CLOSE PROX 2 GA SMA HELIS OPPOSITE DIRECTION OFF SHORE FROM NTD UNDER LOW OVERCAST WITH VISIBILITY 2 PT 5 AND FOG.

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ACFT IS ON AN EXCLUSIVE CONTRACT TO THE U.S. DEPT OF THE INTERIOR; MINERAL MGMNT SVC; FOR USE IN TRANSPORTING GOVERNMENT PERSONNEL TO OFFSHORE OIL DRILLING AND PRODUCTION PLATFORMS OFF THE SOUTHERN CALIFORNIA COAST. AVIATION MISHAP INFO SYS INCIDENT RPT HAS BEEN FILED. I DEPARTED OXR WITH 2 PAXS ABOARD; ENRTE TO OIL PLATFORM TO INVESTIGATE AN OIL SPILL. (THIS PLATFORM IS LOCATED APPROX 15 MI OFFSHORE; S OF LGB). WE TRANSITTED THE NTD CTL ZONE; EBOUND AT 500 FT MSL; IN SPECIAL VFR CONDITIONS; THEN CONTINUED E ALONG THE SHORELINE. (NTD WAS RPTING A MEASURED 700 FT OVCST; VISIBILITY 2 1/2 MI IN FOG). BECAUSE OF THE REDUCED VISIBILITY; I HAD TURNED ALL POS LIGHTS; ANTI-COLLISION STROBE; AND BOTH LNDG LIGHTS; AND WAS WATCHING FOR ANY ON-COMING TFC. THE TERRAIN PREVENTED RADAR ADVISORIES; BUT I CONTINUED TO MONITOR THE MUGU APCH FREQ; VHF 124.700 MHZ; LISTENING FOR TFC. APPROX 8 NM E OF NTD; NEAR SOLOMAR; I SAW ANOTHER HELI APCHING AT THE SAME ALT; HEAD-ON. MY FIRST IMPRESSION WAS OF A MEDIUM OR LARGE HELI; 1 MI DISTANT. I BEGAN A R TURN TO AVOID; PERSUANT TO FAR 91.113(E); BUT THE RAPID RATE OF CLOSURE REVEALED THAT THIS WAS ACTUALLY A SMALLER HELI; VERY MUCH CLOSER. I TOOK VIOLENT EVASIVE ACTION; ROLLING R; THROUGH 90 DEG OF BANK. AT THIS TIME THE SMA Y BEGAN TO BANK R; AND CLRED ALONG OUR L SIDE; AT AN ESTIMATED 75-100 FT LATERALLY; WITH LITTLE OR NO VERTICAL CLRNC. THE SMA Y WAS WHITE WITH A BLUE TAIL BOOM. I DID NOT SEE ANY LIGHTS ON THE SMA Y CERTAINLY THE LNDG LIGHT WAS NOT LIT. I HEARD THE SMA Y CONTACT MUGU TWR; VHF 124.850 MHZ TO REQUEST A SPECIAL VFR CLRNC TO CMA. THE TWR CTLR INSTRUCTED THE HELI TO CONTACT MUGU APCH; AND I HEARD THE APCH CTLR TALKING TO THE SMA Y BUT DID NOT HEAR ANY TRANSMISSIONS FROM THE HELI; DUE TO THE TERRAIN. NO DAMAGE OR INJURIES RESULTED FROM THIS INCIDENT.