Geophysical and Geochemical Exploration of the Pockmark Field in the Gulf of Patras: New Insights on Formation, Growth and Activity

Abstract

The Patras Gulf Pockmark field is located in shallow waters offshore Patras City (Greece) and is considered one of the most spectacular and best-documented fluid seepage activities in the Ionian Sea. The field has been under investigation since 1996, though surveying was partially sparse and fragmentary. This paper provides a complete mapping of the field and generates new knowledge regarding the fluid escape structures, the fluid pathways, their origin and the link with seismic activity. For this, data sets were acquired utilising high-resolution marine remote sensing techniques, including multibeam echosounders, side-scan sonars, sub-bottom profilers and remotely operated vehicles, and laboratory techniques focusing on the chemical composition of the escaping fluids. The examined morphometric parameters and spatial distribution patterns of the pockmarks are directly linked to tectonic structures. Acoustic anomalies related to the presence of gas in sediments and in the water column document the activity of the field at present and in the past. Methane is the main component of the fluids and is of microbial origin. Regional and local tectonism, together with the Holocene sedimentary deposits, appear to be the main contributors to the growth of the field. The field preserves evidence that earthquake activity prompts the activation of the field.

Gas Seepage and Pockmark Formation From Subsurface Reservoirs: Insights From Table‐Top Experiments

“…Long quiescent period of over a decade with no PM geometry change was observed by Brothers et al (2011), implying that it would be extremely hard to observe the short episodic venting during such periods. However, since previous work suggests that stress perturbations accelerate bubble escape from sediments (Katsman, 2019), it is not surprising that most observations of episodic gas emission from PMs, follow stress perturbations, for example, by earthquakes and storms (Christodoulou et al, 2023;Field & Jennings, 1987;Gontz et al, 2001;Hasiotis et al, 1996;Soter, 1999). Based on our experimental observations, we hypothesize that episodicity often characterizes gas seepage from PMs: each seepage event, which also deforms the PM, reflects something akin to a magmatic eruption in a volcano: enough gas overpressure must be accumulated to overcome the overlying layer resistance to deformation and open a fracture, akin to dike opening by magma.…”

Using Distributed Temperature Sensing for Long-Term Monitoring of Pockmark Activity in the Gulf of Patras (Greece): Data Processing Hints and Preliminary Findings

“…The seabed in the area of the pockmark field consists of fine-grained sediments ranging from mud to sandy mud and silt [44]. In the last two decades, the pockmark field has been heavily modified by the construction of the Patras new harbor in the same area, and as a result, 23 pockmarks have been totally or partially covered [38], with possible impacts on its ongoing activity. ranging from mud to sandy mud and silt [44].…”

“…ranging from mud to sandy mud and silt [44]. In the last two decades, the pockmark field has been heavily modified by the construction of the Patras new harbor in the same area, and as a result, 23 pockmarks have been totally or partially covered [38], with possible impacts on its ongoing activity. (c) Detail of the pockmark field and the harbor infrastructure, along with the exact location of the DTS system and its fiber cable.…”

“…The site for the DTS deployment was chosen based on the geological settings in the area and the safety of the system during operation. Based on previously acquired geophysical data [38], a specific pockmark, located at the shallowest part of a pockmark string aligned with the Agia Triada fault (AT.F) inside the Patras harbor, was selected for the deployment of the DTS (Figure 1c). The selected site appears particularly favorable for the DTS deployment and monitoring for the following reasons: (i) the depth is shallow enough (<30 m) and close to the coast to ensure a safe deployment/recovery task; (ii) the fiber cable is running almost parallel and very close to the trace of the active fault of Ag.…”

“…In the context of this work, seafloor temperature data in a selected shallow area (<30 m) consisting of gas seepage-related settings (pockmark and active fault), located within the pockmark field of Patras Gulf (Western Peloponnese, Greece) [37,38], have been acquired for a period of more than 1.5 years, using a 200 m long DTS system. So far, the Patras Gulf pockmark field has been activated at least two times by two strong earthquakes of M5.4 and M6.4 in magnitude, on 14 July 1993 [29] and 8 June 2008 [39], respectively.…”