Determining a propagation velocity for a surface wave

A method of determining a propagation velocity for a surface wave, the method comprising:

• generating by a coherent light source at least a first light spot and a second light spot substantially simultaneously on a surface wherein the first light spot and the second light spot are spatially separated;
• capturing by a camera at least one out-of-focus image of at least a part of the surface comprising the first and second light spots, the out-of-focus image comprising a first light spot image object for the first light spot, and a second light spot image object for the second light spot, the first light spot image object having a first speckle pattern, and the second light spot image object having a second speckle pattern;
• determining the propagation velocity in response to a time difference between speckle pattern changes in the first speckle pattern and the second speckle pattern;
• and
• wherein the determining of the propagation velocity comprises determining the time difference between speckle pattern changes in the first speckle pattern and the second speckle pattern in response to a spatial comparison of the first speckle pattern and the second speckle pattern in one out-of-focus image, wherein said out-of-focus image is captured by a camera comprising rolling shutter for capturing the at least one out-of-focus image.