Right before D-Day, how were naval mines cleared secretly?
score:8
Seems like the wiki article you linked covers it fairly well:
Bombing of Normandy began around midnight with more than 2,200 British, Canadian, and American bombers attacking targets along the coast and further inland
and then:
Minesweepers began clearing channels for the invasion fleet shortly after midnight and finished just after dawn without encountering the enemy.
(all emphasis mine) It wasn't exactly a stealth mission. The bombardment began and provided cover fire to enable the minesweepers to approach.
An interesting discussion of the process from one of the individuals involved can be found here:
Our ship, which was on the port side dropping buoys, had the job of giving a sharp blast on the siren each time a buoy was dropped, so that the buoy layer on the right hand side would drop its buoy at the same moment. The awful noise of the siren made us wonder if the Germans wouldn’t hear us coming, but nothing happened. Our own light coastal forces were out in strength cruising around, waiting to give us cover if we were attacked.
So, not a quiet process, or secret by any means.
The minesweeping process of the time involved cutting loose the mines and then detonating them on the surface with gunfire.But again the bombing taking place might cover any detonations. 
(Image from wikipedia)
So between detonating mines, siren blasts, and overhead bombers attacking the coast, I don't think they were expecting secrecy at this point.
Upvote:1
The sea is large and the Germans were spread very thin defending thousands of miles of coastline from Norway to Southern France.
With Allied naval and air supremacy, they would have no ships nor aircraft regularly monitoring the minefield. Germans surface radar was short ranged. And the sea at night is very, very dark, even with a full moon.
Allied aerial and naval action was kept up all long the coast as part of an operation to confuse the Germans when the real invasion came. By the time June 5th arrived, the Germans did not think it unusual to be bombed, nor would they find it unusual to find small Allied ships near the coast. Even the reports of paratroopers didn't raise the invasion alarm.
p.434 #2 ON 6, Para.16. During the assault phase all available sweepers would be engaged with the mine barrier or inside the assault area. The risk was not great as the British coastal channel was normally constantly searched, the area north of the barrier was not believed to have mines in it, and it was searched during the weeks before NEPTUNE just to make sure.
The map in the question shows the disposition of Allied naval forces after piercing the minefield. The "Northern Limit Of Assault Area" is 49N 40'. The minefield was significantly further out, from 50N to 49N 50'. Further in and German and French coastal shipping would be threatened.
In ON6 [Admiral] Ramsay gave orders for overcoming the German minefields, and especially the barrier known to have been laid from about latitude 50ºN to within seven to ten miles of the Normandy coast. Although to the south of it lay a coastal channel left clear by the enemy for his own use, and which the 'Neptune' plan counted on for the final deployment of the assault and bombarding forces, even this might be mined at the last moment
To avoid tipping off the Germans, the minesweepers were careful to stay just in front of the invasion force, and did not sweep all the way to the coastline.
To prevent the enemy from learning of the Allied approach, or of the area at which the Assault was aimed, until the last possible moment, it was essential that barrier cutting minesweepers should not operate too far in the van of the leading assault ships. The maximum speed of the leading craft in some channels was 5 knots the minimum safe minesweeping speed was 7½ knots. The minesweepers were therefore required to lose approximately an hour and a half. They accomplished this by back-tracking for 40 minutes just before they came within range of enemy radar. Making two 180° turns with sweeps streamed was a difficult manoeuvre, further complicated by the tide.
...The enemy were not alerted by the approaching sweepers, although the 14th minesweeping flotilla, which was operating the lateral convoy route of Force U, and in the first approach channel, was in sight of the French coast from the afternoon of D minus 1.
Inshore waters, lying between the transport area and the assault beaches were not swept during the initial assaults.
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