Apparatus and methods for measuring velocity perturbation in a fluid include one or more sensor vehicle adapted to be deployed in the fluid and a device that obtains position and acceleration data of each sensor vehicle. A physical model of the one or more sensor vehicle behaviour is used to transform the obtained data into a velocity field of the fluid, and output a map of the velocity field in the fluid. Sensor vehicles may be adapted to move passively through the fluid and/or to be transported by the fluid. The apparatus and methods may be used in liquid and gaseous environments, and the velocity field map may be used to track movement of species and particulate matter of interest in the fluid in real time.