Multi-sensor system for airborne geophysical prospecting and method

1. A multi-sensor electromagnetic (EM) system for measuring EM signals, the multi-sensor EM system comprising:

a frame;

a transmitter device attached to the frame and configured to generate a primary EM field;

a receiver device attached to the frame and configured to record a secondary EM field generated by the earth after being excited by the primary EM field; and

a gradient sensor device attached to the frame and configured to record a gradient of the secondary EM field with two parallel coils,

wherein the two parallel coils sandwich a transmitter coil of the transmitter device and the two parallel coils are placed symmetrically about the transmitter coil.

2. The system of claim 1 , wherein the frame is made of hollow tubes that host coils of the transmitter device, receiver device and the gradient sensor device.

3. The system of claim 1 , wherein the transmitter device includes three transmitter coils perpendicular to each other, the receiver device includes three receiver coils perpendicular to each other, and the gradient sensor device includes three gradient coils perpendicular to each other, one of the three gradient coils including the two parallel coils.

4. The system of claim 3 , wherein each of the three gradient coils includes two parallel coils that sandwich a corresponding transmitter coil.

5. The system of claim 1 , wherein the two parallel coils are electrically connected to each other so that currents flowing through the two coils have opposite directions.

6. The system of claim 3 , wherein a receiver coil is a null-coupled coil.

7. The system of claim 6 , wherein the null-coupled coil includes two concentric coils located in a same plane and current flowing through the two concentric coils flow in opposite directions.

8. The system of claim 7 , wherein the two concentric coils are located in a same plane with a corresponding transmitter coil.

9. The system of claim 8 , wherein the two concentric coils are concentric with the transmitter coil.

10. An electromagnetic (EM) surveying system for measuring EM signals, the EM surveying system comprising:

an airborne carrier configured to fly over ground;

a multi-sensor EM system configured to generate a primary EM field and to record a secondary EM field generated by the earth as a response to the primary EM field; and

a towing system connecting the airborne carrier to the multi-sensor EM system

wherein the multi-sensor EM system is also configured to measure a gradient of the secondary EM field with two parallel coils of a gradient sensor device, and

wherein the two parallel coils sandwich a transmitter coil of a transmitter device and the two parallel coils are placed symmetrically about the transmitter coil.

11. The system of claim 10 , wherein the multi-sensor EM system comprises:

a frame;

the transmitter device attached to the frame and configured to generate the primary EM field;

a receiver device attached to the frame and configured to record the secondary EM field; and

the gradient sensor device attached to the frame and configured to record the gradient of the secondary EM field.

12. The system of claim 10 , wherein the carrier is a hybrid air vehicle.

13. The system of claim 11 , wherein the transmitter device includes three transmitter coils perpendicular to each other, the receiver device includes three receiver coils perpendicular to each other, and the gradient sensor device includes three gradient coils perpendicular to each other, one of the three gradient coils including the two parallel coils.

14. The system of claim 13 , wherein each of the three gradient coils includes two parallel coils that sandwich a corresponding transmitter coil.

15. The system of claim 10 , wherein the two parallel coils are electrically connected to each other so that currents flowing through the two coils have opposite directions.

16. The system of claim 13 , wherein a receiver coil is a null-coupled coil and the null-coupled coil includes two concentric coils located in a same plane and current flowing through the two concentric coils flow in opposite directions, and wherein the two concentric coils are located in a same plane with a corresponding transmitter coil and the two concentric coils are concentric with the transmitter coil.

17. A method for recording electromagnetic (EM) signals during an airborne EM survey, the method comprising:

flying a multi-sensor EM system;

generating a primary EM field with a transmitter device attached to a frame of the multi-sensor EM system;

recording, with a receiver device attached to the frame, a secondary EM field generated by the earth after being excited by the primary EM field; and

recording, with two parallel coils of a gradient sensor device attached to the frame, a gradient of the secondary EM field,

wherein the two parallel coils sandwich a transmitter coil of the transmitter device and the two parallel coils are placed symmetrically about the transmitter coil.

18. The method of claim 17 , wherein the transmitter device includes three transmitter coils perpendicular to each other, the receiver device includes three receiver coils perpendicular to each other, and the gradient sensor device includes three gradient coils perpendicular to each other, one of the three gradient coils including the two parallel coils.

19. The method of claim 18 , wherein each of the three gradient coils includes two parallel coils that sandwich a corresponding transmitter coil.

20. The method of claim 17 , wherein the two parallel coils are electrically connected to each other so that currents flowing through the two coils have opposite directions.