## 3 Results
### Pockmark Field
#### 3.1.2 Seismostriagraphy  
The study of the seismic profiles generally showed the existence of two seismic sequences (SS). The upper one, SS I, is an almost acoustically transparent sequence consisting of low amplitude, almost parallel internal reflections. SS I overlies SS II, which is recorded as a single or a series of subparallel high-amplitude reflectors. Beneath SS II, no further penetration of the acoustic waves was achieved (Figure 7). The top reflector of SS II is attributed to the Pleistocene/Holocene boundary [16; 39; 50; 51; 57]. This highly reflective surface may be due to the large change in the geotechnical properties of the sediments, including the possibility of variations in the fluid content. Indeed, sedimentological analyses performed on the sediments from SS I and SS II showed lithological and geotechnical differences between the seismic sequences. The marine sediment samples were retrieved from five drillings conducted within the study area for the construction of the new harbour of Patras [58]. Based on the analysis, the upper sequence (Holocene, SS I) consists of clays with local interlayers of sandy muds, while the  
Figure 6: R.O.V. photos recovered inside selected pockmarks, showing the presence of small holes (diameter about 10–20 cm) in the muddy cover of the pockmarked floor. Figure 7: High-resolution seismic profile showing a typical stratigraphic pattern of the area consisting of two seismic sequences (upper SS I and lower SS II). The sedimentological interpretation of the well drilling [58] is superposed on the seismic profile (ER: Enhanced Reflector; ATZ: Acoustic Turbid Zone; HOL/PL: Holocene Pleistocene boundary; TWTT two-way travel time). base of the sequence (top of Pleistocene, SS II) consists of compacted clay with interlayers of gravels, sandy muds and biogenic clusters. In the later sediment facies, abrupt changes in the geotechnical properties of the sediments (hydraulic index from >100% to 85%, SPT from 2 to 5) were measured. On the other hand, the frequent presence of acoustic patterns related to gas accumulation in the sediments (i.e., Seismically Turbid Zones, Enhanced Reflectors) (see Section 3.1.3) at the top of SS II and locally in the lower part of SS I, together with the large number of pockmarks recorded in the SS I, strongly suggest the presence of fluids at the Pleistocene/Holocene interface, which locally migrated to the SS I. Based on the above, the abrupt change in geotechnical properties, together with the presence of gas-charged sediments on the profiles, suggest that the Pleistocene/Holocene boundary is a gas accumulation horizon. The thickness of the SS I (Holocene) varies from 2 to 34 m (Figure 8). An abrupt increase in Holocene sediment thickness is observed south of the lineament of AT.F (Figure 8). The majority of the pockmarks have been observed in areas where the thickness of the SS I ranges from 12 to 20 m. Only a few pockmarks are obtained in areas where the thickness of SS I is more than 22 m (Figure 8). However, there is a strong relation (R = 0.67) between the pockmark depth and the thickness of the Holocene sequence (SS I), suggesting that the largest pockmarks have been formed in thick sediments (Figure 4d). Based on the thickness of the upper sequence, the acoustic characteristics of the seabed and the type of the deeper strong reflector, five distinct acoustic types [61] were defined. The spatial distribution of the acoustic types is shown in Figure 9.  
Acoustic Type 1 (AT1) consists of an SS I with few, low amplitude and parallel internal reflections, which locally appear acoustically transparent overlying a sequence of parallel high-amplitude reflectors (SS II). Acoustic characteristics indicative of gas are absent in the SS I, suggesting that the gas has probably been accumulated in the Holocene/Pleistocene boundary and has not migrated upwards (within SS I). AT1 is mainly observed in the northern sector of the field, and its spatial distribution correlates well with increases in the Holocene sediment thickness (Figures 8 and 9).