#8,509,033 - Digital Air Gun  (more details)

Current Technology:

In ocean-bottom oil exploration air guns are towed behind marine vessels and triggered periodically to release high pressure air into the water.  The high pressure air creates sound waves that travel to the ocean bottom, through the earth's crust and reflect off layers in the earth's crust.  The pulse required to trigger an air gun solenoid is about 60 to 70 volts after passing through 500 to 1500 meter umbilical cables.  The system had to generate 150V pulses to overcome the long cable length, also using larger gauge wire to carry the pulses.  Also, another wire pair was required to carry sense signals so the system knows when the air gun actually fired.  This created a closed-loop system where the all the air guns in the array could be synchronized to within 100us of a known aim point, providing maximum seismic energy.

Challenge:
The client requested ways of reducing the cable size as a way to reduce system cost and complexity.  Two ways to reduce the cable size were to reduce the wire count and wire size.

• Reducing wire count required combining power and communication to the air gun over common wire pairs.  Off-the-shelf Power Line Communication (PLC) chip sets were looked at.  The main problem with using an existing PLC was the lack of support for triggering the air guns at a precise, synchronous time.

• Overcoming the wire size required putting the circuits needed to generate and control the HV pulse inside the air gun.  This was done so that lower power could be sent over time and stored in the gun.

• Any circuits in the air gun would have to be kept in a leak proof container and withstand 1000 G shocks when the air gun was fired.

• The system would need to be compatible with existing umbilical cables and communicate at 100K baud.

Solution:

• Designed a PLC System to power and communicate with a remote circuit board over small wires.  Power and communication was sent to the board via Voltage Mode, using 100 KHz H-Bridges.  Communication was received from the board in Current Mode, using current sensing transformers.

• 40 volts from the H-bridge was doubled and stored in capacitors internal to the air gun, providing up to 80 volts to trigger the air gun solenoid.

• All components were kept small and surface mounted.  The PCBA and storage caps were mounted in a small compartment with potting to protect the PCBA from sea water exposure and 1000 G shock.

• Developed Software in C and Assembly languages to control communications between controller unit and air-gun unit.

• I measured and recorded the impedance characteristics of an existing umbilical and matching networks were added to the system for impedance matching.