A marine seismic survey method comprising: at one or more spatial locations, activating two or more seismic vibrators in a seismic vibrator array, to be out of phase to produce a source gradient wavefield for surveying a target structure, wherein said activating the seismic vibrators in the seismic vibrator array comprises providing activation signals to the seismic vibrators in the seismic vibrator array, wherein a first output signal produced by one of the at least two seismic vibrators is out of phase with respect to a second output signal produced by another of the at least two seismic vibrators, wherein the first and second output signals are in anti-phase or wherein the first and second output signals have a phase difference whose cosine is less than zero; and at one or more other spatial locations, at a different time, activating the at least two seismic vibrators in the seismic vibrator array to be in-phase to produce an approximately monopolar source wavefield for surveying the target structure, characterized in that the seismic vibrators in the seismic vibrator array are provided at a plurality of depths (D1, D2, D3) in a body of water, wherein seismic vibrators at a first of the plurality of depths emit at a first frequency, and seismic vibrators at a second of the plurality of depths emit at a second, lower frequency, and in that the seismic vibrators at a first depth are controlled to provide the source gradient wavefield and a monopolar source wavefield for different shots, and wherein the seismic vibrators at a second depth being larger than the first depth are controlled to just provide lower frequency monopolar source wavefields for different shots. at one or more spatial locations, activating two or more seismic vibrators in a seismic vibrator array, to be out of phase to produce a source gradient wavefield for surveying a target structure, wherein said activating the seismic vibrators in the seismic vibrator array comprises providing activation signals to the seismic vibrators in the seismic vibrator array, wherein a first output signal produced by one of the at least two seismic vibrators is out of phase with respect to a second output signal produced by another of the at least two seismic vibrators, wherein the first and second output signals are in anti-phase or wherein the first and second output signals have a phase difference whose cosine is less than zero; and at one or more other spatial locations, at a different time, activating the at least two seismic vibrators in the seismic vibrator array to be in-phase to produce an approximately monopolar source wavefield for surveying the target structure, characterized in that the seismic vibrators in the seismic vibrator array are provided at a plurality of depths (D1, D2, D3) in a body of water, wherein seismic vibrators at a first of the plurality of depths emit at a first frequency, and seismic vibrators at a second of the plurality of depths emit at a second, lower frequency, and in that the seismic vibrators at a first depth are controlled to provide the source gradient wavefield and a monopolar source wavefield for different shots, and wherein the seismic vibrators at a second depth being larger than the first depth are controlled to just provide lower frequency monopolar source wavefields for different shots.