P/S wave measurement and compensation

1. A method for use in surveying a subsurface region beneath a body of water by detecting S waves propagating through the subsurface region, the method comprising:

deploying a sensor system onto the seabed, the sensor system comprising a first sensor system in mechanical contact with the seabed and a second sensor configuration attached to the first sensor configuration and floating in the water just above the seabed;

using the first sensor configuration to detect mixed S and P waves on or in the subsurface region;

using the second sensor configuration to detect P waves in the water;

applying scaling and/or conditioning to the detected P waves; and

using the scaled and/or conditioned P waves to compensate the detected mixed S and P waves, and thereby attenuate the effects of P waves in the mixed S and P waves.

2. A method according to claim 1 , wherein the first and second sensor configurations detect wavefield components comprising one or more of mutually orthogonal particle velocities (Vx, Vy, Vz) or particle accelerations.

3. A method according to claim 2 , wherein a sensor configuration detects a particle velocity or particle acceleration using two or more closely spaced hydrophones.

4. A method according to claim 1 , wherein at least one of the first and second sensor configurations detects a pressure.

5. A method according to claim 1 , wherein said step of compensating comprises scaling a component detected by the second sensor configuration to obtain a scaled component, and subtracting the scaled component from a corresponding component detected by the first sensor configuration.

6. A method according to claim 5 , wherein said compensation is applied to wavefield components comprising horizontal particle velocity (Vx and/or Vy) or horizontal particle acceleration.

7. A method according to claim 5 , wherein a component is scaled using a scaling factor corresponding to a water to subsurface density ratio (density 1 /density 2 ).

8. A method according to claim 1 , wherein said first sensor configuration comprises a plurality of geophones in mechanical contact with the subsurface and said second sensor configuration comprises a plurality of hydrophones on the seabed or suspended in the water and, optionally geophones or, accelerometers, suspended in the water.

9. A method according to claim 1 , wherein one or both of said first and second sensors are in mechanical contact with the seabed.

10. A method for use in surveying a subsurface region beneath a body of water by detecting P waves propagating through the body of water, the method comprising:

using the method of claim 1 to detect S waves propagating through the subsurface region, compensated in order to attenuate the effects of P waves; and

applying the compensated S wave to the P waves detected in the water in order to compensate interaction with the former and thereby attenuate the effects of S waves propagating in the subsurface and converted at the water / subsurface interface into P waves propagating in the water or along the seabed.

11. A method according to claim 10 , wherein said step of applying the compensated S wave to the P waves detected in the water comprises applying the compensated S wave to the particle velocity Vz, particle acceleration, and/or displacement in the vertical direction.

12. A method according to claim 10 , wherein said step of applying the compensated S wave to the P waves detected in the water comprises determining parameters of a digital filter using the S wave data, and applying the digital filter to the P wave data.

13. A method of mapping or imaging a subsurface region beneath a body of water, the method comprising detecting S and or P waves according to claim 1 , and using the resulting data to create a map or image of the subsurface region.

14. A method according to claim 1 , wherein said step of applying the compensated S wave to the P waves detected in the water comprises a step of applying the compensated S wave to the particle velocity Vz in the vertical direction including determining a relationship between the vertical particle velocity and a particle velocity, acceleration, and/or displacement in the horizontal direction, applying that relationship to the compensated S wave data, and subtracting the result from the P wave data.