A method for marine seismic surveying includes towing six independent source subarrays 221-1 to 221-6 with a survey vessel. Each subarray has a same total volume, and a same number of source elements. Four sets of three of the six subarrays are sequentially combined to form four sources, and actuated to generate a source signal. Each of the four sets uses three of the six subarrays, and those three are towed at three different selected depths e.g. 3m, 6m and 9m. The source elements of each subarray are arranged at seven different inline positions along a common source cable. The subarrays may be towed to have a same crossline separation. Three may be towed port of a midline, and three starboard. Some of the source elements may include a pair of elements e.g. airguns.
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