Fresnel zone: A pitfall in seismic imaging of mid-ocean ridge magma lenses

We present estimates of the Fresnel zone diameter for the axial magma chamb er lens along the southern and northern East pacific Rise and compare them to the magma lens widths derived from migrated seismic sections. The mean v alues of that diameter are 996m and 1052m and the mean values of the magma lens width are 680m and 1056m respectively. Most individual estimates of th e magma lens width fall below the Fresnel zone diameter. Similar results ar e presented for three other mid-ocean ridges. The registered signal is ther efore a mix of responses from the magma sill and the encasing rock and late ral resolution is not achieved. We conclude that data: analysis procedures that assume a laterally homogeneous Earth, cannot provide reliable estimate s of the magma lens physical properties, including whether it is liquid or not.