The thickness of the Earth´s crust and its structure in general is investigated by seismic refraction measurements. Seismographs are then set up on a line and seismic waves generated at the end of the line and sometimes in several places along the line. Measured seismograms are then arranged in a seismic record section and interpretation software used to generate a synthetic record section for comparison. The ray paths of the seismic waves are curved due to the increasing velocity of the waves with depth. The longer the distance to the seismograph the deeper the ray goes. The diagram below is a famous seismic record section from the investigation of Ingi Bjarnason and others (1993) showing that the Icelandic crust is thicker than had been assumed previously, or 15-23 km in SW-Iceland.
Recent publications of the Institute of Earth Sciences on this topic:
Brandsdóttir, B. and William H. Menke (2008). The seismic structure of Iceland, Jökull (58), 17-34.
Riedel, C. , A. Tryggvason, B. Brandsdóttir, T. Dahm, R. Stefánsson, M. Hensch, R. Böðvarsson, K. S. Vogfjord, S. Jakobsdóttir, T. Eken, R. Herber, J. Hólmjárn, M. Schnese, M. Thölen, B. Hofmann, B. Sigurðsson and S. Winter (2006). First results from the North Iceland experiment. Mar. Geophys. Res., 27, 267-281.doi:10.1007/s11001-006-9007-0.
Hooft, Emilie E. E. , Bryndís Brandsdóttir, Rolf Mjelde, Hideki Shimamura and Yoshua Murai (2006). Asymmetric plume-ridge interaction around Iceland: The Kolbeinsey Ridge seismic experiment. Geochem. Geophys. Geosyst., (G3), 7, Q05015, doi:10.1029/2005GC001123, 26 pp.
Bjarnason, I., W. Menke, Ó. G. Flóvenz, D. Caress (1993). Tomographic image of the Mid-Atlantic plate boundary in Southwestern Iceland. J. Geophys. Res., 98, 6607-6622.