The research principles
Experience with magnetometer searches over old wrecks had led to the conclusion that the remnant magnetism of the iron in such wrecks is frequently very weak, because the metal corrodes rapidly in sea water. Therefore it was desirable to use the most sophisticated measurement instruments possible to detect weak magnetic anomalies at depths down to 70 meters.

Let's have Franck Goddio present his working hypothesis:
I addressed myself to the French Atomic Energy Commission, which provided me with equipment that we had been testing for three years. I knew that the San Diego's fourteen cannons were made of bronze, and would therefore not produce any easily detectable magnetic anomalies. I also had an important document dated July 12, 1601, which included a three-page list of the artillery, weapons, munitions, equipment and provisions on the San Diego supplied by the royal arsenal in Manila.

I noted, in particular, more than one thousand cast or forged iron cannonballs of different sizes, and I estimated that the weight of the iron in these cannonballs must have been a minimum of five tons when new. There were also the anchors, the iron portions of the superstructure and the weapons. But here again, because experience has taught me that small amounts of iron frequently disappear altogether on account of corrosion in sea water, I resolved to focus my search only on the compact mass of iron cannonballs which must have been in the shot locker.

After reading the accounts of the witnesses, I also thought it likely that the San Diego's starboard anchors had remained on the Mauritius. We would therefore have to be prudent in estimating the total residual weight of iron the San Diego might represent. I decided to search for a total residual iron mass on the order of five hundred kilos, taking corrosion into consideration, thereby giving me a wide safety margin.

We found the San Diego!

The research catamaran Kaimiloa was equipped with nuclear resonance magnetometers designed by the French Atomic Energy Commission. These instruments continuously measure the earth's magnetic field with a sensitivity which is one thousand times greater than the equipment currently used by prospecting companies.

Considering the weight of the iron we were looking for, no more than five hundred kilos, we decided to cover the search area in successive passes spaced 30 meters apart in the lengthwise direction of the rectangle, while towing two submerged magnetometer detectors 10 meters above the bottom.

On April 21, 1991, after four weeks of searching, a magnetic anomaly was detected at the geometric center of the rectangle.

Gilbert made the reconnaissance dive, and when he surfaced, his smile told us all we needed to know about the nature of our 'anomaly'. The San Diego was lying underneath the catamaran at a depth of 50 meters, 1.2 kilometers off the eastern coast of Fortune Island."