QIU Jiandong, LIU Jian YUE Nana, WANG Shuang, and MAI Duc Dong



Distribution and Characteristics of Hazardous Geological Features in the Marine Coastal and Offshore Areas of Zhejiang Province, East China Sea

QIU Jiandong1), 2),*, LIU Jian1), 2), YUE Nana3), WANG Shuang1), and MAI Duc Dong4)

1),,,266071,2),,266061,3),,266061,4),,100000,

Newly acquired high-resolution shallow seismic profiles (7069km in length) in the coastal and offshore areas of Zhejiang Province, East China Sea, China, have revealed eight marine hazardous geological features: shallow gas, sand ridges, erosion ditches, scarps, irregular bedrock features, underwater shoals, buried paleo-channels, and submarine deltas. Based on the seismic profiles, we have constructed a marine geological map of these hazardous features. Shallow gas accumulations are common and occur mainly in two separate nearshore regions that cover 4613 and 3382km2respectively. There are also scattered shallow gas accumulations in the offshore area, typically accompanied by paleo-channels that occur mainly in the middle of the study area. Sand ridges, erosion ditches, scarps, and irregular bedrock features are found mainly in the northeast of the study area in association with each other. In the southeastern part of the study area, the sand ridges have a linear form and trend NW–SE, representing the western part of the linear sand ridges in the East China Sea. The maximum slope gradient is 1˚, which suggests that this area is prone to landslides. These hazardous marine geological features are important to marine and engineering activities in this region.

hazardous marine geological features; shallow gas; coastal and offshore areas; East China Sea

1 Introduction

Coastal and offshore areas are vulnerable to the effects of natural and anthropogenic factors, due to the complex interactions between the land, ocean, and atmosphere. Ra- pid global social-economic growth is impacting the surficial geological environment of the Earth, meaning that a number of geological hazards have become more prominent (Bastia., 2011; Cecchini., 2011; Li., 2016). Here, ‘geological hazards’ refer to nature disasters that are caused mainly by geological factors(., an earthquake), whereas ‘hazardous geological features’ refer to a particular geological feature (., a fault) that may lead to a natural disaster. The study of hazardous geological features is one facet of applied geological researches (Hu., 2007).

The identification of hazardous marine geological fea- tures has become increasingly important to mitigate the re-lated risks. This reflects the rapid expansion of the exploitation and utilisation of marine environments, including extraction of marine oil and gas, construction of submarine pipelines, and other engineering constructions along coastal regions. As such, the identification and mapping of hazardous marine geological features are important requirements prior to carrying out marine engineering constructions (Chen., 2014; Brookshire., 2015).

Few researches have been conducted in the coastal and offshore areas of Zhejiang Province, China, and most previous studies focused on the Zhoushan Islands. For example, single-channel seismic data were used to elucidate the Quaternary stratigraphy, paleo-channel distribution, sedimentary environments, and depth to the bedrock near the northern Zhoushan Island (Du., 2008; Zhang., 2011; Liu., 2014). However, little is known about the hazardous marine geological features along the coast of Zhejiang Province. In this paper, we used 7069km long newly acquired high-resolution seismic profiles to investigate the distribution and characteristics of potentially hazardous marine geological features in the coastal and offshore areas of Zhejiang Province. This work might be helpful to mitigate themarine geological disasters.

2 Materials and Methods

The high-resolution shallow seismic profiles were acquired in the coastal and offshore areas of Zhejiang Province in 2012 and 2013 (Fig.1). The seismic reflection data were collected using a seismic system (Sparker) made by Applied Acoustic Engineering Ltd. (AAE; UK), with the pulse energy of 600J, excitation interval of 800ms, and frequency band of 320–2000Hz. The seismic grid system consisted of 50 seismic profiles, including 45 main profilestrending E–W and 5 connecting profiles trending N–S. The grid spacing was 5×10km. In 2013, the seismic reflection data were collected using a seismic system (SIG 2 mille, Boomer) made by SIG, France, with a pulse energy of 250 J and an excitation interval of 500ms. The seismic grid system consisted of 30 seismic profiles, including 16 main profiles trending E–W and 14 connecting profiles trending N–S. The spacing between the main and connecting profiles was 5 and 10km, respectively. Navigation was undertaken using a Differential Global Positioning System (MX575), which is accurate to <5m, and the seismic system was towed at speeds of generally less than 9 kmh−1(5 knots).

Fig.1 Locations of the seismic profile lines and typical profiles (lines in blue and green were acquired in 2012 and 2013, respectively).

All the seismic profiles were processed using the software Qseis as follows: seismic date decoding, positioning data extraction, spectrum analysis, frequency scan, bandpass filter, true amplitude recovery, swell filtering, tow balance, and display.

High-resolution seismic interpretation was based on the recognition of minor seismic discontinuities, boundary surfaces (onlap, downlap, truncation, and toplap), and seismic facies changes. Each seismic unit was correlated throughout the entire seismic grid. Two-way travel times from seismic sections were converted to sediment thicknesses using an acoustic velocity of 1600ms−1based on previous drilling and seismic data.

3 Results

3.1 Classification of Hazardous Marine Geological Features

There are many methods for the classification of hazardous marine geological features, such as the spatial distribution of environmental geological features, composition of materials, causes of environmental geological features (., hydrodynamics), rate of geological environmental change, and degree of potential risk (Li and Yang, 2001; Li., 2002;Liu., 2000; Qiu., 2012; Shmatkova., 2015). In general, any classification scheme should be practical and simple. In this paper, hazardous marine geological features are divided into two types: those exposed on the seafloor and those buried beneath the seafloor. Our detailed classification scheme is provided in Table 1.

3.2 Types and Their Characteristics of Marine Hazardous Geological Features

In general, there are eight types of hazardous marine geological features in the study area: shallow gas, sand ridges, erosion ditches, scarps, irregular bedrock features, underwater shoals, buried paleo-channels, and submarine deltas.

3.2.1 Shallow gas

Shallow gas is relatively common in the study area and is characterised in the seismic data by acoustic blanks, curtains, and disturbances, as well as strong and irregular upper and side-bounding reflection interfaces (Taylor, 1992; Garcia., 2002; Missiaen., 2002; Liu., 2006). According to the reflection characteristics, the shallow gas accumulations can be divided into two types: those that cover large areas and those that are smaller and sporadically distributed. In the nearshore area, shallow gas is distributed over large areas where the water depth is typically <60m. These areas are recognised from acoustic blank areas, as the gas shields the deeper seismic signals due to its high concentration (Figs.2–3). The smaller and sporadically distributed shallow gas occurrences are found mainly in the offshore area, and are characterised by acoustic curtains or disturbances, as the gas concentration is relatively low. The acoustic curtain is formed by the strong horizontal seismic velocity contrast and reflection of the underlying seismic signals. There are clear seismic boundaries on both sides of these occurrences (Fig.4).

Table 1 Classification of hazardous marine geological features in the coastal and offshore areas of Zhejiang Province

Fig.2 Example of shallow gas in a seismic profile (TWTT = two-way travel time).

Fig.3 Example of shallow gas in the study area.

Fig.4 Examples of columnar shallow gas in the study area.

Based on the depth of the top of the gas reservoir, the shallow gas in the nearshore area can be further subdivided into two types (types I and II). Type I gas is shallow and almost exposed on the seafloor, and is found mainly at water depths of <30m. Type II gas is covered by Holocene muddy sediments and occurs mainly at water depths of 30–60m. The depth of the top of the gas reservoir varies from several to tens of metres.

3.2.2 Sand ridges

A sand ridge is a sedimentary body that forms on the seafloor due to the influence of strong tidal currents and exhibits progradation or an ‘S-shape’ reflection structure. These typically develop in a tidal current area characterised by reciprocating flow, at water depths of >50m, and are often associated with erosion ditches. The sand ridges in the study area are 700m wide and <10m high, and occur mainly in the northeast and southeast of the study area. The sand ridges in the northeastern study area have no constant trends (Fig.5), but in the southeast, the ridges strike NW–SE (Fig.6) and represent the western part of the linear sand ridges in the East China Sea (Wu., 2010).

Fig.5 Sand ridges in the northeastern part of the study area.

Fig.6 Sand ridges in the southeastern part of the study area.

3.2.3 Underwater shoals

Underwater shoals comprise muddy sediments or rapidly accumulated sandy deposits. They typically form an underwater platform and are characterised by wedges or lenses in seismic profiles that dip seaward and show obli- que bedding progradation or ‘S-shape’ reflection structure. Underwater shoals have high-frequency, weak-amplitude, and clear bedding seismic reflection characteristics. The inner prograding reflector may converge onto a strong reflecting interface, and the boundary may be downlap or onlap (Fig.7).

Fig.7 Example of an underwater shoal in the study area.

Underwater shoals are typically associated with erosional depressions. In the present study they are distributed in the nearshore area, where the hydrodynamic flow is strong. Although an underwater shoal is similar to a sand ridge in shape, they differ in terms of inner reflections, type of sediments, and location. The internal structure of a sand ridge is marked by cross or oblique bedding, which differs from the progradation or ‘S’ reflection structures in an underwater shoal. Sand ridges are composed of sandy sediments in the northeastern and southeastern parts of the study area, where the Holocene sedimentation rate was low, whereas the underwater shoals that formed above the Holocene deposits are composed of muddy sedi- ments.

3.2.4 Erosion ditches

The erosion ditches are located mainly around the islands within the study area (Figs.2, 4, and 8). There are numerous islands along the Zhejiang coast and, when the tide flows between these islands, they blocked and changed the flow direction. As such, this increases the flow speed and erosional capacity of the tide to form erosion ditches that are a few metres to >30m deeper than the surrounding seafloor.

There are also erosion ditches and associated sand ridges in the northeastern part of the study area (Figs.9– 10), where the sedimentation rate is low and the seafloor is influenced by strong tides.

Fig.8 Examples of erosion ditches in the study area.

3.2.5 Scarps

In the open ocean, landslides may occur when the slope gradient is 1˚–4˚, but landslides can occur even at gradients of 0.01˚in deltas with high sedimentation rates. In coastal zones, scarps are typically found around submarine deltas or erosion ditches, which is also the case in the present study area. Scarps occur at the front of Holocene submarine deltas and on the edges of erosion ditches where the maximum slope gradient is 1˚(Figs.3, 4, and 9). Scarps are typically associated with erosion ditches, and both of these features are located around islands.

Fig.9 Examples of scarps in the study area.

Fig.10 Example of irregular bedrock occurrence in the study area.

3.2.6 Irregular bedrock features

The acoustic impedance between bedrock and overlying unconsolidated sediments is large, which means that the bedrock is usually characterised by strong reflections and acoustic shadows (Chen and Gao, 1997). Therefore, it is straightforward to trace these strong acoustic reflections (Fig.10). The irregular bedrock features are mainly located around the Zhoushan Islands in the eastern part of the study area, and may be exposed on or be up to 100m beneath the seafloor. Large erosion ditches occur around the exposed bedrock.

3.2.7 Buried paleo-channels

In the seismic profiles, the filled and buried paleo- channels exhibit variable reflectance signatures according to the sediment characteristics and hydrodynamic nature, such as complex chaotic-wave, sub-parallel, and high- angle oblique reflection signals. These are due to the different directions of progradation and poorly developed bedding (Zhang., 2007). Given the diverse nature of the paleo-channels and variable angles between the profiles and paleo-rivers, the paleo-channels exhibit different cross-sectional characteristics. In the high-resolution seismic profiles, the paleo-channels are U- or V-shaped, symmetrical or asymmetrical, and the bases are strongly undulating.

Paleo-channels are common in the study area, and formed from thick fluvial deposit during the last glacial period. The absence of large rivers along the Zhejiang coast means that there is no evidence for a trunk river. Small paleo-rivers are found mainly in the middle of the study area, and are characterised by chaotic reflections, discontinuous bedding, and U- or V-shaped cross-sections (Fig.11).

Fig.11 Buried paleo-channels in the study area.

3.2.8 Submarine deltas

Many previous studies (., Xiao., 2005; Liu., 2007) have shown that sediments derived from the Yangtze River are deposited along the coasts of Zhejiang and Fujian provinces due to the influence of coastal currents (Fig.3). This distal clinoform becomes thinner offshore, from 24–30m thickness at 20–30m water depths, to <3m in the offshore area. The clinoform has the SE-dipping (<0.5˚) internal reflectors.

3.3 Distribution of Marine Hazardous Geological Features

Fig.12 shows the marine geological hazard map in the study area. The map documents the types and distribution of marine hazardous geological features, as well as the relationships between them, which can be used for disaster prevention and mitigation (Chiocci and Ridente, 2011; León and Somoza, 2011).

The map in Fig.12 reveals that the shallow gas occurs mainly along the coast in a belt-like shape at depths of several to tens of metres. In addition, there are sporadically distributed shallow gas occurrences around paleo- channels in the offshore area (Figs.4, 6, 8, and 11). The type I and II shallow gas areas are 4613 and 3382km2, respectively.

Sand ridges occur mainly in the northeastern and southeastern parts of the study area. Paleo-channels are located mainly in the middle of the study area, but no trunk river is present. Submarine deltas are distributed in a belt along the Zhejiang coast, with the outer boundary of the belt being sub-parallel to the coastline, and there is a tongue- like distribution between 28˚30΄N and 29˚00΄N. The erosion ditches, scarps, and irregular bedrock features are all located in the northeastern part of the study area, typically near the Zhoushan Islands.

Fig.12 Distribution of marine hazardous geological features in the Zhejiang marine coastal and offshore areas.

4 Discussion

Marine hazardous geological features are important to marine activities and engineering constructions. Shallow gas generates mainly from the decomposition of buried organic materials, which produces methane or biogas that accumulates in sediments. Such sediments are often characterised by high compressibility and low shear strength, which makes the seafloor less stable. When engineering or drilling activities are undertaken in the area with the shallow gas, a blowout may occur, leading to the drillhole collapse and other accidents, such as the subsidence, overturning of the structures and fires. As such, the shallow gas is hazardous when marine piles and drilling holes are constructed (Orange., 2005; Ye., 2003).

Sand ridges, erosion ditches, and scarps make the seafloor uneven, which may affect the maritime transport and the construction of submarine pipelines. Especially in view that sand ridges are typically migrating, they pose a significant threat to marine activities. The sediments in the study area are mainly Holocene mud deposits that are highly susceptible to landsliding.

Bedrock can provide a good foundation for engineering constructions, but if the bedrock is uneven, it may result in structural movement or even collapse. In addition, highly fractured bedrock may be associated with landslides and faults, which may lead to geological disasters. As such, the area with exposed and shallow buried bedrock must be chosen as a construction site only after careful surveying for its stability (Zhang and Zhan, 2010).

The highly variable hydrodynamic conditions in the fluvial sedimentary environments determine the complexity and variability of river deposits. It also produces distinct physical and mechanical properties of sediments, such as the degree of sorting, grain size, density, shear strength, and compressibility. Paleo-channel infill has marked per- meability, which may lead to the local collapse due to slow erosion and overburden loading, resulting in structural instability. Paleo-channel infill may also produce shallow gas due to the high organic content of the river sediments, and such gas is also hazardous during piling and drilling.

5 Conclusions

1) High-resolution seismic profiles acquired in the Zhejiang coastal and offshore areas reveal eight marine hazardous geological features: shallow gas, sand ridges, erosion ditches, scarps, irregular bedrock features, underwater shoals, paleo-channels, and submarine deltas.

2) Shallow gas is widely distributed in the nearshore area and sporadically distributed in the offshore area, where it is often associated with paleo-channels.

3) Irregular bedrock features, erosion ditches, scarps, and sand ridges are mainly concentrated in the northeastern part of the study area. These features are typically associated with each other. Sand ridges are also found in the southeastern part of the study area, and paleo-chan- nels are mainly located in the middle part of the study area.

4) These hazardous geological features should be considered when plans for marine resource development and engineering constructions are made in this area.

Acknowledgements

This study was supported by the China-ASEAN maritime cooperation fund (Comparative Study of Holocene Sedimentary Evolution of the Yangtze River Delta and the Red River Delta), the National Natural Science Foundation of China (Nos. 41306063 and 41330964), and the China Geology Survey (Nos. GZH201200506 and DD20 160145).

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May 7, 2018

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