Distributing a clock in a subterranean survey data acquisition system

1. A method comprising:

determining a first difference between a time that a first network element of a subterranean survey data acquisition network receives a first frame pulse from a second network element of the network and a time that the first network element transmits a second frame pulse to the second network element;

determining a second difference between a time that the second network element receives the second frame pulse and a time that the second network element transmits the first frame pulse;

determining a transmission delay between the first and second network elements based on the first and second differences, wherein determining the transmission delay comprises determining an average of the first and second differences; and

distributing a real time clock over the network based on the determined transmission delay,

wherein at least one of the determining the first difference, the determining the second difference and the determining the transmission delay is performed by a processor-based machine.

2. The method of claim 1 , wherein the act of determining the transmission delay comprises communicating the first difference to the second network element and determining the transmission delay at the second network element based on the first and second differences.

3. The method of claim 1 , wherein the act of determining the first difference occurs at the first network element and the act of determining the second difference occurs at the second network element.

4. The method of claim 1 , further comprising:

sampling a value of the real time clock at the first network element at the time that the first network transmits the second frame pulse to the network.

5. The method of claim 4 , further comprising:

communicating an indication of the value of the real time clock and an indication of the first difference to the second network element; and

based on the second difference, the first difference and the value of the real time clock, determining a value of the real time clock at the second network element.

6. The method of claim 5 , wherein the seismic data acquisition network comprises a synchronous optical network and the act of communicating the indication of the value of the real time clock and the indication of the first difference comprises communicating over an inband data communications channel.

7. The method of claim 1 , further comprising:

communicating acquired seismic measurements over the seismic data acquisition network.

8. The method of claim 1 , further comprising:

towing the first and second network elements on a streamer.

9. The method of claim 1 , further comprising:

determining a third difference between a time that the second network element receives a third frame pulse from a third network element of the seismic acquisition network and a time that the second network element transmits a fourth frame pulse to the third network element;

determining a fourth difference between a time that the third network element receives the fourth frame pulse and a time that the third network element transmits the third frame pulse; and

determining a transmission delay between the second and third network elements based on the third and fourth differences.

10. The method of claim 9 , further comprising:

communicating an indication of the value of the real time clock at the real time clock at the second network element and an indication of the third difference to the third network element; and

based on the third difference, the fourth difference and the value of the real time clock at the second network element, determining a value of the real time clock at the third network element.

11. The method of claim 1 , wherein the subterranean survey data acquisition network comprises a seismic data acquisition network.

12. The method of claim 1 , further comprising timestamping data acquired by at least one of the first network element and the second network element using the real time clock.

13. An apparatus comprising:

a subterranean survey data acquisition network comprising a first network element and a second network element; wherein

the first network element is adapted to:

determine a first difference between a time that the first network element receives a first frame pulse from the second network element and a time that the first network element transmits a second frame pulse to the second network element, and

communicate an indication of the first difference to the second network element; and

the second network element is adapted to:

determine a second difference between a time that the second network element receives the second frame pulse and a time that the second network element transmits the first frame pulse,

based on the second difference and the indication of the first difference communicated from the first network element, determine an average of the first and second differences to determine a transmission delay between the first and second network elements and adjust a distributed real time clock value based on the determined transmission delay.

14. The apparatus of claim 13 , further comprising:

a towed seismic streamer, wherein the subterranean survey data acquisition network is disposed on the streamer.

15. The apparatus of claim 14 , further comprising:

a vessel to tow the streamer.

16. The apparatus of claim 14 , wherein the first network element is further adapted to sample a value of a real time clock at the time that the first network transmits the second frame pulse to the network.

17. The apparatus of claim 16 , wherein the first network element is further adapted to communicate an indication of the value of the real time clock to the second network element.

18. The apparatus of claim 17 , wherein the network comprises a synchronous optical network, and the second network element is adapted to communicate the indication of the value of the real time clock and the indication of the first difference over an inband data communications channel of the network.

19. The apparatus of claim 17 , further comprising: seismic sensors coupled to the first and second network elements.

20. The apparatus of claim 17 , wherein

the network further comprises a third network element; and

the second network element is further adapted to:

determine a third difference between a time that the second network element receives a third frame pulse from the third network element and a time that the second network element transmits a fourth frame pulse to the third network element; and

the third network element is adapted to:

determine a fourth difference between a time that the third network element receives the fourth frame pulse and a time that the third network element transmits the third frame pulse, and

determine a transmission delay between the second and third network elements based on the third and fourth differences.

21. The apparatus of claim 20 , wherein

the second network element is further adapted to communicate an indication of the value of the real time clock at the real time clock at the second network element and an indication of the third difference to the third network element; and

the third network element is further adapted to based on the third difference, the fourth difference and the value of the real time clock at the second network element, determine a value of the real time clock at the third network element.

22. The apparatus of claim 13 , wherein the subterranean survey data acquisition network comprises a seismic data acquisition network.

23. The apparatus of claim 13 , wherein at least one of the first network element and the second network element is adapted to timestamp seismic measurements using the real time clock value.