Surface reflections The amplitude of a reflected signal from a surface depends on incidence angle and refractive index of medium e perpendicular to plane of inciden Where n'is E refractive n cos L n sin I index of E parallel to plane of incidence reflecting E n,2-n sin i nedium E n'cos i+ Normal incidence reflection For normal incidence: the two cases reduce to Pe d E. n'tn E n r E.n'+ Reflection strength will depend on dielectric constants Air E=1; water 80; Dry Sand 3-5; saturated sand 20-30; shale 5-15; silt/clay 5-40; Granite 4-6; Ice 3-4 Reflected strength at least 30% of incident signal 04/1603 12540Lec17† † 04/16/03 12.540 Lec 17 13 Surface reflections E perpendicular to plane of incidence Er Ei = n cosi - n' 2 -n 2 sin2 i ncosi + n' 2 -n 2 sin2 i E parallel to plane of incidence Er Ei = n' 2 cosi - n n' 2 -n 2 sin2 i n' 2 cosi + n n' 2 -n 2 sin2 i n = me refractive index of reflecting medium (m’=m) • The amplitude of a reflected signal from a surface depends on incidence angle and refractive index of medium Where n’ is 04/16/03 12.540 Lec 17 14 • For normal incidence: the two cases reduce to • Reflection strength will depend on dielectric constants: e=1; water 80; Dry Sand 3-5; saturated sand 20-30; shale 5-15; silt/clay 5-40; Granite 4-6; Ice 3-4 Er Ei = 2n n'+n Er Ei = n'-n n'+n Normal incidence reflection – Air – Reflected strength at least 30% of incident signal Perpendicular Parallel 7