The Gassy Sediments of the Cilento Offshore (Southern Tyrrhenian Sea, Italy) and Their Impact on the Marine Hazard Offshore the Cilento Promontory

Abstract

In order to assess their influence on the marine hazard offshore the Cilento Promontory, the gassy sediments of the Cilento offshore have been thoroughly examined using the geological interpretation of a closely spaced grid of Sub-bottom Chirp profiles. Based on the general stratigraphic framework three areas have been previously identified, highlighting the different acoustic features occurring in the Cilento area. The acoustic anomalies include acoustic blanking, shallow gas pockets, and seismic units impregnated of gas, showing distinct acoustic responses. Understanding these anomalies and the related seismo-stratigraphic units in the offshore Cilento Promontory provides a valuable foundation for evaluating marine geohazards and may assist in developing strategies to mitigate geohazards in the Cilento area.

1. Introduction

The aim of this paper is to show the acoustic features on seismic profiles in the Cilento offshore and to evaluate their impact on the marine hazard offshore the Cilento Promontory. This requires reassessment of the geological interpretation of a previously reported grid of closely spaced Sub-bottom Chirp profiles [1], aimed at individuating the main acoustic anomalies and their implications for the marine geohazard offshore the Cilento Promontory. Because shallow gas accumulations can weaken sediments’ shear strength, they present a marine risk to hydrocarbon exploration and development. In Holocene deposits, the presence of shallow gas accumulations and underlying fluid movement indicators may indicate deeper reservoirs [2,3]. This is the case of the Cilento offshore, where a thick Holocene stratigraphic succession, composed of several seismo-stratigraphic units has been detected [1,4]. This succession overlies relict deposits (palimpsest and lowstand) punctuated by significant stratigraphic surfaces, including the ravinement surface and the upper erosional surface of the acoustic basement [5].

Hydrothermal activity, biogeochemical processes, the movement of deep-seated fluid from the Earth’s crust and bedrock, or the release of water from the underlying detritus when the sediments are buried are all common causes of fluids to move through marine sediment [3]. In general, quick sedimentation processes cause fluids to build up beneath the sediment. In clay, the initial porosity, or fluid content between grains, may exceed up to 80%. The pore fluid is then released due to tension from the sediment on top. Significant volumes of secondary fluids, including water and gases, will be supplied by organic matter decomposition, mineral changes, and diagenesis.

If the fluid can escape through the pores in the sediment or along faults and cracks, it spills into the ocean, where it may sustain different bottom habitats. Significant overpressure develops when impermeable sediments obstruct the flow to the seafloor, trapping the fluids [3]. Failure and underwater landslides could result from the seafloor’s stability being compromised if the pressure is greater than the sediment’s strength. Many undersea landslides are believed to be caused by fluid migration and overpressure build-up. Additionally, by leaching chemical compounds, ongoing freshwater circulation may change sediments that are rich in clay. This might weaken the deposits’ cohesiveness and promote instability and liquefaction.

Acoustic anomalies due to gas have been singled out by Hovland and Judd [6]. Methane-generated carbonate cement and rich and diverse fauna are seen in pockmarks, which are compared to seabed seepages in other marine geological contexts. According to Andreassen et al. [7], acoustic masking describes low-very-low amplitude and severely fragmented reflections that result from a decrease in wave propagation speed. While underlying reflections along vertical zones may appear dragged down, the upper limit of gas pipes typically produces a brilliant spot [7,8]. Specifically, the creation of pockmarks on the seafloor due to the fast migration of gas-charged pore fluids is directly related to gas pipes [9,10]. The most common manifestation of acoustic blanking is an enlarged upper border reflector and an undetectable or feeble reflection zone [11,12], and the gas hydrate zone appears above the bottom simulating reflector (BSR) [13,14]. Vertical acoustic blanking (VAB) is a seismic reflection anomaly characterized by a localized loss of amplitude, chaotic reflections, or an acoustically transparent (blank) zone in seismic data. It indicates attenuated acoustic signals, typically caused by shallow gas-charged sediments (e.g., in the North Sea/Baltic Sea) or fluid conduits like gas chimneys [7,8,9,10,11,12,13,14].

Bright spots, heightened reflections, and acoustic blanking are some of the seismic indicators of gas in sediments. Reflections from lower sediment layers cannot be recorded again due to gas’s absorption or reflection of acoustic energy. Missiaen et al. [12] have highlighted these traits, demonstrating seismo-stratigraphic evidence of subsurface gas. Pockmarks are craters formed by the gravitational collapse of a seabed that once covered the fluid reservoir in areas with substantial fluid seepage [15]. Mud volcanoes can also result from the quick discharge of over-pressure material. Unlike pockmarks, mud volcanoes are intricate structures that emerge from the bottom as cone- or shield-shaped mounds [16]. Mud volcanoes, among which diatremes are the most violent and swift, are thought to be the most efficient way to release solid and fluid from deeper deposits to the surface [17,18].

While our previous papers mainly dealt with the depositional environments in the Cilento offshore based on stratigraphic data [4] and with the general seismo-stratigraphic setting and the first occurrence of shallow gas [1], more than the stratigraphic architecture of the relict deposits and their attribution to the MIS stages [5], this paper will focus on the detailed individuation of the acoustic anomalies genetically related to shallow gas based on the geological interpretation and their implications in marine geohazards of this sector of the Eastern Tyrrhenian margin.

In fact, marine geohazards have been deeply analyzed in the frame of the MaGIC Project [19], but it is not an aim of this paper to analyze the whole results on the project in all the Italian seas. Casalbore et al., 2024 [20] have described marine geohazards offshore the Cilento and the Calabro-Tyrrhenian continental margin, but the MaGIc sheet Acciaroli has described deeper areas with respect to the areas covered by this paper. Moreover, the item of shallow gas, which is mainly located in the inner platform, has not been assessed in detail, mainly focusing on the large-scale geomorphological elements of the Tyrrhenian margin.

2. Geological Setting

The Cilento Promontory, a structural high with a relief of up to 1700 m, is located between the Sele Plain–Gulf of Salerno and the Gulf of Policastro. These reliefs are composed of thick successions of siliciclastic and carbonate sequences (“Flysch del Cilento” Auct.), which dip landward in the main carbonate reliefs of the Southern Apennines (“Alburno-Cervati Unit” Auct.). The boundaries of the Cilento Promontory’s structure are defined by Quaternary normal faults. With the exception of the carbonate formations of Palinuro Cape and Bulgheria Mount as well as a few isolated outcrops, the reliefs of the Cilento Promontory are composed of terrigenous rocks that formed in deep basins during the Late Mesozoic and the Late Miocene. The oldest formations belong to the Northern Calabrian Units, which are the highest stratigraphic structural unit in this southern Apennine zone. This is exemplified in the Cilento region by a formation made up of black shales, marls, and marly limestones that range in age from the Malm to the Oligocene and exhibits a deformation grade lower than the units above it (Cilento Flysch). The Pollica, San Mauro, and Monte Sacro Formations make up the Cilento Flysch, which extends upward and has a total thickness of 1500 m (Figure 1).

Several geomorphological studies have been carried out in the Cilento offshore, while no specific studies exist on the gas occurrence, except the paper of Aiello and Caccavale [1] on the same dataset. This sector of the Tyrrhenian margin has been mainly controlled by the sedimentary processes, including the coastal processes, acting in correspondence with the sea cliffs incised in the siliciclastic successions of the Cilento Flysch, the development of marine terraces, often carving the rocky acoustic basement, and the occurrence of important accumulations of relict sands.

Figure 2 shows a comparison between the onshore stratigraphy of the Cilento Group [21] and the offshore stratigraphy based on the Licosa core [22]. The Cilento Group (Cilento Flysch according to Ietto et al.) [23], ranging in age from Langhian to Tortonian [24,25] overlies the Liguride Complex. This group is unconformably overlain by the Gorgoglione Formation (Upper Tortonian). Different turbidite depositional systems have been distinguished in the Cilento Group, including siliciclastic turbidite beds, carbonato-clastic megabeds, and olistostrome beds (Figure 2). Offshore, the Licosa core has shown the stratigraphy of the relict deposits of the Cilento offshore [5]. They consist of a thick seismo-stratigraphic unit, including coarse-grained organogenic sands, whose stratigraphy has been calibrated through the Licosa core (Figure 2). This unit, which is genetically connected to the MIS 2, appears at the sea bottom as sandy ridges that are thought to be submerged beach deposits [5].

3. Materials and Methods

The Sub-bottom Chirp profiles, previously processed by the Seismic Unix software (version SU44R28) [1], have been reassessed based on the criteria of the geological interpretation, focusing on the occurrence of significant acoustic anomalies, genetically related with the occurrence of gas. A sketch map showing the location of the three areas, having different acoustic anomalies, with superimposed seismic profiles has been constructed (Figure 3).

Sub-bottom Chirp Datasonic CAP-6600 Chirp Sub-Bottom Profiler II acoustic profiler model (Datasonic, North Falmouth, MA, USA), installed on the R/V Urania of the National Research Council of Italy (http://www.cnr.it/consulenza/NaveUrania.html; accessed on 27 April 2026), has been used to record the seismic sections. This instrument is a next-generation acoustic sediment profiler, completely replacing the 3.5 kHz Sub-bottom, previously used for these surveys. The Sub-bottom Chirp features a modulated frequency concentrated in two distinct bands (2.57 kHz and 821 kHz), allowing for an investigation depth of 1520 m and a decimeter resolution.

The Seismic Unix software has been used to read the seismic sections. The processing workflow applied to the seismic sections [1] consisted of several steps, including the data conversion from SEG Y to SU, the spectral analysis of the frequencies of the seismograms, in order to reconstruct the distribution of the frequencies, the application of fast Fourier transform (FFT) for the visualization and the analysis of the frequencies of the seismic signal, the application of a high-pass filter with a low-cut frequency at 150 Hz in order to eliminate the seismic noise and the dark signal, the application of a uniform gain on each seismic trace, and the application of a time-variant gain (TVG) in order to improve both the seismic signal of the deeper seismic reflectors and the whole visualization of all the seismic profiles.

Seismic Unix has allowed for improving previous seismo-stratigraphic results, obtained using the Seisprho software (Version 1.2) [26,27], which has been applied to the seismic sections shown by Aiello [28]. The SeisPrho program was developed to allow the user to manually enter the main parameters affecting the data structure in order to read and display data files (Profile, Positioning, Analysis, SEGY File Header, Reflectivity, Bitmaps, Log) [27]. The result of seismic data processing with SeisPrho allows for the rendering of seismic sections in the form of bitmap images. A simple processing was applied to the seismic sections of Cilento, consisting of the application of a linear vertical gain along the entire seismic section, which led to a significant improvement in the quality of the processed seismic data [28]. In particular, the acoustic anomalies related to the gas occurrence have been distinguished from the stratigraphic features based on the criteria of seismic stratigraphy.

A sketch table has been constructed to show the relationships between the geophysical areas, the seismic sections, the acoustic features, and their distribution through shot points on the seismic sections (

Table 1. Geophysical areas, seismic sections, acoustic features of gassy sediments and their distribution through shot points.

Area	Seismic Sections	Acoustic Features	Shot Points
1	B33_1	Acoustic blanking	400–550; 800–1100; 1450–1750
1	B33a_1	Acoustic blanking	350–490
1	B35_1	Acoustic blanking	1250–1450; 1900–2100; 2350–3000
1	B35a_1	Acoustic blanking	0–380; 750–900; 1600–1850; 2600–2800; 3200–3400
1	B36_1	Acoustic blanking	500–600; 1400–1550; 2000–2500; 4200
1	B38	Acoustic blanking	700–800; 1200–1500; 2300–2500; 3200–3600; 4550–5100
1	B39	Acoustic blanking	100–500; 700–750; 1100–1400; 2400–2550; 3100–3400; 3900–4000; 4400–4600; 5000–5400
2	B43	Shallow gas pockets	2900–3200
2	B44
2	B45	Shallow gas pockets	6700–7300; 8200–8800
2	B46	Seismic units impregnated of gas
2	B47	Seismic units impregnated of gas
2	B48	No gas features
2	B49	No gas features
2	B50
2	B50_1	Shallow gas pockets	2000–2500
2	B51	Shallow gas pockets	0–0.1
2	B52	Seismic units impregnated of gas	0–0.18; 0.25–0.45; 0.7–0.85
2	B53	Seismic units impregnated of gas	0.4–0.7; 0.75–1.1
2	B54	Shallow gas pockets; seismic units impregnated of gas	0–0.45; 0.45–0.75; 0.85–0.95
2	B55	Shallow gas pockets; seismic units impregnated of gas	0.25–0.35; 0.45–0.65; 0.65–1.1
2	B56	Shallow gas pockets	0.24–0.45; 0.68–0.88; 0.95–1.5
3	B61	Seismic units impregnated of gas	0.3–0.45; 0.45–1.2
3	B62a	Shallow gas pockets	1.15–1.2; 1.3–1.45
3	B62	No gas features
3	B63	Seismic units impregnated of gas	0.1–0.48; 0.55–0.7
3	B63_1	Seismic units impregnated of gas
3	B64	Shallow gas pockets Seismic units impregnated of gas	0.2–0.5; 0.5–0.7
3	B65	Shallow gas pockets	180–400; 500; 600–800
3	B65_1	Seismic units impregnated of gas	0–0.8
3	B66	No gas features
3	B66_1	Seismic units impregnated of gas	0–0.65; 0.7–1.25
3	B67_1	Shallow gas pockets Seismic units impregnated of gas	0–0.6; 0.6–1.6
3	B68_1	Seismic units impregnated of gas	0–1.1
3	B68a_1	No gas features
3	B68b_1	Seismic units impregnated of gas	0–1600; 1600–6400
3	B69	Seismic units impregnated of gas	0–0.3; 0.4–1.0; 1.0–2.8

).

4. Results

The first area is located offshore from the Licosa Cape Promontory at water depths ranging between 30 and 90 m and consists of seven seismic sections (B33_1, B33a_1, B35_1, B35a_1, B37_1, B38, and B39), whose peculiar acoustic features are represented by both the acoustic blanking, controlled by the occurrence of gas, and by the rocky acoustic basements, representing the seaward prolongation of the Cilento Flysch (Table 1).

The second area is located offshore the northern Cilento Promontory from the seaward prolongation of the Paestum Plain, proceeding southwards up to the Tresino Cape and consists of nine seismic sections (B43, B45, B46, B47, B48, B49, B51, B52, and B53), whose peculiar acoustic features are the shallow gas pockets and a thick sediment filling, organized in several seismo-stratigraphic units, Holocene in age (Table 1).

A third area has been distinguished in the northern Cilento Promontory, from the Paestum Plain to the Licosa Cape Promontory, at water depths ranging between 10 and 60 m and consists of six seismic sections (B61, B62a, B63, B64, B65, and B65_1; Table 1), whose peculiar features are represented by the shallow gas pockets and by the gassy sediments (Table 1).

4.1. Acoustic Anomalies of Area 1

Significant seismic sections of Area 1 are shown and the corresponding acoustic anomalies have been identified. In particular, the seismic profile B33_1 is characterized by three significant areas of acoustic blanking, respectively occurring at the shot points from 400 to 550, from 800 to 1100, and from 1450 to 1750 (Figure 4).

The seismic profile B33a_1 has shown a main zone of acoustic blanking, which is located between the shot points 350 and 490 (Figure 5).

The seismic profiles B35_1 and B35a_1 have revealed numerous zones of acoustic blanking (Figure 6). In particular, the seismic profile B35_1 reveals three main areas of acoustic blanking, respectively located at the shot points 1250–1450, 1900–2100 and 2350–3000 (Figure 6; Table 1). The seismic profile B35a_1 has shown acoustic blanking areas respectively located at the shot points 0–380; 750–900; 1600–1850; 2600–2800; 3200–3400.

Finally, the seismic profiles B38 and B39 have highlighted numerous zones of acoustic blanking (Figure 7). On the seismic line B38, they are located at the shot points 700–800; 1200–1500; 2300–2500; 3200–3600; 4550–5100. On the seismic line B39, they are located at the shot points 100–500; 700–750; 1100–1400; 2400–2550; 3100–3400; 3900–4000; 4400–4600; 5000–5400.

Due to the overall characteristics, the acoustic blanking identified on the seismic profiles of Area 1 could represent VAB (vertical acoustic blanking) [29]. As previously stated, the VAB coincides with vertically disturbed, narrow zones of weak amplitude reflections, interrupting well-layered sub-bottom imagery. They are associated with the presence of free shallow gas, which absorbs or scatters seismic energy, preventing clear imaging of the underlying layers. VAB is a crucial interpretation tool in marine seismic data for identifying potential shallow gas pockets and associated geohazards, particularly in areas like the North Sea [29].

4.2. Acoustic Anomalies of Area 2

The acoustic anomalies of Area 2 are represented by the shallow gas pockets and by the seismic units impregnated of gas. The geological interpretation of the Chirp profiles B46 and B47 has shown a wide seismo-stratigraphic unit, acoustically transparent, interpreted as impregnated by gas due to its acoustic facies (Figure 8).

The geological interpretation of acoustic anomalies on the Chirp profiles B48 and B49 has shown the occurrence of a wide seismo-stratigraphic unit, acoustically transparent, which is interpreted as impregnated of gas (Figure 9). The top of the unit is deeply eroded, and the same unit onlaps the acoustic substratum, gently dipping towards the center of the basin (Figure 9).

The geological interpretation of the seismic profiles B54 and B55 has displayed the occurrence of shallow gas pockets, genetically related to the gas occurrence (Figure 10). In particular, these acoustic anomalies have been detected on the seismic profile B54 at the shot points from 0.55 to 0.7 and from 0.86 to 0.92. On the seismic profile B55, these features appear at the shot points from 0.27 to 0.3, from 0.48 to 0.68 and from 0.8 to 1.0.

4.3. Acoustic Anomalies of Area 3

The acoustic anomalies of Area 3 are represented by the shallow gas pockets and by the seismic units impregnated of gas. The geological interpretation of the seismic profiles B61 and B62 has shown the occurrence of wide shallow gas pockets (Figure 11). On the seismic profile B61, these anomalies are located at the shot points from 0.3 to 0.45 and from 0.45 to 1.2. On the seismic profile B62a, the acoustic anomalies appear at the shot points from 1.15 to 1.2 and from 1.3 to 1.45 (Figure 11).

The geological interpretation of the seismic profiles B63 and B64 has shown the occurrence of a wide seismo-stratigraphic unit, acoustically-transparent, which is interpreted as impregnated of gas (Figure 12). On the seismic profile B63, it has been detected from the shot point 0 to the shot point 0.6, which overlies the top of the acoustic basement. On the seismic profile B64, the unit onlap the top of the acoustic basement at the shot point 2.05, extends up to the shot point 2.8 (Figure 12).

Based on the general seismo-stratigraphic framework discussed by Aiello and Caccavale [1], this unit is mainly composed of coastal and deltaic deposits, Holocene in age and overlies the acoustic basement, genetically related to the Cilento Flysch. The unit is overlain, in turn, by palaeo-channel filling deposits (Figure 12).

5. Discussion and Conclusions

The obtained results have shown the distribution of the three acoustic anomalies in the three areas offshore the Cilento (Figure 1). Area 1 displays mainly acoustic blanking (Figure 4, Figure 5, Figure 6 and Figure 7), involving the seismo-stratigraphic units of the Cilento offshore [1]. These seismo-stratigraphic units have been previously identified as coastal and marine deposits, Holocene in age [1].

Acoustic blanking (acoustic darkening, transparent zones) is a geophysical phenomenon observed in seismic reflection profiles (such as Chirp Sub-bottom profiles) that indicates an area of sediment characterized by a substantial absence of internal reflectors, thereby appearing “transparent” or white in the seismographic field. It is primarily controlled by the presence of shallow gas, often biogenic, within marine or lake sediments.

Regarding the control mechanism, gas bubbles trapped in the sediment pores scatter acoustic energy and dramatically increase signal attenuation, preventing seismic waves from penetrating deep into the sediment and producing stratigraphic reflections. The zone appears as a vertical column or diffuse area where the coherent horizontal stratigraphy suddenly disappears, often overlaid or flanked by highly reflective zones (“bright spots”). Acoustic blanking is a direct indicator of the presence of fluids (gases) that interrupt the acoustic continuity of the sediments.

Area 2 displays shallow gas pockets and seismic units impregnated with gas (Figure 8, Figure 9 and Figure 10). A wide seismo-stratigraphic unit, acoustically transparent, has been recognized and it is probably bounded at the top and the bottom by impermeable layers. It overlies marine and coastal deposits, well stratified, and is overlain in turn, by channel deposits, filling wide palaeo-channels at the erosional unconformity located at the top of the gassy unit (Figure 8, Figure 9 and Figure 10) [1]. Shallow gas is defined as gas-charged sediments occurring within the upper 1000 m of the seafloor [6,9]. The acoustic manifestations of shallow gas are the occurrence of high-reflective patches and layers, also known as acoustic turbidity zones and enhanced reflections [6,9].

Area 3 is characterized by shallow gas pockets and seismo-stratigraphic units impregnated by gas (Figure 11 and Figure 12). Their seismo-stratigraphic framework, previously highlighted by Aiello and Caccavale [1] reveals that they are mainly concentrated in coastal, marine and deltaic seismo-stratigraphic units, which are characteristic of this offshore sector [1]. This is in overall agreement with other shallow gas pockets detected in the North Sea, Irish Sea and other deltaic regions [6,7,8,9]. Seismo-stratigraphic units impregnated with gas are specific sedimentary bodies, delineated by seismic data based on their depositional geometry and acoustic impedance, which contain significant amounts of interstitial gas within their pore spaces. These units are identified by characteristic acoustically transparent seismic facies, controlled by the drastic reduction in acoustic velocity and density when gas replaces pore water. These seismo-stratigraphic units are commonly found in porous, permeable strata like sand-silt mixtures or reservoir sands within deltaic, shoreface, or channel settings. Regarding their seismo-stratigraphic framework these units are mapped within a sequence stratigraphic context (e.g., lowstand or transgressive systems tracts), bounded by chronostratigraphic surfaces that control the trapping of the hydrocarbons. Regarding the marine geohazards, these features are used in petroleum exploration to map reservoir distribution, identify, and quantify potential gas accumulations and fluid migration pathways [2,3].

We have compared our results with other results, previously obtained, on the acoustic anomalies related to gas features in the Tyrrhenian Sea. Passaro et al. [30] have identified 54 fluid emission points, located at water depths ranging between 71 and 158 m and aligned in correspondence to four main clusters. Three of the identified clusters are positioned along the edge of a complex, toe-shaped underwater landscape situated to the southwest of Somma-Vesuvius. These clusters depict a shallow manifestation of partially concealed, merged geological formations, specifically two landslide areas on the flanks and a pyroclastic flow, linked to the volcanic activities of the Late Pleistocene. The fourth AFV cluster was found at a morphological high approximately 8 km to the south of Naples, known as Banco della Montagna. However, the Cilento anomalies appear different, firstly in the depth range, which is shallower, since the fluid emission points in the Naples Bay are located at water depths between 71 m and 158 m depth range. Sediment type is different too, since the Naples Bay has displayed mainly volcaniclastic deposits, while in the Cilento offshore Holocene sedimentary deposits prevail. In both cases, normal faults have not controlled the gas emissions, as shown by the seismic interpretation.

On the contrary, in the Ponza-Zannone volcanic complex a significant structural control has been found [31]. The distribution of these vents is aligned with NE-SW trending faults that bound the Ponza-Zannone structural high, allowing fluids to migrate upward through the shallow fractured basement. Martorelli et al. [31] present proof of an unrecorded hydrothermal area noted for its continuous fluid releases and intricately structured large depressions situated in shallow waters (250 m). Their arrangement implies that the NE-SW oriented faults surrounding the Ponza-Zannone structural elevation and the shallow fractured substrate create ideal conditions for the upward movement of hydrothermal fluids.

Being different from the Cilento anomaly, acoustic anomalies on the northern Campania margin have shown a significant structural control. Based on Sparker profiles located offshore the Volturno river mouth, Misuraca et al. [32] have observed laterally extensive pockets of amplitude anomalies. The seismic characteristics of the external shelf pocket revealed acoustic turbidity, interspersed with vertical areas of amplitude blockage, distinguished at the upper section by strong amplitude disrupted reflectors, which formed a jagged top outline. The presence of a hydrothermal system governed by lithologic discontinuities across the sedimentary pile and the primary orientation of normal to oblique Quaternary-active faults is suggested by fluid escape characteristics associated with an E–W striking fluid front at the outer shelf [32].

Casalbore et al. [33] have highlighted a gas outburst that occurred offshore the Scoglio d’Affrica islet (Tuscan Archipelago, Northern Tyrrhenian Sea) on 16 March 2017. The source area of the outburst, corresponds to a shallow-water mud volcano. The description of the mud volcano and the materials related to the gas eruption that occurred in 2017 offer a valuable understanding of the processes that shape the seafloor and are related to fluid leaks in areas with shallow water. This matter is especially important given the marine risks involved with sudden gas explosions in these depositional environments [33]. Rovere et al. [34] have described numerous unique gas flares, reaching heights of up to 700 m, located on the continental slope of the Paola Basin at depths ranging from 550 to 850 m. Gas flares arise above pockmarks, craters, and mudflows that develop over and along the steep areas of mound structures, aligning with the seismic regions of free gas accumulation beneath the sea floor. Acoustic blanking is dominant in correspondence to these structures. Ferrante et al. [35] have shown gas signatures in the Sicily Channel. Regional amplified reflections exhibiting polarity shifts (illuminated areas) along designated layers can indicate the sideward spread and buildup, while acoustic muddiness and signal voids might arise from the absorption of sound energy in the upper gas-filled sediments, hindering the transmission of signals. Nevertheless, these patterns can also stem from significant variations in acoustic impedance between neighboring materials, even in the absence of gas [35]. Maiorana et al. [36] highlighted seismic signals located below the pockmarks (e.g., seismic chimneys, bright spots) which suggest the presence of fluids that would rise to a few meters’ depth. Drawing from the observations, two origins and their associated ascending mechanisms have been recognized. An analysis of the shape and form of pockmarks has been conducted to clarify their potential relationship with the currents at the seabed. A model of fluid pathways has been developed, uncovering the origin of fluid release at deeper levels in the Adventure Plateau, and offering fresh perspectives on pinpointing routes of fluid leakage. Aiello [37] reevaluated the Sparker seismic images to emphasize the presence of gas and its potential connections with the current bradyseismic crisis. Specifically, formations previously thought to be volcanic dykes, which were associated with extensional features, are now viewed as possible gas infiltrations, potentially linked to the significant gas discharges observed at the Solfatara and Pisciarelli craters during the ongoing bradyseismic crisis. Within this perspective, we propose that the Solfatara–Pisciarelli geothermal system may be inherently connected to the gas releases detected in the Gulf of Pozzuoli, as indicated by seismo-stratigraphic investigations [37].

Marine geohazards strongly impact the marine environment. They consist of earthquakes, volcanoes, tsunamis, submarine mass movements, fluid activity and its manifestations, migrating bedforms and hazards induced from the anthropic activity [1].

The impact of the gassy sediments of the Cilento offshore on the marine geohazards is discussed. The seismo-stratigraphic results obtained in the Cilento offshore based on gas seismo-stratigraphic interpretation are in overall agreement with previously mentioned literature studies [30,31,32,33,34,35,36,37], suggesting that this sector of the Tyrrhenian margin has a significant impact on marine geohazards due to the gas occurrence. Fluid release from the seafloor may pose a geohazard regardless of its size, origin, or location. It may do so directly to drilling operations or seafloor infrastructure, or indirectly through a loss of density and consequently buoyancy that could endanger vessels, rigs, and floating infrastructure above [1,6,7,8].

Acoustic blanking is a key indicator of shallow-charged sediments and in the Cilento offshore it poses a significant hazard for drilling or foundation work. This must be taken into consideration for future planning and coastal management in this area. Indeed, acoustic blanking is crucial for identification of shallow geohazards, such as shallow gas, which can impact offshore engineering projects.

Shallow gas pockets are localized accumulations of gas, trapped in sediments within 1000 m of the seafloor, often before deep-water drilling equipment (BOP) is installed. They are hazards often found below impermeable layers, appearing as high-reflectivity acoustic anomalies, and pose significant blowout risks to offshore drilling due to shallow, pressurized sediments. Regarding the drilling hazard, shallow gas pockets are high-risk because they can flow through the annulus very quickly, destroying wells and causing explosions before the BOP is installed.

Seismo-stratigraphic units impregnated with gas constitute a major marine geohazard, primarily posing risks to offshore engineering, platform stability, and seabed integrity. These gassy sediments, frequently found in Holocene marine deposits and shallow-depth palaeo-channels, can cause sudden, uncontrollable gas releases during drilling or infrastructure installation. This is the case of the Cilento offshore, where a general stratigraphic setting characterized by Holocene marine deposits and palaeo-channels has been detected [1].

The main hazards associated with gas-impregnated seismo-stratigraphic units include the submarine landslides and slope failures, the shallow gas flow, the infrastructure instability, and the seabed pockmarks and leakage. In the first case, gas accumulations control over-pressured fluids, reducing the shear strength of sedimentary deposits, thereby controlling the individuation of pockmarks and submarine landslides [18]. Spatola et al. [18] have studied 5932 pockmarks mapped along the Italian continental margins, mainly on muddy sandy sea bottoms with a low gradient, whose location is genetically related to tectonic lineaments. In the second case, encountering shallow gas pockets during offshore drilling is a primary hazard, responsible for more than 20% of offshore blowouts. These incidents occur suddenly, as shallow sediments often lack the strength to contain the high pressure, resulting in gas flows that can threaten the stability of drilling rigs and platforms. The infrastructure instability is strongly conditioned. The low-strength nature of gas-charged sediment can cause foundation failures for pipelines, wind farm pylons, and subsea structures. Finally, gas escaping through the seabed can create craters (pockmarks), resulting in chaotic seafloor morphology and structural damage, particularly along tectonic faults.

This could be the case of the Cilento offshore, where the detection of gassy sediments has shown the occurrence of significant marine hazard, which must be taken into account for future planning and management of the coastal regions. In Area 1 and Area 3, acoustic blanking and seismo-stratigraphic units impregnated of gas strongly control the instability of the infrastructures, controlling future foundation failures, thereby preventing the installation of pipelines and subsea structures. In Area 2, shallow gas pockets within 50 m of the seafloor pose a direct blowout risk to any future drilling, similar to cases in the North Sea.

The gas origin can be only speculated, if it is biogenic or thermogenic in origin. The acoustic blanking is characterized by sub-vertical sharp lateral boundaries. A comparison of this acoustic facies with others seismic profiles [32] suggests that this could have been induced by the occurrence of subsurface biogenic methane, pervasive to the sedimentary environment (coastal and deltaic deposits) and produced in situ by an almost syn-depositional degradation of organic matter [32]. Regional geological data did not suggest an active geothermal area, thereby preventing a possible thermogenic origin of the gas. While the biogenic gas is produced by bacteria (methanogens) decomposing organic matter in anaerobic, shallow sediments, the thermogenic gas is produced by the thermal cracking of kerogen and heavier hydrocarbons at greater depths and higher temperatures. The investigated water depths confirm the possible biogenic origin of gas in the Cilento offshore.