Modelling of the shallow crustal layer 2A (blue) arrival using the 1-D crfl reflectivity code. (a) and (c) show two P-wave velocity models, where the solid black line represents the crflP-wave velocity model, the blue dashed line shows the on-axis wide-angle P-wave velocity model of Navin (1998), while the red dashed lines and red labels show the P-wave velocity model from the EPR of Harding (1993). Also shown in both (a) and (c) is the CSEM resistivity model (black dash dot line) for the northern end of the RAMESSES AVR (MacGregor 1998). (b) and (d) show crfl synthetic gathers computed using the velocity models shown in (a) and (c) respectively. The gathers have been NMO corrected using a stacking velocity of 1700 m s−1. Both compare favourably with the real data shown in Fig. 11. The preferred model (a) is in good agreement with the results of Harding (1993) and also fits the sub-seabed velocity derived from the wide-angle velocity model of Navin (1998)—confirming that the shallow crustal event (blue) is equivalent to the seismic layer 2A event imaged in many other similar surveys. See text for details. Note that the maximum source-receiver offset for this survey is ∼2.6 km, and that the layer 2A event appears on the far-offset traces only.
