Method of determining a seismic velocity profile

1. A method of determining a seismic velocity, comprising steps of:

(a) providing two or more seismic receivers in the subsurface formation at predetermined distances from one another;

(b) selecting a seismic receiver m from the two or more seismic receivers as the location of a virtual source;

(c) selecting another seismic receiver k from the two or more seismic receivers;

(d) activating a seismic source at a location n, thereby exciting a wave in the subsurface formation;

(e) recording a wave signal trace u nm (t) against time t, at the seismic receiver m;

(f) recording a wave signal trace u nk (t) against time t at the seismic receiver k;

(g) cross correlating the wave signal traces u nm (t) and u nk (t) to obtain u conv nmnk (t);

(h) repeating steps (d) to (g) inclusive, for different locations n;

(i) summing u conv nmnk (t) over all locations n, to obtain a virtual signal trace u vs mk (t) which corresponds to the signal received by the seismic receiver k from the virtual source at the position of seismic receiver m; and

(j) deriving the seismic velocity based on the time of first arrival of the wave in u vs mk (t) and the predetermined distance between the seismic receiver m and the seismic receiver k;

further including, before step (g), at least one of removing part of the wave signal trace u nm (t) that contains a component of the wave field at seismic receiver m traveling in a direction other than from seismic receiver m to receiver k and removing part of the wave signal trace u nk (t) that contains a component of the wave field at the receiver k traveling in a direction other than from seismic receiver m to receiver k.

2. The method according to claim 1 , further comprising repeating the steps (c) to (i) inclusive, using a different seismic receiver k at a different predetermined distance from seismic receiver m, wherein step (j) comprises deriving a seismic velocity profile based on the times of first arrival of the wave in u vs mk (t) for all k.

3. The method according to claim 2 , wherein step (j) comprises performing linear regression analysis through the first arrival times against distance between seismic receiver m and seismic receiver k for all k.

4. The method according to claim 1 , wherein the wave comprises a shear wave.

5. The method according to claim 1 , further comprising the step of gating the signal trace u nm (t) before cross correlating it in step (g).

6. The method according to claim 1 , further comprising weighing the signal trace u nm (t) before cross correlating it in step (g).

7. The method according to claim 1 , wherein the recorded signal traces u ij (t) are pre-processed to separate the contributions from up- and down-going waves.

8. The method according to claim 1 , wherein the recorded signal traces u ij (t) are pre-processed to separate the contributions from P-waves and S-waves.

9. The method according to claim 1 , wherein cross correlating in step (g) includes convolving the time reversal u nm (−t) of signal trace u nm (t) with the signal trace u nk (t).

10. The method according to claim 1 , wherein the seismic velocity is a local seismic velocity in a region of the subsurface formation between the seismic receiver m and the seismic receiver k.

11. The method according to claim 1 , wherein the wave in step j) comprises a shear wave.

12. The method according to claim 1 , wherein the first arrival in step j) comprises a shear wave.

13. The method according to claim 1 , wherein the virtual signal trace u vs mk (t) includes information about a shear wave.

14. A method of determining a seismic velocity, comprising steps of:

(a) providing two or more seismic receivers in the subsurface formation at predetermined distances from one another;

(b) selecting a seismic receiver m from the two or more seismic receivers as the location of a virtual source;

(c) selecting another seismic receiver k from the two or more seismic receivers;

(d) activating a seismic source at a location n, thereby exciting a wave in the subsurface formation;

(e) recording a wave signal trace u nm (t) against time t, at the seismic receiver m;

(f) recording a wave signal trace u nk (t) against time t at the seismic receiver k;

(g) cross correlating the wave signal traces u nm (t) and u nk (t) to obtain u conv nmnk (t);

(h) repeating steps (d) to (g) inclusive, for different locations n;

(i) summing u conv nmnk (t) over all locations n, to obtain a virtual signal trace u vs mk (t) which corresponds to the signal received by the seismic receiver k from the virtual source at the position of seismic receiver m; and

(j) deriving the seismic velocity based on the time of first arrival of the wave in u vs mk (t) and the predetermined distance between the seismic receiver m and the seismic receiver k;

wherein cross correlating the recorded signals is combined with removing part of the virtual signal that contains a component of the wave field at seismic receiver m traveling in a direction other than from seismic receiver m to receiver k, and with removing part of the virtual signal that contains a component of the wave field at the receiver k traveling in a direction other than from seismic receiver m to receiver k.