and a plurality of the gaps whereby the plurality of second velocity activation lines are separated in the crossline direction from the plurality of first velocity activation lines by the plurality of gaps;

(c) performing an imaging survey having an imaging activation pattern extending in the inline direction and the crossline direction and which overlies the velocity activation pattern of the velocity survey in a plan view whereby a lateral offset between the velocity activation pattern and the node patch is greater than a lateral offset between the imaging activation pattern and the node patch, and wherein (c) comprises:

(c1) transporting the array of seismic sources through the survey area while repeatedly activating the array of seismic sources to generate seismic waves to form a plurality of imaging source points forming the imaging activation pattern defining an outer periphery within which each of the plurality of imaging source points are located.

16 . The method of claim 15 , wherein both the first velocity activation line and the second velocity activation line has a minimum crossline width of at least 400 meters.

17 . The method of claim 15 , wherein a ratio of an average crossline width of both the first velocity activation line and the second velocity activation line to an average crossline width of the gap is between 0.10 and 0.40.

18 . The method of claim 15 , wherein each of the seismic sources comprises a seismic airgun.

19 . The method of claim 15 , further comprising:

(d) correcting a seismic velocity model associated with the subterranean formation based on seismic data collected from the velocity activation pattern; and

(e) creating a seismic image of the subterranean formation based on seismic data collected from the imaging activation pattern and the seismic velocity model corrected at (d).