Water-coupled underwater node for seismic surveys

1. A marine node for recording seismic waves underwater, the node comprising:

a spherical body made of a material that has a density equal to or less than a density of the water so that the body is buoyant neutral or positive;

a first sensor located in the body and configured to record three dimensional movements of the node;

a second sensor located in the body and configured to record pressure waves propagating through the water; and

one or more cables connected to the first and second sensors and configured to exit the body to be connected to an external device,

wherein

the body is coupled to the water and the node is buoyancy neutral,

the first sensor includes four individual sensors configured to determine a motion of the body,

each of the four individual sensors has a corresponding sensing axis, and the four axes intersect at a central point, and

the first sensor and the second sensor are arranged inside the spherical body so as to have a first half of the spherical body heavier than a second half thereof thereby favoring a stable vertical orientation of the spherical body with the first half under the second half.

2. The node of claim 1 , wherein the entire body is coupled to the water and not to the seabed.

3. The node of claim 1 , wherein the external device is an anchor.

4. The node of claim 3 , wherein the anchor is fixedly attached to the seabed and the one or more cables acts as a tether to maintain the body at a fixed distance from the anchor.

5. The node of claim 1 , wherein the central point is a center of a tetrahedron and each sensing axis also intersects a corresponding vertex of the tetrahedron.

6. The node of claim 5 , wherein three of the four individual sensors are provided in the first half of the spherical body and the fourth individual sensor is provided in the second half thereof.

7. The node of claim 1 , wherein the second sensor is used as ballast being placed in the first half of the spherical body to orient a predetermined diameter of the body along a gravity direction.

8. The node of claim 7 , wherein the first sensor includes four independent sensors having sensing axes extending from a center to vertices of a tetrahedron and the second sensor is provided diametrically opposed to one of the four independent sensors.

9. A marine node arrangement for recording seismic waves underwater, the marine node arrangement comprising:

a node comprising two or more seismic sensors for recording the seismic waves, a processor, a memory unit, and a power supply, wherein one of the two or more sensors includes four individual sensors configured to determine a motion of the node, each of the four individual sensors has a corresponding sensing axis, and the four axes intersect at a central point; and

a cover configured to provide positive buoyancy to the node,

wherein

the node has a negative buoyancy and the cover has a positive buoyancy such that an overall buoyancy of the marine node arrangement is neutral or negative,

the first sensor and the second sensor are arranged inside the spherical body so as to have a first half of the spherical body heavier than a second half thereof thereby favoring a stable vertical orientation of the spherical body with the first half under the second half, and

each of the two or more seismic sensors is acoustically coupled only to water to receive the seismic waves.