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Forward modelling and identification of shallow gas in the Bohai Bay seabed

Forward modelling and identification of shallow gas in the Bohai Bay seabed The accumulation of shallow gas in the seabed reduces the strength of strata or forms a high‑pressure air sac, endan‑ gering ocean engineering construction. Therefore, it is important to identify the distribution of shallow gas in the sea‑ bed within the study area. Shallow gas increases the soil mass porosity and reduces the acoustic wave velocity, caus‑ ing attenuation by absorbing to high‑frequency components in the acoustic waves. Based on the geological drilling data in the area surrounding an oil platform in Bohai Bay, a stratigraphic model was established for forward analysis, and the results suggest the presence of the phase inversion of reflective waves at the interface between shallow gas and strata and sunken events for the lower shallow gas. According to a survey of stratigraphic profiles surrounding the platform, a seismic attribute analysis of acoustic stratigraphic profile data concerning amplitude, instantaneous phase, and instantaneous frequency was carried out, and characteristics such as disordered weak amplitude reflection, phase inversion, sunken events and indicators, including high‑frequency loss and shallow gas reflection, were identified. Given that the shallow gas reflection is columnar and ended at the top clay strata of the seabed, the shallow gas was probably produced from deep depths. Keywords: Seabed shallow gas, Forward analysis, Instantaneous phase, Instantaneous frequency, Identification of shallow gas Introduction a specific air pressure sac is present. When the overly - Shallow gas refers to gas that accumulates at a depth of ing strata are punctured during ocean engineering, such 1000  m beneath the seabed. Shallow gas is character- as oil platform construction and drilling, the shallow gas ized by small molecules, low density, weak absorbability surges out due to internal pressure and causes blowout and strong diffusivity. It is easily accumulates and trans - accidents. Therefore, identifying the distribution charac - fers in strata. When shallow gas accumulates in strata, teristics of shallow gas within work areas is of great sig- it changes the physical mechanical properties of strata, nificance to the selection and evaluation of construction resulting in increased porosity, reduced compactness, sites in ocean engineering. and poorer strength of the strata (Whelan et  al. 1977; There are two types of shallow gas in the seabed: the first Li et  al. 2013; Wang et  al. 2011, 2021; Shang et  al. 2013; is biomethane, whose main composition is methane. Due Sun and Huang 2014). In the case of an external load, to decomposition by methane bacteria, biomethane is the gas-containing stratum may incur creepage, causing gradually formed from biodetritus and organic substances subsidence or sliding of the foundation. Furthermore, in the strata. Biomethane mainly exists in the shallow in the case of shallow gas with a well overlying the cap, strata. The second is thermogenic methane, which exists in a high-temperature and high-pressure environment at a depth of 2000 m beneath the seabed. It consists of hydro- carbons formed from kerogen cracking and often devel- *Correspondence: yangxiaodi1989@126.com CNPC Research Institute of Engineering Technology, JinTang Road 40#, ops a hyperpressure air sac; sometimes, it also rises and Binhai New District, Tianjin, China Full list of author information is available at the end of the article © The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. Yang et al. Geoenvironmental Disasters (2022) 9:9 Page 2 of 10 transfers through the pores, cracks and broken profiles of where t is time; ρ is the density of the medium; rocks to accumulate in the shallow strata. u, v, andw are displacements in the x, y, andz directions; Currently, acoustic detection is generally adopted for σ , σ , σ , σ , σ , and σ are stresses; and F , F , and xx xy xz yy yz zz x y the identification of shallow gas, where it is based on the F are the stresses in the external force in the x, y, andz characteristics of acoustic stratigraphic profiles, such as directions. acoustic blankets, acoustic disturbances, acoustic curtains, The three-dimensional equation of even isotropous ideal irregular strong reflections at top interfaces, sunken phases elastic media expressed in vector form is: on both sides and sunken events caused by decreases in ∂ S acoustic wave velocity (Woodside et  al. 2003; Yan et  al. 2 (2) ρ = (κ + μ)gradθ + μ∇ S + ρF , ∂t 2007; Gu et  al. 2008, 2009b; Wang et  al. 2014; Yang et  al. 2020, 2015). These amplitude characteristics of the acous - where F = F i + F j + F k is the vector of the external x y z tic wave are mainly utilized in this method. However, as force; S = ui + vj + wk is the vector of the displacement; these characteristics are sufficient conditions for the iden - ∂θ ∂θ ∂θ 2 gradθ = i + j + k is the vector of the gradient; ∇ ∂x ∂y ∂z tification of shallow gas, there might be multiple solutions. is the Laplace operator; ρ is the density of the medium; κ Therefore, multiple seismic attributes are used in this study and μ are Lame constants; and t is time. Equation  (2) is for the identification of shallow gas to improve the accu - rewritten into a plane harmonic equation: racy in the identification of shallow gas. The research pro - cess is shown in Fig. 1. 2πx p(x , t) = p sin − ωt + φ , i 0 (3) Acoustic wave detection technology At present, acoustic detection technology is the main tech- where t is time; p is the acoustic wave signal in the time nology for the detection of shallow gas. For shallow gas domain; p is the acoustic wave amplitude;  is wave- with different causes of formation, their accumulation state length; ω = 2πf is the angular frequency; and φ is the and acoustic wave reflection characteristics in strata also phase shift. In acoustic stratigraphic detection, the acous- vary. Acoustic waves are a form of energy transfer. In media tic wave created by an energy converter can be deemed with different strengths, structures and densities, the trans - the superposition of multiple simple harmonic waves. mission velocity, frequency components, energy decrement and other wave field characteristics of acoustic waves also Seismic attributes change (Lei et al. 2007). Seismic attributes are properties for the description and The transmission equation of acoustic waves in water quantification of seismic data, and they are a subset of all and strata is: information included in raw seismic data. The acquisition of seismic attributes is a process of decomposing seismic ∂σ ∂ u ∂σ xy ∂σ xx xz ρ = + + + ρF data, where each seismic attribute is a subset of seismic  2 x  ∂t ∂x ∂y ∂z ∂σ ∂σ ∂σ yy zy ∂ v xy data. In view of the applied geophysics, seismic attributes ρ = + + + ρF , y (1) ∂x ∂y ∂z ∂t  2 are seismic characteristics for depicting and describing ∂σ yz  ∂ w ∂σ ∂σ xz zz ρ = + + + ρF ∂x ∂y ∂z ∂t geological information, such as the stratigraphic structure, lithology and physical properties. Amplitude Amplitude is the most frequently used seismic attribute. The amplitude of acoustic wave reflection is the convolu - tion of the acoustic wave and the strata’s reflective coef - ficient, reflecting the changes in the strata’s nature. The change in acoustic waves caused by strata is referred to as wave impedance, which is the product of the media’s den- sity ρ and acoustic wave velocity v. When acoustic waves penetrate into the interface of different neighbouring media, reflection and transmission occur. The reflective coefficient of strata is: ρ v − ρ v 2 2 1 1 R = , (4) ρ v + ρ v 2 2 1 1 Fig. 1 Method for the identification of shallow gas Y ang et al. Geoenvironmental Disasters (2022) 9:9 Page 3 of 10 where ρ , v , ρ and v are the density and acoustic wave attenuation is mainly caused by the geometric diffusion 1 1 2 2 velocity of media above and below the interface, respec- of acoustic waves and the geometric structure of media; tively. Strata with different natures often have different intrinsic attenuation is related to the viscoelasticity of densities and acoustic wave velocities. At their interface, rocks, which converts the vibration energy of acoustic the reflective coefficient is not zero. Furthermore, the waves into thermal energy. In porous media filled with greater the difference between strata is, the greater the fluid, intrinsic attenuation is dominant. In porous media, absolute value of the reflective coefficient at the interface the relative movement between solids and fluids (gas and the greater the amplitude of the reflective wave. or water) is the main cause of the energy attenuation of acoustic waves (Li 2015). The porosity of the strata increases when shallow gas Instantaneous phase accumulates, thus influencing the density and acoustic Based on the change in phases when acoustic waves pen- wave velocity in the strata. For the strata, the increased etrate different geologic bodies, the boundary of geologic porosity causes reduced density. However, the acoustic bodies may be identified. The instantaneous phase is the wave velocity in strata is related to the porosity and water resolution of signals: content, where the following empirical formula is avail- 180 g(t) able (Zou et al. 2007, 2008; Long and Li 2015): Ph(t) = arctan , (5) π f (t) v = 1981.7539 × (0.9958 − 0.004n + 0.0002ω) (7) where t is time and g(t) and f (t) are the real and virtual where v is the velocity of the acoustic wave, n is the parts of the signal, respectively. porosity (%) of the strata, and ω is the water content (%). The phase information is irrelevant to the amplitude Biomethane often exists in shallow layers in the form but relates to the transmission phase of the acoustic wave of isolated air sacs or acoustic blankets, with little verti- wavefront. It is a physical property of acoustic waves and cal continuity; thermogenic methane often moves from is not affected by the waveform or amplitude. Further - bottom up in the form of columns, with profound verti- more, it accurately demonstrates the reflection area of cal continuity. Based on the causes of the formation of weak amplitude and can be used for the identification of shallow gas and its ascertained forms, it is assumed that the continuity and boundaries of strata (Mou et al. 2007). biomethane is in the form of an isolated air sac, while thermogenic methane is in the form of vertical columns. Instantaneous frequency To study the acoustic reflection characteristics of shal - The instantaneous frequency is the time derivative of the low gas with 120  m-depth geological drilling data from instantaneous phase, which is related to the frequency Bohai Bay, a forward model of normal strata, a model of spectrum of the acoustic wave. The instantaneous fre - strata with shallow gas air sacs and a model of strata with quency φ(t)(Hz) is the rate of change of the phase over column-shaped shallow gas were established. The drilling time: data are as Table 1. Based on the characteristics of strata, such as the d[Ph(t)] φ(t) = , (6) acoustic wave velocity and wave resistance, the strata are dt divided into six layers whose distribution is as Table2. where t is time; φ(t) is the instantaneous frequency; and The acoustic wave velocity and density of each layer are Ph(t) is the instantaneous phase. calculated via the weighted average. The instantaneous frequency is related to the nature A two-dimensional stratigraphic model was built using of the strata that the acoustic wave passes through. The the data in Table 2, where the width was 2000 m and the instantaneous frequency is a physical property of acous- depth was 120  m, with a mesh of 2  m × 2  m. According tic wave signals related to the density of strata. Gener- to a survey in the Hangzhou subway project (Guo et  al. ally, it may serve as an indicator of the oil and gas zone, 2010), the strata contain shallow gas. Shallow gas mostly fracture zone and thickness of strata. When there is oil or exists in sandy clay, while muddy clay is the overlying cap. gas in strata, the high-frequency components often incur The shallow gas has a maximum pressure of 0.405  MPa; attenuation by absorption (Sager et al. 1999; Orange et al. the strata containing shallow gas have a water content of 2005; Hou et al. 2013; Hu 2010). 7%, a saturation of 20%, a density of 1.49 g/cm , a poros- ity of 49% and an acoustic wave velocity of 1588 m/s. The forward model adopted the acoustic wave equa - Forward modelling of shallow gas tion, with an offset of zero. The Ricker wavelet was During transmission in strata, the attenuation of acoustic used as the excitation wavelet, with a main frequency wave energy may be divided into two parts: nonintrin- of 250  Hz, a phase of zero and a frequency bandwidth sic attenuation and intrinsic attenuation. Nonintrinsic Yang et al. Geoenvironmental Disasters (2022) 9:9 Page 4 of 10 Table 1 Geological drilling data sheet No Depth of bottom Sediment type Water Wet bulk density Proportion Porosity % Acoustic wave layer/m content % N/cm velocity m/s 1 2.4 Soft clay 55 16.5 2.7 62 1506 2 3.3 Medium dense silt 21 20.4 2.71 39 1671 3 14.5 Muddy clay 38 18.5 2.72 51 1588 4 16.7 Silty fine sand 26 19.3 2.69 45 1627 5 19.8 Hard silt 26 19.7 2.7 43 1643 6 22 Dense silt 23 20 2.72 42 1651 7 25.7 Dense silty fine sand 20 20.5 2.68 38 1684 8 28.8 Dense silt 28 19.1 2.68 46 1621 9 34.8 Hard clay 37 18.5 2.69 50 1594 10 40.6 Dense silt 23 19.6 2.71 44 1635 11 49.8 Silty clay 27 20.4 2.71 39 1674 12 55.7 Dense silt 26 19.6 2.7 44 1638 13 58.8 Hard clay 35 18.2 2.71 52 1575 14 61.7 Dense fine sand 24 19.2 2.7 46 1619 15 81.1 Hard silty clay 30 19.8 2.72 43 1644 16 91.7 Dense sandy silt 21 19.9 2.72 42 1645 17 106.6 Hard silty clay 20 20.5 2.72 39 1673 18 111.1 Dense silt 26 19.5 2.71 44 1631 19 120.3 Hard silty clay 21 20.5 2.71 39 1676 Table 2 Stratum division sheet Strata Representative stratum Depth of bottom Formation Acoustic wave velocity Density g/cm layer /m thickness/m m/s Layer‑1 Soft clay 14.5 14.5 1573 1.81 Layer‑2 Silty fine sand 25.7 11.2 1655 1.99 Layer‑3 Hard clay 34.8 9.1 1603 1.87 Layer‑4 Silt 49.8 15.0 1658 2.01 Layer‑5 Dense silt 58.8 9.0 1616 1.91 Layer‑6 Hard silty clay 120 61.2 1661 2.01 of 120  Hz. The continuation length of the wavelet was than that in the upper strata. In Fig.  3b, since a shallow 10  ms. The constructed stratigraphic model is shown in gas air sac was present in layer 2 and the acoustic wave Fig. 2 as follows. Biomethane is designed to exist in layer velocity in shallow gas was smaller than that in layer 2, 2 (silty fine sand) in the form of an air sac (Fig.  2b). Ther - the reflective wave phase at the top interface of the shal - mogenic methane transfers upwards to shallow strata low gas was negative, while that at the bottom interface from deep strata and exists in layers 2, 3, 4, 5 and 6 in the was positive. When the duration of the acoustic wave’s form of columns (Fig. 2c). travel time in shallow gas is greater than that in the The forward acoustic stratigraphic profile is shown in strata, the reflective wave events in the lower shallow gas Fig.  3. Figure  3a shows that there were six continuous are sunken downwards. In Fig. 3c, since penetrating shal- events that matched the interfaces between seawater low gas occurred in the column-shaped strata beneath and the five strata interfaces. The phase of the reflective layer 2, the reflective wave phase at the top interface of wave events was negative at the interface between layer 2 the shallow gas was negative, while the reflection at the and layer 3 and at the interface between layer 4 and layer side interface was not clear. 5, indicating that the reflective coefficient of the strata The reflective wave at the top of the shallow gas in the was negative. Based on the stratigraphic model, veloc- forward formation profile is shown in Fig. 4. ity reversions were present in layers 3 and 5, namely, the When shallow gas is present in the strata, the physical acoustic wave velocity in the lower strata was smaller properties and acoustic reflection characteristics of the Y ang et al. Geoenvironmental Disasters (2022) 9:9 Page 5 of 10 Fig. 2 Stratigraphic model, where a is a normal stratigraphic model; Fig. 3 Forward acoustic stratigraphic profiles, where a is normal b is a biomethane shallow gas model; and c is a pyrolysis shallow gas stratigraphic model; b is a biomethane shallow gas model; and c is a model pyrolysis shallow gas model strata change. In normal strata, the internal nature of the same stratum is often relatively uniform with close wave resistance and a nearly zero reflective coefficient. When the strata contain shallow gas, the density and acous- tic wave velocity of strata are reduced, as is the wave resistance; their reflective coefficient at the interfaces between strata and shallow gas are negative, forming a strong reflective interface and causing a specific shield - ing effect of the lower strata. When acoustic waves pass through strata that contain shallow gas, the attenuation of high-frequency components is increased, the acoustic wave velocity is reduced, and the travel time duration is increased. Seismic attribute analysis reveals that the char- acteristics of shallow gas reflective waves in the acoustic stratigraphic profile are as follows: ① the top interface is a strong reflection with phase reversion, while the lower interface is a weak reflection; ② the reduced acoustic Fig. 4 Partial reflective wave at the top of the shallow gas in the wave velocity results in downwards sunken events; and forward formation profile ③ a relatively low frequency is demonstrated. Yang et al. Geoenvironmental Disasters (2022) 9:9 Page 6 of 10 Analysis of measured seismic materials In the Huanghua Sea of Bohai Bay, there are many shal- low gas layers in the strata. Acoustic stratigraphic profile measurements were carried out for the area surrounding an oil platform. An electric spark was used with an excita- tion energy of 1000 J and a recording duration of 400 ms. Constellation difference GPS was adopted for navigation and positioning; the acoustic wave velocity in strata was 1550  m/s from the material surrounding the area. There was a suspected shallow gas reflection in the acoustic stratigraphic profiles. Since the gas-containing strata are selective in absorbing acoustic wave energy with differ - ent frequencies, the three seismic attributes of amplitude, instantaneous phase and instantaneous frequency were analysed for the acoustic stratigraphic profiles, and shal - low gas was identified with multiple attributes. The sur - vey area was a 1 km × 1 km square, and a grid survey line was adopted (Fig. 5). The coordinates of the centre of the survey area were 38°28′20.1094″N and 117°44′13.9174″E. Two profiles were selected for the analysis of seismic attributes: profile 1 and profile 2, and profile 1 matches Figs. 6, 8, 10 below; profile 2 matches Figs. 7, 9, 11 below. Amplitude characteristics On the amplitude profile, the reflection characteristics are shown in Figs.  6 and 7. Figure  6 shows that there Fig. 6 Acoustic profile 1 in the survey area, and the seismic attribute was a weak reflection area bounded by green lines in is amplitude; the profile, which is likely the reflection of shallow gas. In this area, the energy of the reflection wave was weak, with non-contiguous events, unstable waveforms, dis- surrounding events, with apparent rougher and down- ordered reflection and an insignificant layer structure; wards bent events at the edge; the area extended upwards furthermore, there was apparent interruption with the in the form of a column, with a width of approximately 110 m and a distance of approximately 15 mfrom the top to the seabed. Figure 7 shows that there was a downwards sunken area of events bounded by green lines in the profile that seem - ingly traversed through the entire profile; however, the events on both sides were neat and not staggered, thus ruling out the reflection caused by the fault; the down - wards sunken events were likely caused by a reduced acoustic wave velocity because of shallow gas. The reflec - tive wave energy at the top was stable and had continu- ous events; the reflective wave energy at the bottom was weak and had discontinuous events and unstable wave- forms. Within this area, it extended upwards in a column shape, with a width of approximately 40 m and a distance of approximately 15 m from the top to the seabed. Characteristics of phases The instantaneous phases of the amplitude profiles in Figs.  6 and 7 were calculated, and the instantaneous Fig. 5 Survey area and selected profiles (profile 1 matches Figs. 6, 8 phase profiles are shown in Figs.  8 and 9, respectively. and 10 below; profile 2 matches Figs. 7, 9 and 11 below) Figure  8 shows that there was an area with a disordered Y ang et al. Geoenvironmental Disasters (2022) 9:9 Page 7 of 10 Fig. 9 Acoustic profile 2 in the survey area, and the seismic attribute is the phase for Fig. 7 Fig. 7 Acoustic profile 2 in the survey area, and the seismic attribute is amplitude; Fig. 8 Acoustic profile 1 in the survey area, and the seismic attribute Fig. 10 The acoustic profile in survey area profile1, and the seismic is the phase for Fig. 6 attribute is instantaneous frequency for Fig. 6 Yang et al. Geoenvironmental Disasters (2022) 9:9 Page 8 of 10 of the entire profile was between 50 and 500 Hz, demon - strating a higher frequency in the upper part and a lower frequency in the lower part overall. There was an abnor - mal frequency area bounded by red lines in the middle of the profile. It was column-shaped and had a width of approximately 110  m. The distance from the top to the seabed was approximately 15  m, which was the same as the disordered reflection area in the amplitude profile. In this area, the high-frequency components were lost, demonstrating low-frequency characteristics. The instan - taneous frequencies were 100–150 Hz, while those in the surrounding area were generally higher than 200 Hz. Figure  11 shows that the instantaneous frequencies of the entire profile were between 50 and 500  Hz, dem - onstrating a higher frequency in the upper part and a lower frequency in the lower part overall. There was an abnormal frequency area bounded by white lines in the middle of the profile. It was column-shaped with a width of approximately 40  m. The distance to the seabed was approximately 15  m, which was the same as the sunken area on the amplitude profile. In this area, the high-fre - quency components were lost, demonstrating low-fre- quency characteristics. The instantaneous frequencies Fig. 11 The acoustic profile in survey area profile2, and the seismic were 150–200  Hz, while those in the surrounding area attribute is instantaneous frequency for Fig. 7 were generally higher than 200 Hz. Findings phase bounded by green lines in the middle of the profile. Upon foregoing analysis, there were abnormal areas in It was column-shaped and had a width of approximately the amplitude, phase and frequency profiles with approx - 110  m. The distance from the top to the seabed was imately the same shapes and locations. According to the approximately 15 m, which was the same as the area with geological drilling data, there were mainly sand strata a disordered reflection area on the amplitude profile. In and a few clay strata at depths of 15–120 m beneath the this area, there was no insignificant layered structure; seabed, which were characterized by strong permeability. furthermore, compared with events in the surrounding However, there were mainly muddy clay strata at depths area, the events were significantly rougher. On its top, within 15  m beneath the seabed. Hence, the abnormal there was a staggered and reversed event phase. areas were reflections of shallow gas based on the char - Figure  9 shows that there was a depressed area of acteristics of reflective waves, such as a weak amplitude, events bounded by green lines in the profile. It was col - disordered reflection, phase reversion on the top, sunken umn-shaped with a width of approximately 40  m. The events, and loss of high-frequency components. For the distance from the top to the seabed was approximately acoustic stratigraphic profiles, the spatial sampling rate 15  m, which was the same as the sunken event area on dx = 2  m; the positioning accuracy of the sound source the amplitude profile. At the top of the area, although and hydrophone was 2  m. Based on the interpretation the events were sunken, they were still continuous; in accuracy and onsite measurement accuracy at the cross- the lower part of the area, the events gradually became over points, the accuracy in determining the range of noncontinuous, with a gradually unclear and disordered shallow gas was 3  m. The distribution characteristics of layered structure. On its top, there was a staggered and shallow gas in the survey area are shown in Fig. 12. reversed event phase. Figure  12 shows that the shallow gas was located at the end of a normal fault whose top burial depths were 31–57 m. According to the geological drilling data, there Characteristics of frequency was mainly muddy clay at a depth of 15 m below the sea- The instantaneous frequencies of the amplitude profiles bed, which was an overlying well cap. Based on the seis- in Figs.  6 and 7 were calculated, and the instantaneous mic data of the platform site in past years, shallow gas in frequency profiles are shown in Figs.  10 and 11, respec- the strata at a depth of 60  m surrounding the platform tively. Figure  10 shows that the instantaneous frequency Y ang et al. Geoenvironmental Disasters (2022) 9:9 Page 9 of 10 Huanghua Sea area in Bohai Bay revealed that there were abnormal areas in the seismic attribute profiles. Further - more, they were of the same position and demonstrated a disordered weak amplitude reflection, top phase rever - sion and loss of high-frequency components. Overall, there was shallow gas reflection in the abnormal areas. Therefore, shallow gas may be identified through the analysis of multiple seismic attributes of acoustic stratigraphic profiles, such as the amplitude, phase and frequency. Acknowledgements We thank Xu Hao at the CNPC Research Institute of Engineer Technology for his help in engineering geological borehole data analysis. Author contributions XY was responsible for most of the work of this paper, including data collec‑ tion, data processing, and results analysis. MC was responsible for the field engineering geophysical survey. XL and ZY was responsible for field engineer ‑ Fig. 12 Range of shallow gas distribution ing geological drilling. All authors read and approved the final manuscript. Funding CNPC Scientific research and technology development project ‑ Research on Key Technologies of offshore oil and gas pipeline design and construction appeared in the past 10 years; furthermore, the platform (2019B‑3010). was located in the fault zone of Yangerzhuang-Zhaojiapu in the Chengbei fault terrace. The long-term developed Availability of data and materials The geological borehole data were from actual borehole sampling in Zhaobei fault and Yangerzhuang fault are not only dis- Bohai Bay, which was a full coring borehole. The drilling coordinates were cordogenic faults but also form oil and gas transfer pas- 38°29′28.6651″N, 117°45′15.3138″E; the water depth was 3 m, and the drilling sages with unconformable planes. Therefore, the faults depth was 120 m. The dates of drilling were 2020/11/14–2020/11/25. The seis‑ mic data were from the actual geophysical survey in Bohai Bay, with a survey form oil and gas reservoirs on both sides and in uncon- area of 1 km × 1 km. The location of the survey was surrounded by 4 points: formable areas. The oil reservoir has a burial depth of 38°29′45.3961’’N, 117°44′54.5349″E; 38°29′45.1273″N, 117°45′36.2801″E; 990–1985 m, a porosity of more than 30%, and a perme- 38°29′12.1228″N, 117°45′35.9334″E; 38°29′12.3916″N, 117°44′54.1935″E. The –3 2 survey dates were 2020/10/14–2020/10/19. ability of 1200–1700 × 10  μm . Therefore, the abnormal areas on the profiles were Declarations reflections of shallow gas. However, a large-scale acous - tic blanket requires much gas to maintain its formation Competing interests (Gu et  al. 2009a, b). The shallow gas was likely from the The authors declare that they have no competing interests. in-depth strata and transferred upwards along faults. In Author details recent years, with increased gas pressure, the gas has 1 CNPC Research Institute of Engineering Technology, JinTang Road 40#, Binhai transferred upwards to a location 15 m beneath the sea- New District, Tianjin, China. CNPC Key Laboratory of Marine Engineering, JinTang Road 40#, Binhai New District, Tianjin, China. bed by breaking the resistance of strata in column form. When muddy clay was encountered at the seabed, the gas Received: 15 October 2021 Accepted: 2 April 2022 pressure was equivalent to the resistance of the stratum, forming column-shaped shallow gas in the shallow strata. References Conclusions Gu ZF, Liu HS, Li GF et al (2009a) Genesis of shallow gas in the western area of the South Yellow Sea. Nat Gas Ind 29:26–29 Based on drilling data, a stratigraphic model of shallow Gu ZF, Zhang ZX, Liu HS (2009b) Contrast between traps at the shallow sub‑ gas in the seabed was established for forward simulation bottom depth and the seismic reflection features of shallow gas. Mar analysis of shallow gas in the seabed. 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Acta Oceanogr 29:43–50 Publisher’s Note Springer Nature remains neutral with regard to jurisdictional claims in pub‑ lished maps and institutional affiliations. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Geoenvironmental Disasters Springer Journals

Forward modelling and identification of shallow gas in the Bohai Bay seabed

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Abstract

The accumulation of shallow gas in the seabed reduces the strength of strata or forms a high‑pressure air sac, endan‑ gering ocean engineering construction. Therefore, it is important to identify the distribution of shallow gas in the sea‑ bed within the study area. Shallow gas increases the soil mass porosity and reduces the acoustic wave velocity, caus‑ ing attenuation by absorbing to high‑frequency components in the acoustic waves. Based on the geological drilling data in the area surrounding an oil platform in Bohai Bay, a stratigraphic model was established for forward analysis, and the results suggest the presence of the phase inversion of reflective waves at the interface between shallow gas and strata and sunken events for the lower shallow gas. According to a survey of stratigraphic profiles surrounding the platform, a seismic attribute analysis of acoustic stratigraphic profile data concerning amplitude, instantaneous phase, and instantaneous frequency was carried out, and characteristics such as disordered weak amplitude reflection, phase inversion, sunken events and indicators, including high‑frequency loss and shallow gas reflection, were identified. Given that the shallow gas reflection is columnar and ended at the top clay strata of the seabed, the shallow gas was probably produced from deep depths. Keywords: Seabed shallow gas, Forward analysis, Instantaneous phase, Instantaneous frequency, Identification of shallow gas Introduction a specific air pressure sac is present. When the overly - Shallow gas refers to gas that accumulates at a depth of ing strata are punctured during ocean engineering, such 1000  m beneath the seabed. Shallow gas is character- as oil platform construction and drilling, the shallow gas ized by small molecules, low density, weak absorbability surges out due to internal pressure and causes blowout and strong diffusivity. It is easily accumulates and trans - accidents. Therefore, identifying the distribution charac - fers in strata. When shallow gas accumulates in strata, teristics of shallow gas within work areas is of great sig- it changes the physical mechanical properties of strata, nificance to the selection and evaluation of construction resulting in increased porosity, reduced compactness, sites in ocean engineering. and poorer strength of the strata (Whelan et  al. 1977; There are two types of shallow gas in the seabed: the first Li et  al. 2013; Wang et  al. 2011, 2021; Shang et  al. 2013; is biomethane, whose main composition is methane. Due Sun and Huang 2014). In the case of an external load, to decomposition by methane bacteria, biomethane is the gas-containing stratum may incur creepage, causing gradually formed from biodetritus and organic substances subsidence or sliding of the foundation. Furthermore, in the strata. Biomethane mainly exists in the shallow in the case of shallow gas with a well overlying the cap, strata. The second is thermogenic methane, which exists in a high-temperature and high-pressure environment at a depth of 2000 m beneath the seabed. It consists of hydro- carbons formed from kerogen cracking and often devel- *Correspondence: yangxiaodi1989@126.com CNPC Research Institute of Engineering Technology, JinTang Road 40#, ops a hyperpressure air sac; sometimes, it also rises and Binhai New District, Tianjin, China Full list of author information is available at the end of the article © The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. Yang et al. Geoenvironmental Disasters (2022) 9:9 Page 2 of 10 transfers through the pores, cracks and broken profiles of where t is time; ρ is the density of the medium; rocks to accumulate in the shallow strata. u, v, andw are displacements in the x, y, andz directions; Currently, acoustic detection is generally adopted for σ , σ , σ , σ , σ , and σ are stresses; and F , F , and xx xy xz yy yz zz x y the identification of shallow gas, where it is based on the F are the stresses in the external force in the x, y, andz characteristics of acoustic stratigraphic profiles, such as directions. acoustic blankets, acoustic disturbances, acoustic curtains, The three-dimensional equation of even isotropous ideal irregular strong reflections at top interfaces, sunken phases elastic media expressed in vector form is: on both sides and sunken events caused by decreases in ∂ S acoustic wave velocity (Woodside et  al. 2003; Yan et  al. 2 (2) ρ = (κ + μ)gradθ + μ∇ S + ρF , ∂t 2007; Gu et  al. 2008, 2009b; Wang et  al. 2014; Yang et  al. 2020, 2015). These amplitude characteristics of the acous - where F = F i + F j + F k is the vector of the external x y z tic wave are mainly utilized in this method. However, as force; S = ui + vj + wk is the vector of the displacement; these characteristics are sufficient conditions for the iden - ∂θ ∂θ ∂θ 2 gradθ = i + j + k is the vector of the gradient; ∇ ∂x ∂y ∂z tification of shallow gas, there might be multiple solutions. is the Laplace operator; ρ is the density of the medium; κ Therefore, multiple seismic attributes are used in this study and μ are Lame constants; and t is time. Equation  (2) is for the identification of shallow gas to improve the accu - rewritten into a plane harmonic equation: racy in the identification of shallow gas. The research pro - cess is shown in Fig. 1. 2πx p(x , t) = p sin − ωt + φ , i 0 (3) Acoustic wave detection technology At present, acoustic detection technology is the main tech- where t is time; p is the acoustic wave signal in the time nology for the detection of shallow gas. For shallow gas domain; p is the acoustic wave amplitude;  is wave- with different causes of formation, their accumulation state length; ω = 2πf is the angular frequency; and φ is the and acoustic wave reflection characteristics in strata also phase shift. In acoustic stratigraphic detection, the acous- vary. Acoustic waves are a form of energy transfer. In media tic wave created by an energy converter can be deemed with different strengths, structures and densities, the trans - the superposition of multiple simple harmonic waves. mission velocity, frequency components, energy decrement and other wave field characteristics of acoustic waves also Seismic attributes change (Lei et al. 2007). Seismic attributes are properties for the description and The transmission equation of acoustic waves in water quantification of seismic data, and they are a subset of all and strata is: information included in raw seismic data. The acquisition of seismic attributes is a process of decomposing seismic ∂σ ∂ u ∂σ xy ∂σ xx xz ρ = + + + ρF data, where each seismic attribute is a subset of seismic  2 x  ∂t ∂x ∂y ∂z ∂σ ∂σ ∂σ yy zy ∂ v xy data. In view of the applied geophysics, seismic attributes ρ = + + + ρF , y (1) ∂x ∂y ∂z ∂t  2 are seismic characteristics for depicting and describing ∂σ yz  ∂ w ∂σ ∂σ xz zz ρ = + + + ρF ∂x ∂y ∂z ∂t geological information, such as the stratigraphic structure, lithology and physical properties. Amplitude Amplitude is the most frequently used seismic attribute. The amplitude of acoustic wave reflection is the convolu - tion of the acoustic wave and the strata’s reflective coef - ficient, reflecting the changes in the strata’s nature. The change in acoustic waves caused by strata is referred to as wave impedance, which is the product of the media’s den- sity ρ and acoustic wave velocity v. When acoustic waves penetrate into the interface of different neighbouring media, reflection and transmission occur. The reflective coefficient of strata is: ρ v − ρ v 2 2 1 1 R = , (4) ρ v + ρ v 2 2 1 1 Fig. 1 Method for the identification of shallow gas Y ang et al. Geoenvironmental Disasters (2022) 9:9 Page 3 of 10 where ρ , v , ρ and v are the density and acoustic wave attenuation is mainly caused by the geometric diffusion 1 1 2 2 velocity of media above and below the interface, respec- of acoustic waves and the geometric structure of media; tively. Strata with different natures often have different intrinsic attenuation is related to the viscoelasticity of densities and acoustic wave velocities. At their interface, rocks, which converts the vibration energy of acoustic the reflective coefficient is not zero. Furthermore, the waves into thermal energy. In porous media filled with greater the difference between strata is, the greater the fluid, intrinsic attenuation is dominant. In porous media, absolute value of the reflective coefficient at the interface the relative movement between solids and fluids (gas and the greater the amplitude of the reflective wave. or water) is the main cause of the energy attenuation of acoustic waves (Li 2015). The porosity of the strata increases when shallow gas Instantaneous phase accumulates, thus influencing the density and acoustic Based on the change in phases when acoustic waves pen- wave velocity in the strata. For the strata, the increased etrate different geologic bodies, the boundary of geologic porosity causes reduced density. However, the acoustic bodies may be identified. The instantaneous phase is the wave velocity in strata is related to the porosity and water resolution of signals: content, where the following empirical formula is avail- 180 g(t) able (Zou et al. 2007, 2008; Long and Li 2015): Ph(t) = arctan , (5) π f (t) v = 1981.7539 × (0.9958 − 0.004n + 0.0002ω) (7) where t is time and g(t) and f (t) are the real and virtual where v is the velocity of the acoustic wave, n is the parts of the signal, respectively. porosity (%) of the strata, and ω is the water content (%). The phase information is irrelevant to the amplitude Biomethane often exists in shallow layers in the form but relates to the transmission phase of the acoustic wave of isolated air sacs or acoustic blankets, with little verti- wavefront. It is a physical property of acoustic waves and cal continuity; thermogenic methane often moves from is not affected by the waveform or amplitude. Further - bottom up in the form of columns, with profound verti- more, it accurately demonstrates the reflection area of cal continuity. Based on the causes of the formation of weak amplitude and can be used for the identification of shallow gas and its ascertained forms, it is assumed that the continuity and boundaries of strata (Mou et al. 2007). biomethane is in the form of an isolated air sac, while thermogenic methane is in the form of vertical columns. Instantaneous frequency To study the acoustic reflection characteristics of shal - The instantaneous frequency is the time derivative of the low gas with 120  m-depth geological drilling data from instantaneous phase, which is related to the frequency Bohai Bay, a forward model of normal strata, a model of spectrum of the acoustic wave. The instantaneous fre - strata with shallow gas air sacs and a model of strata with quency φ(t)(Hz) is the rate of change of the phase over column-shaped shallow gas were established. The drilling time: data are as Table 1. Based on the characteristics of strata, such as the d[Ph(t)] φ(t) = , (6) acoustic wave velocity and wave resistance, the strata are dt divided into six layers whose distribution is as Table2. where t is time; φ(t) is the instantaneous frequency; and The acoustic wave velocity and density of each layer are Ph(t) is the instantaneous phase. calculated via the weighted average. The instantaneous frequency is related to the nature A two-dimensional stratigraphic model was built using of the strata that the acoustic wave passes through. The the data in Table 2, where the width was 2000 m and the instantaneous frequency is a physical property of acous- depth was 120  m, with a mesh of 2  m × 2  m. According tic wave signals related to the density of strata. Gener- to a survey in the Hangzhou subway project (Guo et  al. ally, it may serve as an indicator of the oil and gas zone, 2010), the strata contain shallow gas. Shallow gas mostly fracture zone and thickness of strata. When there is oil or exists in sandy clay, while muddy clay is the overlying cap. gas in strata, the high-frequency components often incur The shallow gas has a maximum pressure of 0.405  MPa; attenuation by absorption (Sager et al. 1999; Orange et al. the strata containing shallow gas have a water content of 2005; Hou et al. 2013; Hu 2010). 7%, a saturation of 20%, a density of 1.49 g/cm , a poros- ity of 49% and an acoustic wave velocity of 1588 m/s. The forward model adopted the acoustic wave equa - Forward modelling of shallow gas tion, with an offset of zero. The Ricker wavelet was During transmission in strata, the attenuation of acoustic used as the excitation wavelet, with a main frequency wave energy may be divided into two parts: nonintrin- of 250  Hz, a phase of zero and a frequency bandwidth sic attenuation and intrinsic attenuation. Nonintrinsic Yang et al. Geoenvironmental Disasters (2022) 9:9 Page 4 of 10 Table 1 Geological drilling data sheet No Depth of bottom Sediment type Water Wet bulk density Proportion Porosity % Acoustic wave layer/m content % N/cm velocity m/s 1 2.4 Soft clay 55 16.5 2.7 62 1506 2 3.3 Medium dense silt 21 20.4 2.71 39 1671 3 14.5 Muddy clay 38 18.5 2.72 51 1588 4 16.7 Silty fine sand 26 19.3 2.69 45 1627 5 19.8 Hard silt 26 19.7 2.7 43 1643 6 22 Dense silt 23 20 2.72 42 1651 7 25.7 Dense silty fine sand 20 20.5 2.68 38 1684 8 28.8 Dense silt 28 19.1 2.68 46 1621 9 34.8 Hard clay 37 18.5 2.69 50 1594 10 40.6 Dense silt 23 19.6 2.71 44 1635 11 49.8 Silty clay 27 20.4 2.71 39 1674 12 55.7 Dense silt 26 19.6 2.7 44 1638 13 58.8 Hard clay 35 18.2 2.71 52 1575 14 61.7 Dense fine sand 24 19.2 2.7 46 1619 15 81.1 Hard silty clay 30 19.8 2.72 43 1644 16 91.7 Dense sandy silt 21 19.9 2.72 42 1645 17 106.6 Hard silty clay 20 20.5 2.72 39 1673 18 111.1 Dense silt 26 19.5 2.71 44 1631 19 120.3 Hard silty clay 21 20.5 2.71 39 1676 Table 2 Stratum division sheet Strata Representative stratum Depth of bottom Formation Acoustic wave velocity Density g/cm layer /m thickness/m m/s Layer‑1 Soft clay 14.5 14.5 1573 1.81 Layer‑2 Silty fine sand 25.7 11.2 1655 1.99 Layer‑3 Hard clay 34.8 9.1 1603 1.87 Layer‑4 Silt 49.8 15.0 1658 2.01 Layer‑5 Dense silt 58.8 9.0 1616 1.91 Layer‑6 Hard silty clay 120 61.2 1661 2.01 of 120  Hz. The continuation length of the wavelet was than that in the upper strata. In Fig.  3b, since a shallow 10  ms. The constructed stratigraphic model is shown in gas air sac was present in layer 2 and the acoustic wave Fig. 2 as follows. Biomethane is designed to exist in layer velocity in shallow gas was smaller than that in layer 2, 2 (silty fine sand) in the form of an air sac (Fig.  2b). Ther - the reflective wave phase at the top interface of the shal - mogenic methane transfers upwards to shallow strata low gas was negative, while that at the bottom interface from deep strata and exists in layers 2, 3, 4, 5 and 6 in the was positive. When the duration of the acoustic wave’s form of columns (Fig. 2c). travel time in shallow gas is greater than that in the The forward acoustic stratigraphic profile is shown in strata, the reflective wave events in the lower shallow gas Fig.  3. Figure  3a shows that there were six continuous are sunken downwards. In Fig. 3c, since penetrating shal- events that matched the interfaces between seawater low gas occurred in the column-shaped strata beneath and the five strata interfaces. The phase of the reflective layer 2, the reflective wave phase at the top interface of wave events was negative at the interface between layer 2 the shallow gas was negative, while the reflection at the and layer 3 and at the interface between layer 4 and layer side interface was not clear. 5, indicating that the reflective coefficient of the strata The reflective wave at the top of the shallow gas in the was negative. Based on the stratigraphic model, veloc- forward formation profile is shown in Fig. 4. ity reversions were present in layers 3 and 5, namely, the When shallow gas is present in the strata, the physical acoustic wave velocity in the lower strata was smaller properties and acoustic reflection characteristics of the Y ang et al. Geoenvironmental Disasters (2022) 9:9 Page 5 of 10 Fig. 2 Stratigraphic model, where a is a normal stratigraphic model; Fig. 3 Forward acoustic stratigraphic profiles, where a is normal b is a biomethane shallow gas model; and c is a pyrolysis shallow gas stratigraphic model; b is a biomethane shallow gas model; and c is a model pyrolysis shallow gas model strata change. In normal strata, the internal nature of the same stratum is often relatively uniform with close wave resistance and a nearly zero reflective coefficient. When the strata contain shallow gas, the density and acous- tic wave velocity of strata are reduced, as is the wave resistance; their reflective coefficient at the interfaces between strata and shallow gas are negative, forming a strong reflective interface and causing a specific shield - ing effect of the lower strata. When acoustic waves pass through strata that contain shallow gas, the attenuation of high-frequency components is increased, the acoustic wave velocity is reduced, and the travel time duration is increased. Seismic attribute analysis reveals that the char- acteristics of shallow gas reflective waves in the acoustic stratigraphic profile are as follows: ① the top interface is a strong reflection with phase reversion, while the lower interface is a weak reflection; ② the reduced acoustic Fig. 4 Partial reflective wave at the top of the shallow gas in the wave velocity results in downwards sunken events; and forward formation profile ③ a relatively low frequency is demonstrated. Yang et al. Geoenvironmental Disasters (2022) 9:9 Page 6 of 10 Analysis of measured seismic materials In the Huanghua Sea of Bohai Bay, there are many shal- low gas layers in the strata. Acoustic stratigraphic profile measurements were carried out for the area surrounding an oil platform. An electric spark was used with an excita- tion energy of 1000 J and a recording duration of 400 ms. Constellation difference GPS was adopted for navigation and positioning; the acoustic wave velocity in strata was 1550  m/s from the material surrounding the area. There was a suspected shallow gas reflection in the acoustic stratigraphic profiles. Since the gas-containing strata are selective in absorbing acoustic wave energy with differ - ent frequencies, the three seismic attributes of amplitude, instantaneous phase and instantaneous frequency were analysed for the acoustic stratigraphic profiles, and shal - low gas was identified with multiple attributes. The sur - vey area was a 1 km × 1 km square, and a grid survey line was adopted (Fig. 5). The coordinates of the centre of the survey area were 38°28′20.1094″N and 117°44′13.9174″E. Two profiles were selected for the analysis of seismic attributes: profile 1 and profile 2, and profile 1 matches Figs. 6, 8, 10 below; profile 2 matches Figs. 7, 9, 11 below. Amplitude characteristics On the amplitude profile, the reflection characteristics are shown in Figs.  6 and 7. Figure  6 shows that there Fig. 6 Acoustic profile 1 in the survey area, and the seismic attribute was a weak reflection area bounded by green lines in is amplitude; the profile, which is likely the reflection of shallow gas. In this area, the energy of the reflection wave was weak, with non-contiguous events, unstable waveforms, dis- surrounding events, with apparent rougher and down- ordered reflection and an insignificant layer structure; wards bent events at the edge; the area extended upwards furthermore, there was apparent interruption with the in the form of a column, with a width of approximately 110 m and a distance of approximately 15 mfrom the top to the seabed. Figure 7 shows that there was a downwards sunken area of events bounded by green lines in the profile that seem - ingly traversed through the entire profile; however, the events on both sides were neat and not staggered, thus ruling out the reflection caused by the fault; the down - wards sunken events were likely caused by a reduced acoustic wave velocity because of shallow gas. The reflec - tive wave energy at the top was stable and had continu- ous events; the reflective wave energy at the bottom was weak and had discontinuous events and unstable wave- forms. Within this area, it extended upwards in a column shape, with a width of approximately 40 m and a distance of approximately 15 m from the top to the seabed. Characteristics of phases The instantaneous phases of the amplitude profiles in Figs.  6 and 7 were calculated, and the instantaneous Fig. 5 Survey area and selected profiles (profile 1 matches Figs. 6, 8 phase profiles are shown in Figs.  8 and 9, respectively. and 10 below; profile 2 matches Figs. 7, 9 and 11 below) Figure  8 shows that there was an area with a disordered Y ang et al. Geoenvironmental Disasters (2022) 9:9 Page 7 of 10 Fig. 9 Acoustic profile 2 in the survey area, and the seismic attribute is the phase for Fig. 7 Fig. 7 Acoustic profile 2 in the survey area, and the seismic attribute is amplitude; Fig. 8 Acoustic profile 1 in the survey area, and the seismic attribute Fig. 10 The acoustic profile in survey area profile1, and the seismic is the phase for Fig. 6 attribute is instantaneous frequency for Fig. 6 Yang et al. Geoenvironmental Disasters (2022) 9:9 Page 8 of 10 of the entire profile was between 50 and 500 Hz, demon - strating a higher frequency in the upper part and a lower frequency in the lower part overall. There was an abnor - mal frequency area bounded by red lines in the middle of the profile. It was column-shaped and had a width of approximately 110  m. The distance from the top to the seabed was approximately 15  m, which was the same as the disordered reflection area in the amplitude profile. In this area, the high-frequency components were lost, demonstrating low-frequency characteristics. The instan - taneous frequencies were 100–150 Hz, while those in the surrounding area were generally higher than 200 Hz. Figure  11 shows that the instantaneous frequencies of the entire profile were between 50 and 500  Hz, dem - onstrating a higher frequency in the upper part and a lower frequency in the lower part overall. There was an abnormal frequency area bounded by white lines in the middle of the profile. It was column-shaped with a width of approximately 40  m. The distance to the seabed was approximately 15  m, which was the same as the sunken area on the amplitude profile. In this area, the high-fre - quency components were lost, demonstrating low-fre- quency characteristics. The instantaneous frequencies Fig. 11 The acoustic profile in survey area profile2, and the seismic were 150–200  Hz, while those in the surrounding area attribute is instantaneous frequency for Fig. 7 were generally higher than 200 Hz. Findings phase bounded by green lines in the middle of the profile. Upon foregoing analysis, there were abnormal areas in It was column-shaped and had a width of approximately the amplitude, phase and frequency profiles with approx - 110  m. The distance from the top to the seabed was imately the same shapes and locations. According to the approximately 15 m, which was the same as the area with geological drilling data, there were mainly sand strata a disordered reflection area on the amplitude profile. In and a few clay strata at depths of 15–120 m beneath the this area, there was no insignificant layered structure; seabed, which were characterized by strong permeability. furthermore, compared with events in the surrounding However, there were mainly muddy clay strata at depths area, the events were significantly rougher. On its top, within 15  m beneath the seabed. Hence, the abnormal there was a staggered and reversed event phase. areas were reflections of shallow gas based on the char - Figure  9 shows that there was a depressed area of acteristics of reflective waves, such as a weak amplitude, events bounded by green lines in the profile. It was col - disordered reflection, phase reversion on the top, sunken umn-shaped with a width of approximately 40  m. The events, and loss of high-frequency components. For the distance from the top to the seabed was approximately acoustic stratigraphic profiles, the spatial sampling rate 15  m, which was the same as the sunken event area on dx = 2  m; the positioning accuracy of the sound source the amplitude profile. At the top of the area, although and hydrophone was 2  m. Based on the interpretation the events were sunken, they were still continuous; in accuracy and onsite measurement accuracy at the cross- the lower part of the area, the events gradually became over points, the accuracy in determining the range of noncontinuous, with a gradually unclear and disordered shallow gas was 3  m. The distribution characteristics of layered structure. On its top, there was a staggered and shallow gas in the survey area are shown in Fig. 12. reversed event phase. Figure  12 shows that the shallow gas was located at the end of a normal fault whose top burial depths were 31–57 m. According to the geological drilling data, there Characteristics of frequency was mainly muddy clay at a depth of 15 m below the sea- The instantaneous frequencies of the amplitude profiles bed, which was an overlying well cap. Based on the seis- in Figs.  6 and 7 were calculated, and the instantaneous mic data of the platform site in past years, shallow gas in frequency profiles are shown in Figs.  10 and 11, respec- the strata at a depth of 60  m surrounding the platform tively. Figure  10 shows that the instantaneous frequency Y ang et al. Geoenvironmental Disasters (2022) 9:9 Page 9 of 10 Huanghua Sea area in Bohai Bay revealed that there were abnormal areas in the seismic attribute profiles. Further - more, they were of the same position and demonstrated a disordered weak amplitude reflection, top phase rever - sion and loss of high-frequency components. Overall, there was shallow gas reflection in the abnormal areas. Therefore, shallow gas may be identified through the analysis of multiple seismic attributes of acoustic stratigraphic profiles, such as the amplitude, phase and frequency. Acknowledgements We thank Xu Hao at the CNPC Research Institute of Engineer Technology for his help in engineering geological borehole data analysis. Author contributions XY was responsible for most of the work of this paper, including data collec‑ tion, data processing, and results analysis. MC was responsible for the field engineering geophysical survey. XL and ZY was responsible for field engineer ‑ Fig. 12 Range of shallow gas distribution ing geological drilling. All authors read and approved the final manuscript. Funding CNPC Scientific research and technology development project ‑ Research on Key Technologies of offshore oil and gas pipeline design and construction appeared in the past 10 years; furthermore, the platform (2019B‑3010). was located in the fault zone of Yangerzhuang-Zhaojiapu in the Chengbei fault terrace. The long-term developed Availability of data and materials The geological borehole data were from actual borehole sampling in Zhaobei fault and Yangerzhuang fault are not only dis- Bohai Bay, which was a full coring borehole. The drilling coordinates were cordogenic faults but also form oil and gas transfer pas- 38°29′28.6651″N, 117°45′15.3138″E; the water depth was 3 m, and the drilling sages with unconformable planes. Therefore, the faults depth was 120 m. The dates of drilling were 2020/11/14–2020/11/25. The seis‑ mic data were from the actual geophysical survey in Bohai Bay, with a survey form oil and gas reservoirs on both sides and in uncon- area of 1 km × 1 km. The location of the survey was surrounded by 4 points: formable areas. The oil reservoir has a burial depth of 38°29′45.3961’’N, 117°44′54.5349″E; 38°29′45.1273″N, 117°45′36.2801″E; 990–1985 m, a porosity of more than 30%, and a perme- 38°29′12.1228″N, 117°45′35.9334″E; 38°29′12.3916″N, 117°44′54.1935″E. The –3 2 survey dates were 2020/10/14–2020/10/19. ability of 1200–1700 × 10  μm . Therefore, the abnormal areas on the profiles were Declarations reflections of shallow gas. However, a large-scale acous - tic blanket requires much gas to maintain its formation Competing interests (Gu et  al. 2009a, b). The shallow gas was likely from the The authors declare that they have no competing interests. in-depth strata and transferred upwards along faults. In Author details recent years, with increased gas pressure, the gas has 1 CNPC Research Institute of Engineering Technology, JinTang Road 40#, Binhai transferred upwards to a location 15 m beneath the sea- New District, Tianjin, China. CNPC Key Laboratory of Marine Engineering, JinTang Road 40#, Binhai New District, Tianjin, China. bed by breaking the resistance of strata in column form. When muddy clay was encountered at the seabed, the gas Received: 15 October 2021 Accepted: 2 April 2022 pressure was equivalent to the resistance of the stratum, forming column-shaped shallow gas in the shallow strata. References Conclusions Gu ZF, Liu HS, Li GF et al (2009a) Genesis of shallow gas in the western area of the South Yellow Sea. Nat Gas Ind 29:26–29 Based on drilling data, a stratigraphic model of shallow Gu ZF, Zhang ZX, Liu HS (2009b) Contrast between traps at the shallow sub‑ gas in the seabed was established for forward simulation bottom depth and the seismic reflection features of shallow gas. Mar analysis of shallow gas in the seabed. 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Acta Oceanogr 29:43–50 Publisher’s Note Springer Nature remains neutral with regard to jurisdictional claims in pub‑ lished maps and institutional affiliations.

Journal

Geoenvironmental Disasters – Springer Journals

Published: Apr 21, 2022

Keywords: Seabed shallow gas; Forward analysis; Instantaneous phase; Instantaneous frequency; Identification of shallow gas

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