American intelligence knew that numerous unmined areas existed off the objective beachheads, but invasion planners thought it best to at least determine the exact boundaries of any minefields that existed in the areas. Accordingly, "Zane" and four of her sisterships (, , and ) —were to sweep an area extending from the 100 fathom (183 m) curve toward Port Purvis on Florida Island, but split into two groups near Gavutu Island to head simultaneously toward beach "Red," clearing a wide stretch through Lengo Channel toward Indispensable Strait. Japanese shore batteries opened up on the ships, but their fire was erratic and did no damage. By 1550 on the day of the first landings, 7 August, the area had been thoroughly swept. No mines were found.