Influence of shallow gas on the geotechnical properties of pockmark soil: A case study in the East China Sea

Applied Ocean Research 93 (2019) 101966

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Influence of shallow gas on the geotechnical properties of pockmark soil: A case study in the East China Sea

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Hongyue Suna, Zhongxuan Chena, Xianghua Laib, , Xin Yana, Taojun Hub a b

Ocean College, Zhejiang University, Zhejiang 310058, China Laboratory of Engineering Oceanography, Second Institute of Oceanography, MNR, No. 36 Baochubei Road, Xihu District, Hangzhou, Zhejiang 310012, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Pockmark Shallow gas Geotechnical properties Pockmark soil zoning MIP-CPT

Several incidents involving damage to submarine pipelines indicate that there will be potential hazards for many submarine structures if the geotechnical properties of the soil in pockmarks remain unclear. Based on a geophysical survey, geological drilling, in-situ measurement, and shallow gas eruption experiment, two large pockmarks near the Zhongjieshan Archipelago in the East China Sea were analyzed comprehensively. The geophysical and in-situ data indicated that the soil below the two pockmarks contains free or dissolved shallow gas, which continues to migrate upward from the deep zones, but there is no high-pressure gas reservoir in the pockmark soil. In-situ piezocone data, geotechnical results, and shallow gas eruption experiments have demonstrated that the pockmark soil has strengthened mechanical properties and zoning characteristics. After analyzing the pockmark soil in the area where the study was conducted, it was concluded that the pockmark soil in this area is not suitable for the accumulation of high-pressure, shallow gas. It also was concluded that the pockmark soil had stronger mechanical properties than virgin sediment due to the compaction of the soil caused by the eruption of the shallow gas. The zoning characteristics of pockmark soil are due to the regional differences in the ability of shallow gas to carry soil particles, which is a new finding that is worthy of attention in off-shore engineering.

1. Introduction A pockmark is a submarine depression that is distributed extensively in the global ocean [33]. Gay et al. [19] and Pilcher and Argent [41] summarized several mechanisms for developing and maintaining pockmarks, including the presence of buried channels (e.g., [21]), mud diapirs (e.g., [13]), and sediment slumps (e.g., [57]). Pockmarks, which also are recognized extensively to have formed as a result of the seepage of fluid and gas through the seabed [22,23,25,32], sometimes clearly are associated with the underlying gas chimneys [28]. Shallow gas is a type of geologic body that commonly is found below submarine pockmarks, and it usually is referred to as a gas that accumulates in sediments that are as much as 1000 m below the seafloor [12], and methane generally is the most important component [16,46]. In academia, the general consensus is that the eruption of the shallow gas on the seafloor will lead to the generation of pockmarks [32]. However, there have been no extensive studies concerning the influence of the activity of the shallow gas on the geotechnical properties of the pockmark soil. However, in recent years, several incidents involving damage to submarine pipelines in the East China Sea have

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been considered to be related to the complex geological conditions in the pockmark areas where the pipelines are located (Jia et al., 2010 [15,29,55,58]. Thus, it can be anticipated that, in the future, there will be potential hazards for many submarine structures if the geotechnical properties of the pockmarks associated with free or dissolved gas remain unclear, because submarine pipelines that radiate from the landing-place to the open ocean will not have enough area to bypass some nearshore pockmarks. In addition, these pockmarks are important in climatic studies that assess the release of methane from the ocean to the atmosphere as a greenhouse gas [1,26]. The western coastal area of the East China Sea where the Zhongjieshan Archipelago is located is an area in which various seeprelated sedimentary features, such as pockmarks, shallow gas (acoustic blanking and gas chimney), acoustic plumes, diapirs, and mud volcanoes have been found over the decades, especially during exploration mapping at water depths between 10 and 50 m [4,27,43,52,54–56]. In this study, our focus was on the northern waters of the Zhongjieshan Archipelago, where very high-resolution bathymetry data acquired during the Dq-1 cruise shows the presence of two large pockmarks. (Note: Dq is the abbreviation for the Chinese Phonetic Symbols ‘Donghai

Corresponding author. E-mail addresses: [email protected] (H. Sun), [email protected] (Z. Chen), [email protected] (X. Lai), [email protected] (X. Yan), [email protected] (T. Hu).

https://doi.org/10.1016/j.apor.2019.101966 Received 17 May 2019; Received in revised form 29 August 2019; Accepted 11 October 2019 0141-1187/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. Sonar image and location of the study area.

zone, the Earth's crust is less uniform, and the land is more undulating [20,27,30]. The sediments on the two sides of the fault zone have undergone different geological histories, and the northern part of the fault experienced a relatively strong Longjing movement after Eocene during which a series of overthrusts were folded by squeezing and squeezed anticlines were developed locally [20,30]. According to the previous studies, shallow gas is distributed throughout the west coastal areas of the East China Sea where the study area was located. Lee et al. [36] and Ye et al. [55] found that, in the Yangtze River's subaqueous delta near the study area, shallow gas mainly was distributed in the delta-front facies and prodelta facies where the depth of the water is less than 25 m. However, in the southeastern estuary of the Yangtze River, shallow gas is distributed in the region where the depth of the water exceeds 35 m, and it can even be found on the shelf. The various geophysical data acquired in three cruises from 2014 through 2015 (Dq-1, Dq-3, Dq-4) of the two pockmarks indicated the distribution of the shallow gas or possibly the stage of evolution. In addition, the in-situ data acquired in the Dq-2 cruise and drilling samples recovered in Dq-5 cruise showed the occurrences of shallow gas (free or dissolved) at several sites in the pockmarks (Fig. 2). Based on the results of the surveys, dense free gas accumulated within the layers of sediment that were a few meters thick and located at shallow depths below the floor of the pockmarks, and dissolved methane almost filled the sediments that were within 30 m below the pockmark. (Note that the maximum effective survey depth was only 30 m below the seafloor.)

Qiancengqi’.) Various methods of investigation, i.e., geophysics, geotechnics, and in-situ measurements, were used to determine whether the morphology and geology of these pockmarks could be controlled by shallow gas dynamics. The process of shallow gas eruption on the seafloor was simulated through experiments to validate the assumptions and the working hypothesis about the possible link between the geotechnical properties of pockmark soil and the activity of shallow gas. 2. Geological setting The study area was close to the southeast end of the Yangtze River Delta since the Holocene era, and it covered an area of about 5 km2 where the average depth of the water was about 28 m (Fig. 1.c). The topography of the seafloor in the study area is relatively flat and often encounters strong tidal currents exceeding 3 m/s [27,50]. According to the results of the research related to the sedimentary geology in this area, it is generally known that loose sediment, coastal facies, sea-land interaction facies, and loose marine sediments have been formed in the study area since the Quaternary period [36,53]. According to tectonic geological information, the E-W fault zone extends from the Zhoushan Archipelago to the Okinoerabu Island of Japan on the south side of the study area, and it divides the geological structure of the East China Sea into two parts, and, on the northern side of the study area, the Mohorovičić discontinuity (Moho) is deeper, the Earth's crust is continuous, and the landform is more flat. On the south side of the fault 2

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Fig. 3. Diagrammatic sketch of the MIP-CPT (Drawn based on the customer technical manual of Fugro [17]).

Fig. 2. Layout of the MIP-CPT sites and the location of the drilling in the study area.

strength of the soil ([35,51]; Lunne, 2010; [39])). In addition, dynamic and static pore pressure measurements provided improved stratigraphic information [5,9]. The MIP was designed for the in-situ screening of volatile organic compounds (VOCs) in the subsurface, and the MIP sensor also can detect gases, including methane (CH4) and hydrogen sulphide (H2S) at very low concentrations. When free and dissolved gas is present during the push, it will diffuse across the semi-permeable MIP membrane and travel through capillaries in the cable up to the controller and the detection unit on the vessel or jack-up deck, where it is measured continuously by three gas detectors, i.e., a Photo Ionization Detector (PID), a Flame Ionization Detector (FID), and a Dry Electrolytic Conductivity Detector (DELCD). Methane is detected only by the FID, while H2S is detected by both the DELCD and the PID (Fig. 3).

3. Tools and methods The geophysical, geotechnical, and in-situ data used in this study were acquired within the framework of the comprehensive survey of shallow gas in the coastal area of the East China Sea. (The Dq-1 cruise occurred in March 2014; the Dq-2 cruise occurred in June 2014; the Dq3 cruise occurred in October 2014; the Dq-4 cruise occurred in May 2015; and the Dq-5 cruise occurred in July 2015.) 3.1. Geophysical exploration Sub-bottom profiler (SBP) data were acquired by using a CAP-6600 Chirp Ⅲ sub-bottom profiler, and the SBP survey lines covered the two pockmarks and the areas surrounding them. For the seismic exploration, a Generator-Injector (GI) airgun source was used that had a capacity of 60 in3, the shot interval was 25 m, and the survey lines covered the two pockmarks. The 24-channel seismic system consisted of a solid-state cable with a channel spacing of 12.5 m and high-resolution seismometers. Combined with the SBP data, the information of the strata in the study area since Quaternary was completely identified.

3.3. Drilling samples and laboratory tests In the Dq-5 cruise, the G1 well was drilled in the south part of the Pockmark A, where the free gas reservoir was found by the MIP-CPT survey, and the drilling depth of G1 was70 m. In order to determine the key mechanical and physical parameters of the pockmark soil, all cores were extracted without being disturbed, and a series of geotechnical tests was conducted. For comparison with G1, the N1 well was drilled about 300 m from G1 in the WWN direction from G1 and about 150 m from the outer edge of Pockmark A. N1 was drilled to a depth of 50 m (Fig. 2). The sampling and testing were conducted in accordance with the code for the investigation of geotechnical engineering (GB500212001) promulgated by the former Ministry of Construction of China, which is now the Ministry of Housing and Urban-Rural Development of China.

3.2. Membrane interface probe-cone penetration testing In-situ measurements were conducted using Membrane Interface Probe-Cone Penetration Testing (MIP-CPT). A string of 35-mm diameter steel sounding rod was connected to the MIP-CPT cone, and it was pushed below the seafloor by the rotating action of four wheels of the subsea unit. The MIP-CPT cone consisted of two submodules, i.e., the Membrane Interface Probe (MIP) and piezocone testing (CPTU). By interpreting the cone resistance (qc) and the sleeve friction (fs), the CPTU data provided detailed information about the in-situ mechanical characteristics of the soil in the study area. (According to academic consensus, the qc value had a positive correlation with the shear

3.4. Shallow gas eruption experiment The idea of this experiment was derived from the previous experiments on Piercement structures [40]. The experimental setup consisted 3

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Fig. 4. Shallow gas eruption experiment diagrammatic sketch.

4. Results

of a transparent container that consisted of two parallel glass plates that were 0.1 m apart, as shown in Fig. 4.a. The 1-m wide glass plates were sealed at the bottom and at the sides, and they were open at the top. Air was injected into the container and into the bed of the material through an inlet that was 0.05 m above the bottom of the container to prevent airflow from being focused along the walls of the container. The material in the container was divided into three layers, i.e., 1) a 0.5-m thick sand layer was laid on the bottom to simulate the reservoir, 2) a 0.16-m thick silty clay layer was laid on the sand layer to simulate the cap layer, and 3) the top layer was a water layer that was 0.3-m deep. the ratios of the thicknesses of the layers were consistent with the data that were obtained in the study area. The inlet air pressure was controlled at five different settings, i.e., 50, 40, 30, 20, and 10 kPa, and another experiment was conducted without input air as a control experiment to simulate the environment without the shallow gas. During the process of injecting the air, siphon devices were used to simulate the undersea currents, and they carried the ejected particles out at the bottom of the water layer. The time that air was injected for each experiment was determined by whether the ‘pockmark’ was formed; the injection of air was terminated when the experimental pockmark no longer formed during the experiment. The methods for collecting the experimental data were as follows (Fig. 4.b): (1) the morphological changes that occurred during the formation of the experimental pockmark were recorded by a high-definition camera; (2) using the sieving method to measure the gradations of the soil particles of the experimental pockmark, the sampling sites were divided into a depression area and a peripheral area; and (3) using the Mini Penetrometer (MPT) to measure the mechanical characteristics (maximum cone resistance) of the topsoil of the experimental pockmark 24 h after the end of the experiment. The operation of the MPT test was based on Chinese industry standards [7], and the MPT sites were divided into the depression area, the margin of the depression area, and the peripheral area. The test was performed five times in each area, and the average of the five results was used.

4.1. Detection of shallow gas Generally, shallow gas has the following reflection characteristics in SBP and seismic profiles, i.e., acoustic turbidity, enhanced reflection, acoustic blanking, bright spot velocity pull-down, and gas chimneys (Judd et al., 1992). So that the methane concentrations on the site could be estimated, we performed laboratory gas calibration studies with the MIP-CPT. To simulate field conditions, the MIP submodule was connected to a gasproof syringe via a sample tube loop that contained neat methane gas and dissolved methane, respectively. The response of the FID signal was recorded repeatedly in several test runs, and the strengths of the response signals that were obtained corresponded to the different states of the dissolved methane and the free gas (Table 1). 4.1.1. Pockmark A Fig. 1.a shows that Pockmark A was approximately an oval shape, with a north-south length of about 400 m, an east-west width of about 150 m, and a depth of about 1 m. The SBP profile in Fig. 5.a clearly shows two sets of the reflection features of the shallow gas under Pockmark A, i.e., (1) cloudy turbidity, seabed depression, enhanced reflection, acoustic blanking, and velocity pull-down; (2) an acoustic plume with gas chimneys. Based on the acoustic theories of Robert J. Urick [49] and Barger [2], the first set of reflections showed that there was an accumulation of gas in the sediment under Pockmark A, and this gas absorbs most of the acoustic energy and keeps leaking into the seawater; the gas chimneys of the second set of reflections provide Table 1 . Criterion results from the calibration of the laboratory gas.

4

Shallow gas state

FID (mV)

Free gas Medium-high concentration of dissolved methane (15–30 mL/L) Low concentration of dissolved methane (< 15 mL/L)

>800 200–800 100–200

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Fig. 5. SBP interpretation of shallow gas distribution in the study area.

Pockmark A, with a distance of about 6 km. It has an approximately rectangular shape with a north-south length of about 300 m, an east–west width of about 250 m, and a depth of about 1 m. In the SBP profile of Pockmark B (Fig. 5.b), the shallow gas reflection features clearly can be observed, as was the case in Pockmark A, i.e., cloudy turbidity, depression of the seabed, enhanced reflection, acoustic blanking, velocity pull-down, and an acoustic plume [2,49]. The layout of the MIP-CPT sites in Pockmark B was different from that of Pockmark A, i.e., all four sites were arranged in the depression area, with ZK12 and ZK13 on the symmetrical axis of the rectangular depression and ZK14 and ZK15 arranged on the inner border of the southern edge of the depression (Fig. 2.b). According to the FID logs of these sites and the criterion results from the calibration of the laboratory gas, dissolved methane was found at all four test sites (Fig. 6.b). ZK12 and ZK13 had methane-saturated pore water over the entire depth that was investigated, as described above, and there were small, free, gas bubbles in ZK13, as suggested by the FID spikes on the gas profile. The methane concentrations were lower in ZK14 and ZK15.

evidence of the upward migration of shallow gas from the deep stratum, and the gas that breaks through the seafloor creates an acoustic plume in the SBP profile. The sites of the MIP-CPT were determined based on the geophysical results from three sites in the depression area of Pockmark A, i.e., ZK2, ZK4, and ZK11, from five sites in the acoustic blanking area outside the depression area, i.e., ZK1, ZK5, ZK7, ZK8, and ZK10), and the last three sites, i.e., ZK3, ZK6, and ZK9), were arranged in the peripheral area (Fig. 2). According to the FID logs of these sites and the criterion results from the calibration of the laboratory gas (Table 1 and Fig. 6.a), free gas was detected at depths below 25 m in ZK2, as expected. The gas layer was at least 4.4 m thick, as was concluded from the dropping of the FID signals at the depth of 29.4 m, but it might as well continue beyond the final test depth of 30 m. ZK4 duplicated the result of ZK2, and added a free gas layer between the depths of 17.5 and 23.1 m. Apart from these, ZK7 provided a signal peak at the depth of 17 m, which shows a free gas layer that is 0.6 m thick. In addition, Pockmark A and its adjacent area that was covered by the tests indicated that dissolved methane (at various concentrations) was encountered, and there was a clear relationship between the depth of the dissolved methane and the locations, i.e., the closer to the depression area, the sooner the FID detected the high concentrations, i.e., the vertex of the high concentration layer was closer to the surface of the seafloor, and the FID logs of the peripheral sites (ZK3, ZK6, ZK9) indicated merely low concentrations.

4.1.3. Seismic data The high-energy seismic waves emitted by the G.I airgun were able to penetrate some of the strata where the SBP profile was not clear [3,38]. Even so, the seismic waves still were shilded strongly by the shallow gas, and the acoustic turbidity and gas chimneys continue from the bottom of the pockmarks to exceed the Tg interface (quaternary bedrock basement identified by the multi-channel seismic survey) in Fig. 7. In addition, some new seismic shields appeared in the

4.1.2. Pockmark B Fig. 1.c shows that Pockmark B is in the EES direction from 5

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Fig. 6. FID signal feedback of shallow gas concentration in the study area.

modulus (Es), consolidation quick direct test cohesion (Ccq), and internal friction angle (Φcq) (Fig. 8). The MIP-CPT data logs from the same pockmark were combined on the same chart for comparison (Figs. 9 and Fig. 12), with the data logs from the depression area marked with blue, the data logs from the acoustic blanking area outside the depression area marked with red, data logs from the inner verge of the depression area marked with orange (Pockmark B), and the green logs represent the data from the peripheral area. In order to compare the data logs more accurately, the

Pleistocene stratum, and future surveys of the shallow gas should be supplemented by more diversified means to get better data. 4.2. Geotechnical information Geotechnical tests for the drilling samples recovered from G1 and N1 provided eight key physical and mechanical parameters, i.e., water content (ω), void ratio (e), saturation (Sr), coefficient of horizontal permeability (kh), coefficient of vertical permeability (kv), oedometric 6

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Fig. 7. Multi-channel seismic profile in the study area.

consistently into 3 segments, i.e., 1) in the range of depths from 1 to 9 m, the qc values of the blue logs are higher than the qc values of the green logs by about 0.1 MPa; 2) starting from the depth of 9 m, the qc values of the blue logs appear to decline collectively, and the blue logs begin to overlap the green logs until the depth reaches 16 m; 3) starting at a depth of 16 m, the qc values of the blue logs showed a second collective decline, and the qc values of the blue logs began to be less than the qc values of the green logs by about 0.1 MPa until the end of the test. By focusing on the fs values, the situation of the blue logs echo the situation when compared with the qc values, i.e., in the range of depths from 1 to 16 m, the fs values of the blue logs are higher than the fs values of the green logs and the differences increase with depth, but, starting from the depth of 16 m, the fs values of the blue logs decrease. Compared to the other logs, the red logs (including qc and fs) had striking fluctuations from depths of 1 to 14 m, indicating that the sediments encountered by the probe were more heterogeneous [10,37]. Also, the qc and fs values of the red logs were significantly greater than the qc and fs values of other logs at these depths, indicating that this is a special area. Fig. 10 compares these data logs according to the distance from the depression area, and the figure shows that the logs (including qc and fs) of ZK1 and ZK10, which were far away from the depression area, were similar to those from the periphery of Pockmark A in the depth range from 0 to 7 m. In connection with Fig. 6, the FID signals of ZK1 and ZK10 only feedback very low concentrations of dissolved methane at these depths. In contrast, when the FID signals begin to feedback medium-high concentrations of dissolved methane, the two black logs show the same performance as other red logs (Fig. 10). Fig. 11 shows that the pore water pressure logs of all sites coincided, and no excess pore water pressure was found. Therefore, no highpressure gas reservoir was detected in the study area [8,45]. This result is consistent with the results of the above geotechnical tests (Fig. 8).

seafloor of the non-depression area was chosen as the 0 m datum level, so the starting depth of the data logs from the non-depression area was unified as 0 m, and the starting depth of the data logs from the depression area was unified as −1 m. Compared with the geotechnical data, the in-situ data were linear and had many sites, which can better present the complex characteristics of the natural sediments [11, 14]. 4.2.1. Pockmark A First, the drilling samples were classified according to the code for the investigation of geotechnical engineering (GB50021-2001) promulgated by the Ministry of Housing and Urban-Rural Development of China. In G1, the drilling samples taken from depths between 3.2 m and 25.2 m were silty clay, and the samples from greater depths were mostly clay. In N1, the drilling samples taken from depths of 6–35.6 m showed that the strata are interbedded with silt clay and clay, and the drilling samples taken from depths of 38.2–50 m were clay. Fig. 8 shows that the samples from N1 mostly were saturated, while the samples from G1 had lower ω and Sr values than the samples from N1. These results were consistent with the distribution of the shallow gas in the study area, as mentioned above. And the Sr values of G1 remained about 90% from depths of 0–30 m. Then, in contrast with the kh and kv values of the drilling samples, the samples from G1 had better horizontal and vertical permeabilities than the samples from N1 within 30 m below the seafloor, and this advantage was more obvious in the stratum within 10 m below the seafloor. It shows that G1 did not meet the formation with the ability to seal gas from depths of 0–30 m. Comparing the Es values of the drilling samples, it was observed that, in the strata within 40 m below the seafloor, the Es values of the samples from N1 generally were smaller than the Es values from G1, and the difference could exceed 1 MPa. Consistent with this, the e values of the samples from G1 were significantly lower than those from N1, which indicated that the strata drilled by G1 were more compact. In terms of shear strength, the samples from G1 had better shear strength indexes (Ccq and Φcq) based on our observation of the results of the consolidation quick direct test, which was consistent with the results of the following CPTU. Fig. 9 shows that the qc (left logs) and fs (right logs) values of the three areas (red, blue, and green logs) had significant differences based on the lateral comparison. The green logs (including qc and fs) increased linearly with depth and had good consistency, which indicated that the soil in the peripheral area had not been disturbed significantly, so its properties are consistent [5,34]. Therefore, green logs can be used to represent the mechanical characteristics of the soil in virgin sediment. By focusing on the qc values, the blue logs can be divided

4.2.2. Pockmark B Fig. 12 shows the qc and fs values versus depth obtained from the four sites of pockmark B and, similar to the case of the pockmark A, there were some differences between the orange logs and the blue logs. The blue data logs show the qc and fs values higher these values were for the orange data logs at depths of 0–2 m below the seafloor, similar to pockmark A. After the depths exceeded 2 m below the seafloor, the orange logs of the fs values had larger fluctuations and higher detection values than the blue logs, but the orange logs of the qc values had neither large fluctuations nor higher detection values. The orange logs and blue logs were no longer different beginning at 19 m below the seafloor, and their growths became linear. 7

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Fig. 8. Geotechnical results of drilling samples.

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Fig. 9. Comparison of the cone resistance qc and sleeve friction fs data logs of the Pockmark A.

(Fig. 13.c), the air pressure that accumulated in the gas bulge has exceeded the sealing pressure of the cap layer, and a gas breakthrough appears above the gas bulge, presenting a violent eruption, and the particles that are carried away by the gas quickly make the water layer turbid. With the release of the gas, the gas bulge disappears, and the gas breakthrough gradually expands into a funnel-shaped pockmark prototype, and the experiment enters the third stage (Fig. 13.d). In the third stage, the ability of the escaping gas to carry particles is weakened

4.3. Experimental results Fig. 13 shows a set of experimental photographs when an air pressure of 50 kPa was selected to describe the process by which an experimental pockmark was formed. At the initial stage of the experiment (from Fig. 13.a to b), the air pumped into the container rapidly formed a gas bulge that lifted the cap layer up and above the reservoir like the formation of the diapir [24,25]. In the second stage of the experiment 9

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Fig. 10. Comparison of cone resistance qc and sleeve friction fs data logs of the acoustic blanking area outside the depression area of Pockmark A.

pockmark are carried by the continuous undersea current, so the backfilling of particles on the edge of the pockmark that occurred in the experiment usually will not occur in the measurement of an actual image [22].

by the release of the pressure previously accumulated in the gas bulge and the expansion of the pockmark, so that some coarse particles in the water layer begin to sink and fall back. The particles deposited at the bottom of the pockmark will weaken the energy of the escaping gas, thereby slowing down the expansion of the pockmark and causing even more particles to sink. Ultimately, due to the dual effects of the expansion of the pockmark and the backfilling of particles, the energy of the escaping gas is insufficient to expand the pockmark continuously, and the experimental pockmark is finalized (Fig. 13.e). In addition, because of the siphon device used to simulate the undersea current, a considerable portion of the particles is transported out of the container instead of falling back. After the gas is stopped and the water in the container is siphoned off, the experimental pockmark shows a profile similar to the profiles that occur in nature, as shown in (Fig. 13.f. However, in nature, the particles that are deposited outside the

4.3.1. Gradation change in the particles of soil Fig. 14 shows the gradation curves of the surface soil of the pockmarks formed by each experiment, and it was found that the soil samples taken from different positions of the pockmark have characteristics that can be followed regularly. After each experiment, the proportion of coarse particles (i.e., particles larger than 0.074 mm) in the blue line is increased compared to the black line, but there is a negative correlation between the magnitude of the increase and the air pressure. Each time there was a slight difference between the green line and the black line, it was observed that the green line gradually moved 10

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the depth range from 7 to 14 m, where medium-to-high concentrations of dissolved methane were detected. Therefore, the acoustical blank area obtained by geophysical exploration roughly denotes the plane range of the pockmark soil [55]. 5.2. Shallow gas in the pockmark soil Although the results of the seismic survey showed that gas is still being transported to the pockmarks in the study area (Fig. 7), the pore water pressure data indicated that there is no high-pressure gas reservoir in the pockmark soil (Fig. 11). Therefore, even the free gas is still found under the pockmarks, so it can be inferred that, after the pockmarks were formed, the high-pressure gas reservoirs (or gas diapir) that were present in the sediment near the seafloor have been released, and the pockmark soil is no longer conducive for the accumulation of shallow gas. This inference coincides with the results of the geotechnical tests shown in Fig. 8, i.e., the samples from G1 have relatively high horizontal (kh) and vertical (kv) permeability coefficients at depths from0 to 30 m. In the shallow gas eruption experiment, it was found that, when the experimental pockmark was formed, the gas could still enter the water layer through the pockmark soil using the migration paths formed during the eruption rather than forming another gas bulge. Accordingly, the destruction of the cap layer caused by the activity of the shallow gas was assumed to be the major factor in improving the permeability of the pockmark soil. The activity of the gas will destroy the natural sequence of the virgin sediment, and the strong airflow will carry a lot of soil particles, thereby leaving migration paths in the soil. If the soil particles that are carried out are not washed away by the current, the loose deposits formed by backfilling cannot have a good ability for sealing gas in a short period of time. The acoustic plume with gas chimneys in the SBP profiles (Fig. 5.a) demonstrate the existence of such migration paths in the pockmark soil of the study area, but, admittedly, more detailed research is needed to prove this deduction. For example, the distribution and concentration of shallow gas can be assessed by in situ measurements of the velocity of the seismic wave [42,48].

Fig. 11. Comparison of the pore water pressure u2 data logs of the Pockmark A.

from below the black line to above the black line as the gas pressure increased.

5.3. Mechanical characteristics of pockmark soil Previous studies have classified soil that contains shallow gas as a kind of weak soil for the following reasons, i.e., during the storage of the gas, the soil expands and the soil skeleton is damaged; this slows the consolidation process of the soil layer due to the effect of its own weight, and this increases the compressibility of the soil layer and reduces the shear strength of the soil ([47]; Ye et al., 2012). This classification can be proved by the descent of the blue logs (including the qc and fs values) in the free-gas-bearing strata (Fig. 9.a). However, after a series of studies of the pockmarks in the study area, the results indicated that it is too general to classify pockmark soil as weak soil. On the contrary, the pockmark soil formed by the shallow gas eruption exhibited strengthened mechanical properties. According to the data acquired from geotechnical tests (Fig. 8), the drilling samples from depression area G1 had lower e values and higher Es values than the samples from the peripheral area (N1). Therefore, the pockmark soil with stronger mechanical properties is more compact than the virgin sediment. Based on this finding in addition to the loss of soil particles, the compression deformation of virgin sediments in shallow gas activity also is a cause of the formation of pockmarks in the study area. This inference is consistent with Fredlund's bivariate strength theory of unsaturated soils [18]. The net normal stress (σ*) is the difference between the total stress (σ) and the air pressure in the pores (ua):

4.3.2. MPT results The histograms in Fig. 14 are the MPT data acquired from the experiment, and the gray part below each column represents the MPT value at that location from the non-gas controlled experiment. By observing the histograms, it was apparent that the MPT values of the experimental group were significantly higher than values of the control experiment, and the relationship between the MPT values and the air pressure was not an ordinary positive or negative correlation. The air pressure corresponding to the peak of the blue column was 30 kPa, and then the blue column decreased as the air pressure increased. Therefore, the pockmark soil formed by the shallow gas eruption also exhibited strengthened mechanical characteristics similar to what occurred in the study area, thereby proving that the occurrence was not accidental. 5. Discussion 5.1. Range of pockmark soil The survey results acquired from ZK1 and ZK10 proved that the range of pockmark soil has an inheritance relationship with the preoccurrence range of shallow gas (Figs. 6 and 10). The qc and fs values reflect the mechanical characteristics of virgin sediment at depths from 0 to 7 m, where very little, if any, dissolved methane is detected. These values also reflect the mechanical characteristics of pockmark soil in

σ * = σ − ua

(5-1)

Matrix suction (s) is the difference between the pore air pressure 11

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Fig. 12. Comparison of cone resistance qc and sleeve friction fs data logs of the Pockmark B.

ua - uw, the gas in the soil is released completely to saturation, after which the net normal stress can be expressed by the effective stress, σ -uw, and the matrix suction becomes 0:

(ua) and the pore water pressure (uw):

s = ua − u w

(5-2)

The relationship between the variation of void ratio of the soil (Δe) and the variation of stresses is as follows:

Δe = a v × Δσ * + as × Δs

(5-3)

Δσ * = (σ − u w ) − (σ − ua) = ua − u w

(5-4)

Δs = ua − u w

(5-5)

Therefore, Formula (5-3) can be changed to:

where Δσ* is the variation of net normal stress; Δs is the variation of matrix suction; av is the coefficient of compressibility corresponding to the net normal stress; and as is the coefficient of compressibility corresponding to matrix suction. Assuming that the initial matrix suction is

Δe = a v × ua − u w + as × ua − u w = (a v + as ) × (ua − u w ) According to the formula of soil compression deformation: 12

(5-6)

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Fig. 13. Shallow gas eruption experiment process diagram.

S=

Δe ×H 1 + e0

mechanical properties than the soil in the depression area (blue logs) within the depth range from 1 to 14 m (Fig. 9). According to the best of the author's knowledge, this phenomenon has never been recorded in previous studies. But, to date, the number of pockmarks studied by academic researchers is less than one-thousandth (0.1%) of the total pockmarks in the world, so it cannot be ruled out that this phenomenon is universal. If this is a common phenomenon, then the zoning characteristics of the mechanical properties of pockmark soil must be due to regional differences in the process of the formation of pockmarks. Accordingly, the process diagram of the shallow gas eruption experiment to observe the difference between the depression area and the non-depression area during the formation of the experimental pockmark (Fig. 13) provided support for this hypothesis in that, during the formation of the pockmark, the cap layer in the non-depression area was not blown away by the air current. From the results of MIP-CPT, the red logs and the green logs overlapped roughly in the depth range from 0 to 1 m, which illustrates that the topsoil of the acoustic blanking area outside the depression area (red logs) in the study area was still virgin sediment (Fig. 9). Thus, the results of the experiment were consistent with the situation in the study area. It can be reasoned that the soil under the topsoil of the acoustic blanking area outside the depression area did not lose a large amount of soil particles during the process of forming the pockmark, as did the soil under the cap layer of the non-depression area in the experiment, which also can be corroborated by the geomorphology of the study area. Therefore, it can be speculated that there are two main causes for the zoning characteristics of the mechanical properties of pockmark soil (Fig. 9). Since a lot of soil particles were not lost, the soil of the acoustic blanking area outside the depression area had a larger weight than the soil in the depression area, which explains why the red logs have greater qc and fs values at depths from 1 to 14 m; when the shallow gas was being released, it did not have the ability to carry soil particles in the soil of the subsequent non-depression area, so it only could squeeze some soil particles to form migration paths to the outlet, and the

(5-7)

where S is the amount of deformation of the unit thickness; e is the natural void ratio; and H is the thickness of the soil. By substituting Eq. (5-6) into Eq. (5-7) we get:

S=

(a v + as ) × (ua − u w ) ×H 1 + e0

(5-8)

Eq. (5-8) shows that the release of gas will lead to the compression deformation of virgin sediments, and the thickness of the deformation is correlated positively with the thickness of the gas-bearing soil and the initial matrix suction (i.e., the pressure of the gas reservoir). Thus, it can be proved that the strengthening of the mechanical properties of the pockmark soil in the study area was related closely to the compression deformation of the soil caused by the eruption of the shallow gas. This inference coincides with a recent survey in the Yellow Sea of China [31]. In addition, the MPT data indicated that the mechanical property of the pockmark soil formed by the eruption of the shallow gas has the optimal experimental value at the corresponding air pressure, and there is a negative correlation between the proportion of coarse particles in the depression area and the pressure of the input gas. Thus, it can be speculated that the change in the gradation of the particles caused by the loss of soil particles was one of the reasons for the change in the mechanical properties of the pockmark soil [6,44]. However, the scale of the shallow gas pressure is not the only factor that can influence the particle gradation of the pockmark soil. So, the multiple influences of the activity of the shallow gas on the mechanical properties of the pockmark soil are complex, and it is impossible to reach a definitive conclusion based only on the results of this study. 5.4. Zoning characteristics of pockmark soil As is known from the results of MIP-CPT, the soil in the acoustic blanking area outside the depression area (red logs) had stronger 13

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Fig. 14. Results of the shallow gas eruption experiment at different inlet air pressures.

significant fluctuations of the red logs reflect this inhomogeneity of the disturbed soil. According to the speculation described above, there should be a transitional zone in the marginal slope of the depression area. The converging gas has the ability to carry the soil particles in the transitional zone, so squeezing soil particles is not the sole way for the gas to form migration paths in the soil. The gas still cannot break through the cap layer of the transition zone, so the topsoil of the transition zone is still virgin sediment; since the soil below the topsoil of the transition zone has the simultaneous release of gas and the loss of soil particles, the mechanical characteristics of this part of the soil should be similar to that of the depression area. Fortunately, ZK14 and ZK15 are located at the verge of the depression area, and the MIP-CPT data of Pockmark B can be used to verify this deduction (Fig. 12). In the depth range from 0 to 2 m, the qc and fs values of the orange logs (ZK14

and ZK15) are lower than those of the blue logs (ZK12 and ZK13); in the depth range from 2 to 9 m, the qc values of the orange logs basically overlap the values of the blue logs, but the fluctuation of the fs values was more obvious. Thus, the characteristics of the data of ZK14 and ZK15 corresponded to the deduction of the transition zone mentioned above, which indicated that the transition zone exists objectively. In summary, the speculation about the causations for this new phenomenon of the zoning characteristics of pockmark soil was consistent with the existing results of the investigation in the study area. Thus, Fig. 15 shows the zoning of pockmark soil according to the results of this study. But to completely prove the validity of our results, more investigations are needed in the future, including drilling sampling in the acoustic blanking area outside the depression area and investigating more pockmarks. 14

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Fig. 15. Schematic diagram of zoning characteristics of pockmark soil and corresponding representative MIP-CPT sites.

6. Conclusion [2]

This article is based on the use of two pockmarks near Zhongjieshan Archipelago in the East China Sea as the objects of this research. The influence of shallow gas activity on the geotechnical properties of pockmark soil were analyzed comprehensively by using a geophysical survey, geological drilling, in-situ measurement, and a shallow-gas eruption experiment. The following results were obtained. The pockmark soil in the study area was not suitable for the accumulation of high-pressure, shallow gas, and one of the main causes is that the eruption of the gas can leave migration paths in the soil. The eruption of shallow gas can enhance the mechanical properties of pockmark soil, and the cause of such eriptions is related closely to the compaction of the soil caused by the eruption of shallow gas. In addition, the mechanical properties of the pockmark soil in the study area exhibited the zoning phenomenon, and this is a new finding. After analysis, it was decided that the reason this occurred was related to the imbalance of the loss of soil particles during the eruption of the shallow gas. According to this result, pockmark soil is partitioned by region, which is instructive for future research. Due to the limitation of the scale of this research, we were unable to investigate additional pockmarks, so additional in situ surveys are needed to validate the findings presented in this article and to improve the analyses that were made. However, it is apparent that our findings and analyses have brought us a step closer to fully understanding the geotechnical properties of pockmarks.

[3]

[4]

[5]

[6]

[7] [8]

[9]

[10]

[11]

[12]

Acknowledgment

[13]

The authors acknowledge the financial support of the National Key R&D Program of China (Project no. 2016YFF0202903).

[14]

[15]

Supplementary materials [16]

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.apor.2019.101966.

[17] [18]

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