Abstract: An injection apparatus and use thereof that comprises a plurality of spaced ejection ports from which the tracer composition can be ejected so as to be injected directly into a chosen layer of a stratified flow, and that in use can be adjusted such that the ports lie each within the appropriate layer. Each port is connectable to a source of tracer composition and most conveniently the source is the combination of a reservoir and a syringe-like device which can draw a suitable amount of the composition from the reservoir and then drive it to, and eject it from, the associated port into the chosen layer.