Comparison of marine controlled-source electromagnetic data acquisition systems by a reservoir sensitivity index:analyzing the effect of water depths

During the past ten years, a marine controlled source electromagnetic（CSEM） method has been developed rapidly as a technology for hydrocarbon exploration. For shallow water environments, two CSEM data acquisition systems： Seabed Logging（SBL） and towed streamer electromagnetics（TSEM） have been developed in recent years. The purpose is to compare the performance of the SBL and TSEM systems at different water depths. Three different methods for the comparison are presented. The first method is a quick one dimensional sensitivity modelling. As a result, the sensitivity of marine CSEM data increases with water depth for the SBL system. Further, the sensitivity decreases with the increasing water depth for the TSEM system. The two other methods use two dimensional synthetic data from a simple 2-D isotropic model. The second method is a reservoir sensitivity index（RSI） method which has been developed to provide a quick comparison of the two systems. The RSI is calculated as the amplitude of the scattered field dividing by data uncertainty. From the calculations, it is found that with the increasing water depth RSI increases for the SBL system, while it decreases for the TSEM system. The third method uses Occam＇s inversion, and applies an anomaly transverse resistance（ATR） ratio for evaluating the resulting resistivity image. In shallow water environments, the resolution of the CSEM inversion results is good for both the SBL and TSEM systems. In deep water environments, the resolution of the CSEM inversion is better for the SBL system than for the TSEM system. The ATR ratios of the resistivity images show the similar conclusion. The SBL data acquisition system has an advantage in deep water environments. The TSEM system, on the other hand, is preferable for the shallow water environments.

Effects of Uncertainties in the Position and Orientation of Both the Transmitter and Receivers on Marine Controlled-Source Electromagnetic Data

Simulation and interpretation of marine controlled-source electromagnetic(CSEM) data often approximate the transmitter source as an ideal horizontal electric dipole(HED) and assume that the receivers are located on a flat seabed.Actually,however,the transmitter dipole source will be rotated,tilted and deviated from the survey profile due to ocean currents.And free-fall receivers may be also rotated to some arbitrary horizontal orientation and located on sloping seafloor.In this paper,we investigate the effects of uncertainties in the transmitter tilt,transmitter rotation and transmitter deviation from the survey profile as well as in the receiver's location and orientation on marine CSEM data.The model study shows that the uncertainties of all position and orientation parameters of both the transmitter and receivers can propagate into observed data uncertainties,but to a different extent.In interpreting marine data,field data uncertainties caused by the position and orientation uncertainties of both the transmitter and receivers need to be taken into account.

A marine gravimeter based on electromagnetic damping and its tests in the South China Sea

A new gravity sensor based on electromagnetic damping for the JMGrav marine gravimeter is presented.The new gravity sensor considered the advanced construction methods of the electromagnetic damping system of the gravimeter.The design features of the new system are discussed and the research survey data in the South China Sea are shown.Numerical simulations are applied to model the magnetic and mechanical characteristics of the system using finite element analysis and to evaluate the force distribution and the resulting damping effects.The performance characteristics of the system were tested on a motion simulator in laboratory,and the gravimeter was subjected to vertical accelerations of up to 100 Gal in 1-1000 s.It was found that the amplitude reduction of vertical accelerations in 3-15 s is 30-45 dB,with a time lag of 2-5 s,while the effect on gravity in period greater than 600 s is less than 0.5 dB,with a time lag of less than 100 s.The accelerations cause discrepancies of approximately only 1 mGal between the static value and the mean dynamic value.The sea tests were conducted in September 2020.Gravity measurements were taken with a JMGrav marine gravimeter onboard the R/V Dongfanghong 3,and the effective survey line exceeded 2000 km.Completely irregular accelerations with peaks up to 100 Gal yielded a reduction of approximately 40 dB in amplitude.The survey data were evaluated using ocean gravity field models and grid line tests.The results show that the accuracy of the gravity measurements is better than 2 mGal.

New correction method for controlled-source electromagnetics source effects

A novel method for source effect correction based on integral equation method is proposed.By taking the electrical horizontal field Ex of current source as an example,the correction method is validated using both simulated data and field data.The results show that the correction method is feasible and effective for isotropic media.When the field data are processed,the correction method normalizes the sources with different geological structures,which eliminates the geological difference among sources,and retains the geological difference among receivers.The correction results are in line with the expectation in whole.

Controlled-source electromagnetic data processing based on gray system theory and robust estimation

We propose a novel method that combines gray system theory and robust M-estimation method to suppress the interference in controlled-source electromagnetic data. We estimate the standard deviation of the data using a gray model because of the weak dependence of the gray system on data distribution and size. We combine the proposed and threshold method to identify and eliminate outliers. Robust M-estimation is applied to suppress the effect of the outliers and improve the accuracy. We treat the M-estimators of the preserved data as the true data. We use our method to reject the outliers in simulated signals containing noise to verify the feasibility of our proposed method. The processed values are observed to be approximate to the expected values with high accuracy. The maximum relative error is 3.6676%, whereas the minimum is 0.0251%. In processing field data, we observe that the proposed method eliminates outliers, minimizes the root-mean-square error, and improves the reliability of controlled-source electromagnetic data in follow-up processing and interpretation.

3D forward modeling and response analysis for marine CSEMs towed by two ships

A dual-ship-towed marine electromagnetic （EM） system is a new marine exploration technology recently being developed in China. Compared with traditional marine EM systems, the new system tows the transmitters and receivers using two ships, rendering it unnecessary to position EM receivers at the seafloor in advance. This makes the system more flexible, allowing for different configurations （e.g., in-line, broadside, and azimuthal and concentric scanning） that can produce more detailed underwater structural information. We develop a three-dimensional goal-oriented adaptive forward modeling method for the new marine EM system and analyze the responses for four survey configurations. Ocean-ottom topography has a strong effect on the marine EM responses; thus, we develop a forward modeling algorithm based on the finite-element method and unstructured grids. To satisfy the requirements for modeling the moving transmitters of a dual-ship-towed EM system, we use a single mesh for each of the transmitter locations. This mitigates the mesh complexity by refining the grids near the transmitters and minimizes the computational cost. To generate a rational mesh while maintaining the accuracy for single transmitter, we develop a goal-oriented adaptive method with separate mesh refinements for areas around the transmitting source and those far away. To test the modeling algorithm and accuracy, we compare the EM responses calculated by the proposed algorithm and semi-analytical results and from published sources. Furthermore, by analyzing the EM responses for four survey configurations, we are confirm that compared with traditional marine EM systems with only in-line array, a dual-ship-towed marine system can collect more data.

南海神狐海域天然气水合物及气源条件海洋可控源电磁成像

地震剖面的似海底反射(BSR)指示了天然气水合物的底边界,并已得到钻探的验证。南海神狐海域是天然气水合物(简称水合物)研究的热点区域。然而,由于地质构造的复杂性且钻井数量有限,仅仅依靠地震与钻探研究不能完全描述区内天然气水合物及气源通道的分布。含水合物地层比围岩具有更高的电阻率,海洋可控源电磁(MCSEM)数据获得的电阻率能为水合物和气源通道检测提供重要依据。本文在神狐海域开展MCSEM探测研究,沿一条测线采集了15个站位的海洋可控源电磁场数据,通过数据处理与二维反演,实现了研究区海底二维电阻率成像。然后,通过灵敏度分析,检验了MCSEM反演结果的可靠性。结合SH-W07-2016随钻测井(LWD)和地震数据,对二维电阻率剖面进行解释,推断了剖面上水合物和游离气分布,划分了两种形式的水合物储层结构,分析了其构造成因。

Sensitivity of Marine Controllable Source Electromagnetic Soundings for Identifying Plume Migration in Offshore CO_(2) Storage

Offshore carbon dioxide(CO_(2)) storage is an effective method for reducing greenhouse gas emissions. However, when using traditional seismic wave methods to monitor the migration of sequestration CO_(2) plumes, the characteristics of wave velocity changes tend to become insignificant beyond a certain limit. In contrast, the controllable source electromagnetic method(CSEM) remains highly sensitive to resistivity changes. By simulating different CO_(2) plume migration conditions, we established the relevant models and calculated the corresponding electric field response characteristic curves, allowing us to analyze the CSEM's ability to monitor CO_(2) plumes. We considered potential scenarios for the migration and diffusion of offshore CO_(2) storage, including various burial depths, vertical extension diffusion, lateral extension diffusion,multiple combinations of lateral intervals, and electric field components. We also obtained differences in resistivity inversion imaging obtained by CSEM to evaluate its feasibility in monitoring and to analyze all the electric field(Ex, Ey, and Ez) response characteristics. CSEM has great potential in monitoring CO_(2) plume migration in offshore saltwater reservoirs due to its high sensitivity and accuracy. Furthermore, changes in electromagnetic field response reflect the transport status of CO_(2) plumes, providing an important basis for monitoring and evaluating CO_(2)transport behavior during storage processes.

海洋电磁发射机可控源整流电路动态性能优化

海洋电磁发射机在非理想状况工作时母线会产生冲击电压,严重时损坏开关管和滤波电容,为此提出一种混合控制策略。首先,基于dq坐标系的前馈解耦控制策略分析海洋电磁发射机可控源整流电路的动态性能,其次,通过理论证明负载扰动和无功电流iq≠0影响控制系统的动态性能,然后提出在dq电流解耦控制中引入瞬态直接电流思想的改进q轴混合控制方法,在dq电流解耦控制中引入负载电流前馈和无功电流iq≠0环节,改善了d轴电流id的响应速度,提高了控制系统的动态性能。最后,实验结果对比表明,混合控制方法在负载突变、直流侧电压给定值突变和船上柴油发电机电压跌落3种情况下,直流侧电压超调差最小,与最长调整时间相比减少了65%左右。

基于混合优化算法的电磁监测裂缝参数识别

压裂实时监测是水力压裂效果评价和工程参数优化的重要保障措施之一。传统电磁监测裂缝参数识别方法的准确性和精度难以保证,影响了监测效果。为了提高裂缝参数识别能力,提出了一种基于海洋捕食者密度聚类混合优化算法的电磁监测裂缝参数识别方法。利用海洋捕食者算法(MPA)进行多次寻优,以每次寻优结果作为初始数据集,然后,利用密度聚类算法(DBSCAN)进行聚类,构建中间样本数据集,最后,抽取该样本数据中值作为最终输出结果。采用Rastrigin函数进行测试,分析混合优化算法寻优能力。测试结果表明,相对粒子群优化算法(PSO),MPA算法单次寻优效果较佳。但两种算法寻优结果均具有较强随机性,其中,PSO和MPA算法50次寻优精度分别为10^(-7)~10^(2)和10^(-10)~10^(-2),而改进的混合优化算法寻优效果更稳定,寻优精度达10^(-7)。构建缝长、方位压裂模型并进行了数值模拟实验,结果表明,在噪声低于15%时,缝长和方位识别平均绝对误差分别小于1 m和1°。利用改进的算法对四川盆地某井页岩气压裂电磁监测实测数据进行分析,确定了裂缝改造的长度(缝长)与方位。实例分析结果验证了改进算法的可行性和有效性。

单摆永磁直驱式波浪能发电装置的性能研究

为解决海洋仪器的长期能源供应问题,基于永磁材料的电磁特性,结合易于起振的摆式俘能机制,提出了一种与小体积球形浮标相结合的永磁式波浪能发电装置。详细论述了装置的发电原理和设计方案,推导了装置在波浪激励下的运动模型。依靠AQWA与Maxwell软件进行了联合仿真并对装置进行了参数优化,最终仿真结果表明:在波浪入射角度β=0°,波浪周期T=3.5s,波浪高度H=0.6m的波况下摆锤做周期T_(p)=2s,幅值A_(p)=15°的往复振荡运动,单相绕组产生的感应电动势最大可达27.7V,最佳输出功率为2.07W,可有效满足海洋仪器的供电要求。

Snubber Circuit for Marine Controlled-Source Electromagnetic Transmitter

A high-power marine controlled-source electromagnetic transmitter(HP-MCSET)transmits a highfrequency conversion current on the sea floor.Some problems exist when the direct-current to alternating-current(DC-AC)launch bridge(LB)is used in the marine controlled-source electromagnetic transmitter(MCSET).There is a high voltage peak in the LB when the insulated gate bipolar transistor(IGBT)is turned on and off.In some cases,the voltage stress of the IGBT can be exceeded,which may cause IGBT damage.Because the rise of the current steepness is relatively low and the output voltage has a voltage peak in the LB,a snubber circuit is added to the IGBT to suppress the voltage peak to improve the output current and voltage waveform.The suppression of the voltage peaks is analyzed and compared for several groups of snubber circuits.To meet the performance requirements of the MCSET,the optimal snubber circuit is selected to effectively suppress the voltage peaks at an output current of 1 kA.This method is verified by using a 70 kW MCSET and the experimental waveforms are provided.The simulation of the inductance obstruction load in seawater is necessary to determine the conditions for actual marine environment experiments.

海洋可控源电磁法与地震全波形二维联合反演研究

为降低单一地球物理反演方法的局限性以及反演中所存在的多解性等问题,开展了海洋可控源电磁法(MCSEM)与地震全波形的二维联合反演研究。MCSEM采用数据空间OCCAM反演算法,地震全波形采用梯度法反演算法,引入交叉梯度函数实现两种物性参数结果的相互耦合;开发出一套二维联合反演算法,并通过3组理论模型算例验证了算法的准确性。研究结果表明:MCSEM联合反演结果相对于单方法反演结果有显著改善和提升,主要体现在异常体的形态刻画、结构构造以及物性数值的恢复,这说明全波形的反演方法能够提高MCSEM反演结果的可靠性。

起伏地形的2.5维主轴各向异性海洋CSEM反演研究

笔者利用OCCAM方法实现了MPI并行的2.5维海洋CSEM反演算法。正演采用基于非结构网格的有限元方法,在精确模拟海底起伏地形和构造模型的同时,对核心区域和扩展区域进行不同尺度的剖分以减少反演参数。水平海床和起伏海床的理论模型算例表明,沿测线发射的2.5维剖面数据对垂直各向异性模型的恢复效果优于水平各向异性模型。复杂海底地形模型的试算结果表明,带地形沉积层的存在会对反演结果产生较大影响,在实际资料解释中应该引起重视。

磁流体推进器优化设计及对比研究

为降低船舶推进噪声,提升船舶隐蔽性能,本文针对主要的磁流体推进器拓扑进行了优化设计与对比研究,对这几种磁流体推进器的拓扑结构分别进行参数化建模,建立对应的解析模型。基于解析模型计算每种磁流体推进器的性能参数,全面分析了结构参数和激励特性对于磁流体推进器性能的影响性。并且基于结构参数与激励特性的分析,在一定的约束内完成各类拓扑结构的优化设计。根据每种磁流体推进器性能最优的解重新建立模型,考虑实际应用时的场景,对比分析这些磁流体推进器的性能特点,并指出各类的磁流体推进器类型提升性能的方向。

船舶甲板机械电动机的应用与适应性设计制造

主要讲述船舶甲板机械电动机的应用和设计特点,在电磁设计,材料选择,适应性设计,制造工艺的控制和电机的防水设计方面分别进行论述。说明船舶甲板机械电动机需要注意的特殊设计要点。

Precision of meshfree methods and application to forward modeling of two-dimensional electromagnetic sources

Meshfree method offers high accuracy and computational capability and constructs the shape function without relying on predefined elements. We comparatively analyze the global weak form meshfree methods, such as element-free Galerkin method （EFGM）, the point interpolation method （PIM）, and the radial point interpolation method （RPIM）. Taking two dimensional Poisson equation as an example, we discuss the support-domain dimensionless size, the field nodes, and background element settings with respect to their effect on calculation accuracy of the meshfree method. RPIM and EFGM are applied to controlled- source two-dimensional electromagnetic modeling with fixed shape parameters. The accuracy of boundary conditions imposed directly and by a penalty function are discussed in the case of forward modeling of two-dimensional magnetotellurics in a homogeneous medium model. The coupling algorithm of EFG-PIM and EFG-RPIM are generated by integrating the PIM or RPIM and EFGM. The results of the numerical modeling suggest the following. First, the proposed meshfree method and corresponding coupled methods are well-suited for electromagnetic numerical modeling. The accuracy of the algorithm is the highest when the support-domain dimensionless size is 1.0 and the distribution of field nodes is consistent with the nodes of background elements. Second, the accuracy of PIM and RPIM are lower than that of EFGM for the Poisson equation but higher than EFGM for the homogeneous medium MT response. Third, RPIM overcomes the matrix inversion problem of PIM and has a wider selection of support-domain dimensionless sizes as compared to RPIM.

Investigation of the oil-seawater mixed flow under an electromagnetic field

The electromagnetic separation method is a new approach to treat ship-based marine oily wastewater,in which oil droplets are dispersed in seawater(oil-seawater mixed flow).In order to clarify the separation process and determine the separation characteristics,the flow field and volume fraction of the oil droplets of the oil-seawater mixed flow under an applied electromagnetic field with different operating conditions were investigated by 2D numerical simulations with the Eulerian model.The results show that:(1)the downward Lorentz force causes seawater to flow downwards and the oil droplets to move upwards due to the electromagnetic separation force in the effective section of the separation channel;(2)the volume fraction of the oil droplets at the top of the outlet section increases with the current density,magnetic field,and the diameter of the oil droplet and decreases with the inlet velocity of the oily seawater.The results provide useful guidance for the design of electromagnetic separation devices of the oil-seawater mixed flow.

Electromagnetic fields from a horizontal electrical dipole buried in ocean

Marine controlled source electromagnetic signal could be used in mineral resource exploration,reservoir appraisal and communicative technique in ocean. It's necessary to study the electromagnetic generated by MCSEM. The propagation of the electromagnetic fields from a controlled source in the marine environment was studied with virtual interface method combined with discrete complex image method. Transmitter of finite length current source is approximated by dipole (HED) . A three-layered model is accepted,with sea water as intermediate conductive layer under air and a relatively high resistive seabed as basement,possibly containing a hydrogen layer of higher resistivity. The electromagnetic fields in whole space thus computed show that: (1) the spatial distribution of field component depends on its type; (2) inline Ex component is more sensitive to reservoir layer than that in broadside; (3) The airwave affects marine electromagnetic (MEM) exploration when sea water is relatively shallow; in the case of deep water MEM exploration,the airwave influence could be neglected; and (4) an appropriate frequency should be selected in order to balance the signal strength and electromagnetic induction effect.

一种MCSEM数据噪声压制方法

针对海洋可控源电磁(MCSEM)信号在勘探中极易受各种噪声干扰,影响后期反演以及数据处理准确性的问题,提出一种注意力机制引导的卷积自编码器海洋可控源电磁数据消噪方法.首先基于自编码器,构建基于卷积自编码器的海洋可控源电磁数据消噪网络,然后根据数据中存在噪声的特点对其进行优化,加深网络深度、引入注意力机制,使网络能更关注数据中的有效信号特征,增强特征提取能力,构建网络模型,实现对海洋可控源电磁数据噪声的压制.实验结果表明,在对海洋可控源电磁数据噪声压制中,该方法比db8小波消噪方法和变分模态分解消噪方法信噪比更高、均方误差更低,同时应用到实测数据中仍能较完整地保留信号特征并增加偏移距的可解释范围,证明了该方法在海洋可控源电磁数据噪声压制中的有效性.