The 42-year-old, from Draperstown in Co Derry, has been missing since Sunday morning, November 25.
Her black Mitsubishi Warrior vehicle was found in a layby close to Foyle Bridge.
Foyle Search and Rescue say on their Facebook page on Wednesday afternoon: “Volunteers have continued with standard Search Operations today for a missing person.
“Our Drone Team launched a UAV Unmanned Aerial Vehicle which enables larger areas to be searched from the air which are closely monitored from a base station.
“Our UAV Team were supported today by a mobile unit, shore Team, Jet Ski & boat crews.”
The PSNI have also issued a fresh appeal for information about Helena McElhennon.
They want anyone who was in the vicinity of Foyle Bridge on Sunday morning and who may have dashcam footage to get in touch with police in Derry on 101 ref 771 of 24/11/2019.
Helena is described as being 5″4 in height, of medium build with short, dark hair and possibly wearing a maroon coloured cardigan or long, brown dress.
As searches continue, FSR continue to appeal to those involved in take care and follow their safety advice:•Always stay in a group & let someone know the area you are searching and notify them when you have completed your search.
•Persons searching should wear appropriate clothing for the weather conditions.
•It is recommend that persons searching near the water’s edge wear a Life Jacket & that all other searchers wear high-vis clothing to enable boat crew to observe them on river banks.
•If you have access to a pole or walking stick please use it to test the ground in front of you.
•Always carry a mobile phone.
•Be careful along the river’s edge. Search between the river’s edge and high water line.
•If you approach rocks or large tress, try to walk around, not over, unless this means going to close to the edge.
•DO NOT enter the Glar/mud. You may become stuck & need rescued.
•DO NOT wade or enter the water.
•DO NOT cross over any railings under any circumstances.
•DO NOT cross over onto or walk along Rail Tracks
•In the event of finding any items of clothing etc. DO NOT TOUCH or remove. Please contact the PSNI on 101 who will in turn alert Foyle Search & Rescue on 02871 313800.•Check the tidal tables and weather forecast prior to commencing the search. If you need any advice on tides please contact Foyle Search.
•People who are searching are reminded that the river banks are extremely hazardous after dark.
•Please when searching along railings or on any of the Bridges DO NOT lean over the railings as CCTV operators can
not differentiate between a genuine searcher or a person who may be in need of help requiring immediate activation of the emergency services.”