Borehole Gravity Enewetak Atoll

Borehole Gravity measurements were made in a series of shallow wells straddling the OAK crater created by a nuclear test on the coral atoll of Enewetak in Micronesia. The survey was made to determine the density changes in the reef due to the blast.

This was the first Borehole Gravity survey conducted from a floating drill ship in a shallow lagoon in shallow wells. The survey had to contend with the rise and fall of the ship due to tide and waves entering the lagoon and the ship operating noises transmitted through the wireline cable to tool. The depth of the tool in the hole was also affected by the vertical motion of the ship.

The gravity tool motion in the well was stopped using a casing clamp operated by the tool's mass and controlled by the tool's direction of travel. The depth of the tool in the well was monitored depth referencing system consisting of an aircraft cable attached to the well casing running over a pulley system to a spring beside the wireline hoist.

The crater was adjacent to the edge of the reef which dropped vertically into the ocean depths, presenting a significant density contrast close to the series of wells. This necessitated a three dimensional structural modeling correction to the data.

This survey is described in more detail in U.S. Geological Survey open-file report, 87-665.

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