At long distances from a marine seismic shot, the recorded signal is dominated by reflections and refractions within the water layer. This guided wave signal is complex and is often referred to as normal or harmonic modes. Using LOFS field data from the Valhall Field we identify 5 modes. The observed time-frequency plots fits reasonably well with the simple period equation assuming that the dominant signal is caused by the group velocity. A comparison of 3 vintages of estimated normal mode curves for the Valhall field example representing sea bed locations shifted by 1 km indicates that minor shifts in group velocity minima for the various modes are detectable. A synthetic example where the seabed velocity gradually changes from 1800 to 1600 m/s confirms that the normal mode curves are sensitive to these changes and hence, it is likely to assume that normal modes might be exploited to detect lateral changes in seabed properties.