(Guangzhou Marine Geological Survey Guangzhou 5 10760)

About the author: Ma, male, born in 1968, 1990, graduated from China Geo University (Wuhan) with a master's degree in engineering, and is a senior engineer engaged in marine environmental geology, disaster geology and comprehensive geology and geophysics. E-mail: SZ-m @163.com.

The safe positioning and stable construction of offshore oil drilling platform are closely related to the engineering geological conditions of seabed in the well site area. Geophysical exploration technology, as a comprehensive science and technology, plays an irreplaceable role in marine engineering geology and marine disaster geological investigation. Practice has proved that the comprehensive investigation of geophysical exploration means such as sounding, side-scan sonar scanning, shallow stratum profile, single-channel earthquake, high-resolution 2D earthquake and marine magnetic survey can reveal the topographic changes and potential geological disaster factors in the sea area around the drilling platform site.

Field investigation of the platform; Marine geophysical exploration; Marine geological disasters

1 preface

With the development of China's economy and the need of strategic reserve, the focus of crude oil exploration and development in China has gradually shifted from land to sea. China offshore is rich in seabed mineral resources, with proven oil resources reaching 246× 108 t and natural gas15.79×10/2m3, accounting for 23% of the total oil and gas resources in China. However, in oil and gas development, it has been repeatedly damaged by marine geological disasters, and the uneven bearing stratum has repeatedly caused the drilling platforms in Bohai Sea and Pearl River Mouth Basin to tilt and shift, which has caused huge economic losses to the country.

The disaster investigation of drilling platform site should be carried out before oil drilling. It is necessary to detect faults and shallow gas reservoirs in order to deal with disasters such as derrick collapse, blowout, fire and oil spill during drilling or oil production, and to investigate geotechnical engineering problems related to drilling platform foundation to avoid accidents and disasters. According to the data, during the period of 1955 ~ 1980, there were 3 ~ 4 accidents that seriously damaged the foundation of drilling ships in the United States every year, resulting in huge economic losses and casualties. Site investigation of marine structures is to determine the engineering geological conditions that affect the design, layout, construction and safe operation of engineering structures such as fixed platforms and submarine pipelines. 1969, Hurricane Camille hit the Mississippi River Delta, causing a large area of soil to slide on the seabed, causing damage to three platforms, and the loss exceeded 1 billion dollars [1]. It can be seen that the field investigation of offshore oil drilling platform plays an important role in oil well drilling and development. The development of offshore oil in China started late, and it was not until the early 1980s that China really started offshore engineering geological exploration. In the past ten years, we have done a lot of experimental work in the field investigation of oil drilling platforms, and with the continuous progress of investigation technology, the research is advancing to the deep sea.

The design and construction of offshore platform need to carry out marine engineering geological survey on the platform site, including seabed topography, seabed surface layer and shallow stratum structure. From the aspects of topography, sediment characteristics, geological age, etc., the seabed stability is analyzed and calculated by using the measured and platform design marine hydrological data and the physical and mechanical parameters of the soil in the site, and the seabed stability of the site is evaluated on the basis of analysis and research.

2 Geological types of common marine disasters

Common geological types of marine disasters [2-5] are as follows:

Active faults, earthquakes and volcanoes, etc. They may not only cause direct damage to the seabed structure, but also induce disasters such as landslide, turbidity current and sand liquefaction.

Landslide, collapse, turbidity current and debris flow, etc. Their activities may directly damage drilling platforms and submarine pipelines.

Submarine sand dunes, submarine sand waves, tidal sand ridges, scouring troughs, concave-convex lands and shallow valleys are geomorphological disasters, and their distribution is related to meteorological and hydrological conditions.

Shallow gas, mud diapir, weak interlayer, liquefiable sand layer, etc. They exist in Quaternary shallow strata in the form of confined fluids and plastic bodies. When the foundation of submarine structure comes into contact with these geological bodies, disasters may occur.

Buried ancient rivers, buried ancient lakes and swamps, buried undulating bedrock surface, buried coral reefs, etc. Generally, it is a lens in shallow stratum. When the pile foot of drilling platform is inserted into different geological bodies, the platform will be skewed or even overturned due to uneven stress.

Study on the application of geophysical methods in platform site investigation

3. 1 seabed topography detection

Submarine topography detection includes single-beam sounding, multi-beam sounding and lateral sonar. It is a geophysical method to study the nature and integrity of rock and soil by detecting the propagation characteristics of sound waves in underwater or rock and soil media, but the frequency and intensity of sound waves used by the two are different. High frequency can improve the resolution, while low frequency can improve the range and penetration depth of sound waves [6 ~ 9]. At present, many detection systems adopt dual-frequency or multi-frequency probe structure to improve the detection ability of instruments.

3. 1. 1 single beam detection and multi-beam detection

The single-beam sounding system uses its transducer to emit an acoustic pulse from the water surface to the seabed, and the acoustic wave is reflected at the underwater interface and then returned to the transducer to be received. Through the transformation of time function, a group of time-discrete digital quantities are formed and processed in real time, and the continuous undulating seabed profile on the measuring line is directly displayed on the recording paper. It reflects the convex and concave nature, height difference and extension range (development scale) of the seabed surface morphology.

Multi-beam bathymetric system is a complex system composed of multiple sensors, which can form dozens to hundreds of strip bathymetric data and hundreds or even thousands of backscattering data on the survey profile, and can obtain wide seabed scanning and high density survey points. It is characterized by full coverage, high precision, high density and high efficiency. The bathymetric data reflect the fluctuation, elevation difference and extension of the seabed surface, and the topographic map of the seabed in the measured sea area can be made by computer processing and drawing technology.

3. 1.2 side scan sonar scanning

Side-scan sonar technology uses the principle of backscattering of incident sound waves by underwater objects to detect seabed morphology, which can directly provide active acoustic imaging. Side sonar is a kind of high-resolution and multi-purpose underwater acoustic equipment, which is widely used in ocean mapping and underwater target detection (such as detecting underwater ships, planes, missiles, torpedoes and mines). ), delimitation of continental shelf and marine exclusive economic zone, marine geology, marine engineering, port construction and waterway dredging.

Side-scan sonar adopts deep towed side-scan sonar system, with dual-frequency frequency 100/500 kHz, measuring range 100/200 m and towed body distance 10 ~ 30 m, which can obtain various target detection objects on the seabed surface, and the obtained sonar images are of high quality, and can distinguish pipelines and cables on the seabed surface. The height of an underwater object can be determined according to the height of the object. The synchronous operation of several geophysical methods can be mutually verified (Figure 1).

Figure 1 Geological types of disasters shown by side-scan sonar and single-channel seismic profile

3.2 Shallow detection

3.2. 1 shallow stratum profile measurement

Shallow stratum profile measurement system is one of the important methods to detect shallow structure, seabed sedimentary characteristics and seabed surface mineral distribution within 30 m below the seabed. The transmission frequency of shallow stratum profile system is low, generally between 2.5 and 23 kHz, and the energy of sound wave generated by electric pulse is large, and the transmitted sound wave has strong penetrating power, which can effectively penetrate tens of meters of stratum on the seabed [10 ~1], and the stratum resolution is above 8 cm. It can provide vertical profile information of the stratum directly below the survey ship, and accurately reflect the stratum interface and possible submarine geological disaster factors, such as shallow gas, shallow faults, ancient rivers or other objects (such as pipelines). The penetration depth of shallow profiler varies with working frequency and seabed sediment type.

The shallow stratum profile measurement system adopts SES-96 parameter shallow profile system of INNOMAR Company in Germany, and is connected with surge compensation system, which can output water depth data. The transmission power is 18 kw, the main frequency is 100 kHz, and the difference frequency is 4 ~ 12 kHz. The difference frequency of 8 kHz is generally used in platform site investigation, and the detected strata have high resolution. Shallow water can detect pipelines and can be mutually verified with magnetic survey.

3.2.2 Single channel seismic profile measurement

Single-channel seismic recording system consists of single-channel data acquisition and processing system, seismic source system, signal receiving cable and EPC recorder. It is mainly used to understand the structural and sedimentary characteristics of the middle and shallow layers within 200 meters below the seabed.

The working principle of single channel seismic exploration technology and oil and gas seismic exploration technology is the same. Single-channel seismic exploration uses small source energy, wide frequency band (tens of Hz to thousands of Hz) and high main frequency (hundreds of Hz to thousands of Hz). Generally, electric sparks and air guns are used as seismic sources, with energy ranging from tens of joules to thousands of joules and penetration depth ranging from tens of meters to hundreds of meters.

The most commonly used seismic sources at sea are air guns and electric spark sources, and electric spark sources are generally used for on-site investigation of platforms. The seismic source system consists of a seismic source control box and sound source equipment (electrodes and acoustic pulse generators).

For example, the British CSP 1500 seismic source system mainly includes CSP 1500 seismic source control box, SQUID500 electrode, SQUID2000 electrode or AA200 BOOMER, and the excitation level of the seismic source is 100 ~ 1500J, so the time required for repeated excitation is short. The French SIG800J seismic source system adopts 120 or 200 fishbone EDM electrodes, and the energy output is 270J, 540 J and 800J. In the platform investigation, the excitation energy of 250~800J is generally selected, and the excitation interval is 0.5 s (Figure 2). In the GEO-SPARK 10kJ seismic source system in the Netherlands, the energy output of GEO-SPARK2×800 electrode is between100 ~100000J, with the maximum working depth of 4500 m and the maximum penetration depth of 750 ms, which can meet the needs of deep water well field investigation.

We choose French hydrophones SIG 16 4.8. 12 and SIG1612.12.34, British AAE20 single channel signal receiving cable and Dutch GEO-Sense signal receiving cable, and the detector is 0.15.

The recording instrument is matched with the seismic source and hydrophone through delph seismic data acquisition system. The system can not only actively control the excitation times of the seismic source per second, but also record the longitude and latitude coordinates of each shot trajectory at any time through the connection with GPS navigation system, which is convenient for accurate positioning. The dynamic range of the instrument is 90db, the analog-to-digital conversion is 16 bits, and it has a high sampling frequency. When used with BOOMER seismic source, its sampling rate is as high as 6000 ~ 10000 Hz, which is more conducive to the reception of high-frequency effective signals. In the process of single-channel seismic data acquisition at sea, the distance between traces can be adjusted by controlling the speed of the survey ship. The slower the ship speed and the smaller the channel spacing, the better the continuity of seismic wave group. When the seismic source is excited twice per second, the measured hull sails at a speed of 3.5 knots, and the seismic trace spacing is less than 1 m, which shows that this method is more suitable for high-precision shallow seismic exploration.

In the data processing flow, effective methods and techniques are adopted to separate signals from noise, which can weaken the influence of interference waves such as multiples and diffraction, and further improve the signal-to-noise ratio and resolution of single-channel seismic records. Figure 3 (left) clearly shows the shallow gas and its rising along the fault. The reflected wave circled by the red ellipse has strong amplitude and the reflected in-phase axis is reversed, which has obvious anti-phase characteristics. Figure 3 (right) shows various forms of buried ancient rivers.

Fig. 2 Single channel seismic profile

Shallow gas and buried ancient river channel shown by single seismic profile.

3.3 High-resolution 2D multi-channel seismic profile measurement

Usually, 48 or 96 multi-channel seismic cables are used to collect high-resolution 2D seismic data. In order to avoid the suppression of high-frequency components by ghost reflection, the sinking depth of seismic source and geophone cable is relatively shallow, generally 3m for seismic source and 4 m for cable. The seismic source is generally a small-capacity GI air gun source or a sleeve gun combination source to ensure the generation of high-frequency seismic wavelets. The frequency band of seismic data collected by this method can reach 20 ~ 350 Hz, which is much higher than that of conventional seismic data (20 ~ 50 Hz), which can fully meet the needs of identifying thin layers and stratigraphic structures and improve the accuracy.

3.4 Marine magnetic survey

Magnetic method is a detection method to find useful minerals, find out underground structures and solve other geological problems by using magnetic field changes (magnetic anomalies) caused by magnetic differences between underground rocks and minerals or geotechnical media. Magnetic force is an effective means to solve the problem of detecting magnetic objects in engineering geological exploration. In various investigations, we use GS880 cesium optical pump magnetometer and SeaSPY marine magnetometer, and adopt different investigation methods for different research purposes, all of which can achieve satisfactory results. Its advantage is that it can not only detect magnetic anomalies exposed to the seabed, but also be effective for magnetic anomalies covered below the seabed.

In the application of this survey, submarine cables have been laid (including submarine communication cables, power cables and optical cables, etc.). ) in the sea area where submarine optical cables are routed. After years of changes, the coordinates of these submarine cables in this sea area have changed, and it is not clear whether some of them still exist. In addition, when laying submarine cables in the past, there was a big error in the locator. In order to find out the exact location where the submarine cable meets the optical cable route, it is necessary to detect the optical cable route. In the platform site investigation, the marine magnetometer produced by MarineMagnetics Company of Canada was used for investigation, combined with lateral sonar and shallow profile for detection. Fig. 4 shows the curve of magnetic anomaly detected when the magnetometer probe passes through the cable. Generally, the amplitude of magnetic anomaly of the cable and platform scanned by the side sonar can reach tens to hundreds of nT.

Fig. 4 Pipelines, cables and production platforms detected by shallow profile, magnetic force and side scan sonar.

4 Conclusion and discussion

There are two main methods for geological survey of platform site: one is geophysical method, and the other is geological sampling method. At present, the widely used geophysical methods are: single-beam sounding or multi-beam sounding, side-scan sonar, shallow profile detection, single-channel seismic, high-resolution 2D seismic and magnetic survey. With the support of high-precision positioning system, the above six underwater detection systems can enable us to obtain three-dimensional engineering geological conditions in the platform site, especially the shape, scale, location and development trend of various disaster geological phenomena that endanger engineering construction. Its advantages are economy and rapidity, and each geophysical exploration method has its own advantages and limitations in solving a certain geological problem. Therefore, according to the purpose and requirements of the investigation, a variety of methods should be used to conduct a comprehensive investigation, so that the advantages of each method can complement each other and achieve the best results. Based on more than 20 years' practical experience, using high-resolution seismic comprehensive shallow geophysical exploration technology, 2 ~ 3 m long geological gravity sampling and shallow geological drilling are simultaneously carried out at the well site and the predicted anchor position. The combination of geophysical exploration and geological sampling is an effective investigation method to understand marine geological disaster factors, disaster types and marine engineering geology, which can provide information to the owner economically and quickly.

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Marine geophysical survey technology and its application in well site survey

Ma Zhongsheng

(Guangzhou Marine Geological Survey, Guangzhou, 5 10760)

Abstract: The safety of offshore oil drilling platform is closely related to the seabed engineering geological conditions of the well site. Geophysical technology plays an irreplaceable role in marine engineering and disaster geological investigation. Practice has proved that comprehensive investigation with geophysical instruments such as echo sounder, side-scan sonar, shallow stratum profiler, single channel earthquake, high-resolution 2D earthquake and marine magnetometer can effectively identify the topography and potential geological disasters in the well site area.

Key words: well site survey, marine geophysical survey and submarine geological disasters