海底大地电磁数据采集器

进行海底大地电磁数据采集，需借助于专用的水下探测设备 . 虽然大地电磁测深法无论是仪器或是测量手段在陆地上的应用已较为成熟，但把该方法移 植到海洋中，还需解决一些与海洋探测有关的工程技术问题. 主要包括：水下电场信号的提 取，海底环境的监测，仪器在非实时监控运行中的纠错对策以及测量部件的密封承压等等 . 在研制海底大地电磁数据采集器的过程中，采用先进的材料工艺，高精度的电子线路，可 靠的软件编程以及牢固的器件安装结构，有效地解决了上述技术问题. 用所研制的仪器首次 在我国海域进行了探测试验.     

The marine controlled source electromagnetic response of a steel borehole casing: applications for the NEPTUNE Canada gas hydrate observatory

Gas hydrates are a potential energy resource, a possible factor in climate change and an exploration geohazard. The University of Toronto has deployed a permanent seafloor time‐domain controlled source electromagnetic (CSEM) system offshore Vancouver Island, within the framework of the NEPTUNE Canada underwater cabled observatory. Hydrates are known to be present in the area and due to their electrically resistive nature can be monitored by 5 permanent electric field receivers. However, two cased boreholes may be drilled near the CSEM site in the near future. To understand any potential distortions of the electric fields due to the metal, we model the marine electromagnetic response of a conductive steel borehole casing. First, we consider the commonly used canonical model consisting of a 100 Ωm, 100 m thick resistive hydrocarbon layer embedded at a depth of 1000 m in a 1 Ωm conductive host medium, with the addition of a typical steel production casing extending from the seafloor to the resistive zone. Results show that in both the frequency and time domains the distortion produced by the casing occurs at smaller transmitter‐receiver offsets than the offsets required to detect the resistive layer. Second, we consider the experimentally determined model of the offshore Vancouver Island hydrate zone, consisting of a 5.5 Ωm, 36 m thick hydrate layer overlying a 0.7 Ωm sedimentary half‐space, with the addition of two borehole casings extending 300 m into the seafloor. In this case, results show that the distortion produced by casings located within a 100 m safety zone of the CSEM system will be measured at 4 of the 5 receivers. We conclude that the boreholes must be positioned at least 200 m away from the CSEM array so as to minimize the effects of the casings.     

A new marine controlled‐source electromagnetic receiver with an acoustic telemetry modem and arm‐folding mechanism

We developed a new marine controlled‐source electromagnetic receiver for detecting methane hydrate zones and oil and gas reservoirs on the seafloor, which is not imaged well by seismic reflection surveys. To determine the seafloor structure, the electromagnetic receiver should have low noise, power consumption, clock drift error, and operating costs while being highly reliable. Because no suitable receiver was available in our laboratory, we developed a new marine controlled‐source electromagnetic receiver with these characteristics; the receiver is equipped with acoustic telemetry modem and an arm‐folding mechanism to facilitate deployment and recovering operations. To demonstrate the applicability of our new receiver, we carried out a field experiment offshore of Guangzhou in the South China Sea, where methane hydrates have been discovered. We successfully obtained controlled‐source electromagnetic data along a profile about 13 km long. All six new receivers were recovered, and high‐quality electromagnetic data were obtained. Relatively high apparent resistivity values were detected. The results of the offshore field experiment support the claim that the electromagnetic data obtained using the new receiver are of sufficient quality for the survey target.     

A first application of a marine inductive source electromagnetic configuration with remote electric dipole receivers: Palinuro Seamount,Tyrrhenian Sea

We study a new marine electromagnetic configuration that consists of a ship‐towed inductive source transmitter and a series of remote electric dipole receivers placed on the seafloor. The approach was tested at the Palinuro Seamount in the southern Tyrrhenian Sea, at a site where massive sulphide mineralization has been previously identified by shallow drilling. A 3D model of the Palinuro study area was created using bathymetry data, and forward modelling of the electric field diffusion was carried out using a finite volume method. These numerical results suggest that the remote receivers can theoretically detect a block of shallowly buried conductive material at up to ～100 m away when the transmitter is located directly above the target. We also compared the sensitivity of the method using either a horizontal loop transmitter or a vertical loop transmitter and found that when either transmitter is located directly above the mineralized zone, the vertical loop transmitter has sensitivity to the target at a farther distance than the horizontal loop transmitter in the broadside direction by a few tens of metres. Furthermore, the vertical loop transmitter is more effective at distinguishing the seafloor conductivity structure when the vertical separation between transmitter and receiver is large due to the bathymetry. As a horizontal transmitter is logistically easier to deploy, we conducted a first test of the method with a horizontal transmitter. Apparent conductivities are calculated from the electric field transients recorded at the remote receivers. The analysis indicates higher apparent seafloor conductivities when the transmitter is located near the mineralized zone. Forward modelling suggests that the best match to the apparent conductivity data is obtained when the mineralized zone is extended southward by 40 m beyond the zone of previous drilling. Our results demonstrate that the method adds value to the exploration and characterization of seafloor massive sulphide deposits.     

我国海底大地电磁探测技术研究的进展

几年以前 ,我国尚未独立开展过海洋电磁探测 ,没有获取海洋岩石层电性资料的技术手段。然而 ,地下岩层的电性参数较之其他物性参数能更好地反映岩石的性质以及岩石所处的物理状态 ,因此迫切需要发展海底大地电磁探测技术。海底大地电磁探测是把仪器布置在海底 ,采集海底大地电磁场数据 ,从而研究海底以下不同深度上介质导电性的分布规律 ,达到了解地下不同深度地质情况的目的。经过两年的努力 ,我们已基本实现了海底大地电磁探测技术的研究目标     

一维电阻率各向异性对海洋可控源电磁响应的影响研究

地下介质的电阻率常常表现为各向异性, 海底裂隙地层和层状沉积序列可能形成宏观电阻率各向异性.在解释海洋电磁资料时, 电阻率各向异性的影响不应该被忽略, 否则可能会得到错误的海底地电模型.作者编写了电阻率任意各向异性一维层状介质海洋可控源电磁场计算程序, 计算了电阻率各向异性层状模型的海洋可控源电磁响应, 讨论了覆盖层和高阻储层分别具有电阻率各向异性时的电磁场响应特征.     

基于改进的接收点插值算法的频率域海洋可控源电磁法2.5维正演

在海洋可控源电磁法勘探中,接收站常置于海底.在进行海洋电磁场模拟时,由于海水和海底介质存在显著电性差异,这给海底接收点处场值的求取带来困难.本文提出一种新的接收点插值算法,该算法考虑到海底电场法向分量不连续性问题,用法向电流分量进行插值以准确求取海底任意接收点处电磁场值.本文利用交错网格有限差分法实现了二维介质中频率域海洋可控源法（CSEM）正演.对构造走向做傅里叶变换,将三维电磁模拟问题转换为波数域2.5维问题,即三维场源激励下针对二维地电模型的电磁模拟问题.使用交错网格有限差分法,基于一次场/二次场分离方法导出波数域二次电场离散形式,并进一步求得波数域电磁场.采用本文提出的改进的插值算法可求得海底任意接收点处波数域电磁场,采用傅里叶逆变换对波数域电磁场进行积分可得到接收点处空间域电磁场.模型算例表明,与常规的线性插值和严格插值算法相比,本文提出的改进的插值算法具有更高的精度.

     

深水大地电磁数据采集的若干理论要点与仪器技术

深水环境下进行大地电磁数据采集,信号幅值微弱,高频分量被严重衰减;仪器承载的环境压力巨大,且伴随底流推曳、淤泥吸附等不利因素;海上作业情况复杂多变,带给设备投放与回收诸多困难.针对这些特殊问题,从理论上分析了大地电磁场在海水及其以下层状介质中的传播规律,计算了电场与磁场随不同水深的衰变比值,研讨了与仪器上浮速度相关的物理因素;从技术上阐述海底大地电磁仪的工作原理,包括信号传感器、数据采集器、声控释放单元以及机械组装部件等等.整套仪器信号分辨率达到nV级,频带宽度为10~0.3×10-3 Hz,最大工作水深4000 m.该仪器搭载“海洋六号”科学考察船,在我国南海中沙群岛海域进行了国内首次深水环境下的大地电磁数据采集试验.结果显示,所研发的仪器性能指标吻合先前的理论推算值,达到设计要求.我国的海底大地电磁探测技术已完全享有自主的知识产权.     

海底油气藏及天然气水合物的时频电磁辨识

提出了伪随机多频海洋电磁法观测方案.采用伪随机多频信号作为激励场源,多偏移距同线偶极-偶极同时观测,相关辨识海底地电系统的频率特性和冲激响应,可以在时间域和频率域同时辨识海底高阻薄层.在时间域,瞬变冲激时刻可以直接指示海底地层电导率的变化;在频率域,利用多个频率的电场响应计算的频散率及其道闻变化量,相对相位道间变化量对高阻薄层有很好的反映.从而实现对海底油气及天然气水合物的多参数辨识.     

海洋拖曳式水平电偶源数值模拟与电场接收机研制

海洋天然气水合物资源调查中,电磁法已成为地震勘探最有效的补充手段之一,而我国海洋水合物电磁探测中的拖曳式电场接收机此前是空白.本文从方法原理、仪器设备和海洋试验等方面,对海洋拖曳式水平电偶极-偶极方法进行研讨.建立三维电阻率模型,进行数值模拟,讨论收发距、发射频率、拖曳装置离海底高度等参数与电场异常响应的关系,针对特定模型给出了最佳观测参数以指导仪器硬件设计与海洋测试.在此基础上,采用高级嵌入式计算机的智能化控制、前端低噪声斩波放大、高精度同步采集等技术,研制了由拖体、承压舱、采集电路、低噪声电场传感器等组成的海洋拖曳式水平轴向电场接收机,其系统噪声达到2 nV/√HZ@(1~100Hz).将自主研制的电场接收机用于国内首次拖曳式电偶极-偶极方法的海洋试验与评估,并对海试资料进行了处理与分析.研究结果表明,所研发的仪器达到了设计指标,能够用于海洋天然气水合物调查.     

海底可控源电磁接收机及其水合物勘查应用

海洋可控源电磁法在国外已成为海底天然气水合物调查的有效手段之一.为实现我国海域深水条件下水合物的海洋可控源电磁探测,本文从方法原理出发,采用低功耗嵌入式控制、前端低噪声斩波放大、高精度时间同步和水声通讯等技术,设计并开发了由承压舱、玻璃浮球、采集电路、电场与磁场传感器、姿态测量装置、声学释放器、USBL定位信标、测量臂、水泥块等部件组成的海底可控源电磁接收机,实现了海洋微弱电磁场信号的高精度采集.海底可控源电磁接收机具有高可靠性、低噪声、低功耗和低时漂的特点.利用研制的海底可控源电磁接收机,在琼东南海域进行水合物勘查,采集得到了可靠的人工源电磁场数据.通过数据处理及反演,获得了研究区海底的电阻率模型,结合地震资料,对高阻异常体进行推断解释,其结果为天然气水合物钻探井位布置提供了电性依据.

     

基于非结构有限元的时间域海洋电磁三维反演

海洋电磁法是一种有效的钻前储层评价手段,可识别出海底构造是否储油气,从而减少干井率降低勘探成本.近年来,频率域三维海洋电磁反演得到了快速发展,但受到空气波的影响,其在浅海环境中无法取得很好的效果.为解决这一问题,本文研究时间域海洋电磁数据三维正反演.正演模拟和计算中,我们选择基于非结构网格的矢量有限元方法.该算法中空间离散采用非结构四面体网格,可很好地拟合复杂海底地形条件和地下复杂结构;而对于时间离散,我们采用无条件稳定的后推欧拉方法,以确保任意时间步长数值计算的稳定性.反演计算中,灵敏度信息采用伴随正演隐式进行计算;同时,依据时间域反演方法的特点采用L-BFGS方法计算模型修正量.通过利用合成数据反演结果证明了本文提出的三维时间域反演方法可用于复杂海底环境,特别是在浅海环境下的有效性.

     

Two types of marine controlled source electromagnetic transmitters

Marine controlled source electromagnetic methods are used to derive the electrical properties of a wide range of sub‐seafloor targets, including gas hydrate reservoirs. In most marine controlled source electromagnetic surveys, the deep‐tow transmitter is used with a long horizontal electric dipole being towed above the seafloor, which is capable of transmitting dipole moments in the order of up to several thousand ampere‐metres. The newly developed deployed transmitter uses two horizontal orthogonal electrical dipoles and can land on the seafloor. It can transmit higher frequency electromagnetic signals, can provide accurate transmission orientation, and can obtain higher signal stacking, which compensates for the shorter source dipole length. In this paper, we present the study, key technologies, and implementation details of two new marine controlled source electromagnetic transmitters (the deep‐tow transmitter and the deployed transmitter). We also present the results of a marine controlled source electromagnetic experiment conducted from April to May 2014 in the South China Sea using both the deep‐tow transmitter and the deployed transmitter, which show that the two types of marine transmitters can be used as effective source for gas hydrate exploration.     

A numerical comparison of time and frequency‐domain marine electromagnetic methods for hydrocarbon exploration in shallow water

In shallow water the frequency domain controlled source electromagnetic method is subject to airwave saturation that strongly limits the sensitivity to resistive hydrocarbon targets at depth. It has been suggested that time‐domain CSEM may offer an improved sensitivity and resolution of these deep targets in the presence of the airwave. In order to examine and test these claims, this work presents a side‐by‐side investigation of both methods with a main focus on practical considerations, and how these effect the resolution of a hydrocarbon reservoir. Synthetic noisy data for both time‐domain and frequency domain methods are simulated using a realistic frequency dependent noise model and frequency dependent scaling for representative source waveforms. The synthetic data studied here include the frequency domain response from a compact broadband waveform, the time‐domain step‐response from a low‐frequency square wave and the time‐domain impulse response obtained from pseudo‐random binary sequences. These data are used in a systematic resolution study of each method as a function of water‐depth, relative noise and stacking length. The results indicate that the broadband frequency domain data have the best resolution for a given stacking time, whereas the time‐domain data require prohibitively longer stacking times to achieve similar resolution.     

电导率各向异性的海洋电磁三维有限单元法正演

本文提出了一种基于非结构化网格的海洋电磁有限单元正演算法.为了回避场源奇异性, 文中选用二次场算法, 将背景电阻率设置为水平层状且各向异性, 场源在水平层状各向异性介质中所激发的一次场通过汉克尔积分得到.基于Coulomb规范得到二次矢量位和标量位所满足的Maxwell方程组, 通过Galerkin加权余量法形成大型稀疏有限元方程, 采用不完全LU分解(ILU)预条件因子的quasi-minimum residual(QMR)迭代解法对有限元方程进行求解得到二次矢量位和标量位; 进而, 利用滑动平均方法得到二次矢量位和标量位在空间的导数, 由此得到二次电磁场; 通过一维模型对算法的可靠性进行验证, 与此同时, 针对实际复杂海洋电磁模型, 比较有限元模拟结果与积分方程模拟结果, 进一步验证算法精度.若干计算结果均表明, 文中算法具有良好的通用性, 适用于井中电磁、航空电磁, 环境地球物理等非均匀且各向异性介质中的电磁感应基础研究.     

一维垂直各向异性介质频率域海洋可控源电磁资料反演方法

本文提出了一维垂直各向异性(VTI)介质倾斜偶极源频率域海洋可控源电磁(CSEM)资料高斯-牛顿反演方法.在电阻率各向异性介质水平偶极源和垂直偶极源海洋CSEM正演算法的基础上,利用欧拉旋转方法,实现了各向异性介质倾斜偶极源海洋CSEM正演算法.海洋可控源电磁场关于地下介质横向电阻率(ρ_h)和垂向电阻率(ρ_v)的偏导数(即灵敏度矩阵)是解析计算的,结合垂直各向异性介质横向电阻率与垂向电阻率的关系,将各向异性率融入到正则化因子选择中,实现了正则化因子的自适应选择.理论模型合成数据和实测资料反演算例表明,我们提出的反演方法能够较准确的重构海底围岩和基岩的各向异性电阻率以及高阻薄层的埋藏深度、厚度和垂向电阻率.     

层状垂直各向异性介质海洋CSEM发射源及接收站姿态和位置对电磁响应影响研究

众所周知,电阻率各向异性对海洋可控源电磁（CSEM）响应有显著影响.在模拟和解释海洋可控源电磁资料时,通常假设接收站布放于平稳的海底,发射源以理想的水平电偶极源形式按预设路径拖曳前进.然而,在实际海洋作业中,自由下沉的接收站可能沉降在局部倾斜的海底面上,在海底的方位可能是任意取向的.同时,由于洋流等因素的影响,发射源偶极子会发生旋转、倾斜,其位置也会偏离预设的拖曳路径,所以发射源和接收站姿态的变化都将对电磁资料产生影响.本文计算了电阻率各向异性介质中,发射源任意姿态任意位置激发、接收站任意姿态任意位置接收的海洋CSEM响应,分析观测系统引起数据误差的机制,分别在电阻率各向同性、围岩电阻率各向异性和高阻储层电阻率各向异性三种模型中,讨论发射源和接收站的姿态及位置变化对响应的影响.计算结果表明,电磁场振幅误差主要来源于发射源和接收站的倾斜角,而相位的变化为发射源位置和接收站倾斜角、位置共同影响的结果.

     

无网格局部Petrov-Galerkin法大地电磁场二维正演模拟

有限差分法和有限单元法在大地电磁场数值模拟中已经得到了广泛的应用,但其数值结果的精度在很大程度上依赖于网格的离散程度.当模拟起伏地形、弯曲界面等复杂地电模型大地电磁场响应时,常常需要花费大量的时间以便得到较合理的离散网格.无网格局部Petrov-Galerkin法（MLPG）不同于有限差分法和有限元法,其形函数和权函数脱离了网格的束缚.本文详细推导了二维大地电磁场边值问题的弱式形式,并将其离散为局部积分域内的表达形式.通过模拟二维海洋地电模型大地电磁场响应,并与结构网格有限元结果进行对比,验证了本文算法和程序的正确性及精度.设计了一个含有弯曲界面的二维地电模型,讨论了不同离散网格对MLPG无网格法模拟结果的影响,并与结构有限元法结果进行了比较,结果表明MLPG无网格法模拟结果受离散网格影响较小.最后利用MLPG无网格法计算了两个海洋起伏地形模型的大地电磁响应,讨论了海底起伏地形对大地电磁响应的影响.

     

Design,modelling and imaging of marine seismic swarm surveys

Autonomous marine vehicles instrumented with seismic sensors allow for new efficient seismic survey designs. One such design is the swarm survey, where a group, or swarm, of slow moving autonomous marine vehicles record seismic data from shots fired by a source vessel sailing around circles within the swarm. The size of the swarm is dictated by the maximum offset requirement of the survey, and it can be shaped to acquire wide‐ and full‐azimuth data. The swarm survey design equation describes the relationship between the source and receiver positions of the survey and the subsurface coverage or fold. It is used to adapt the swarm to the seismic survey requirements and to calculate survey duration time estimates as function of available equipment. It is shown that a survey conducted by a slowly moving swarm requires six times fewer shots than an equivalent seabed node survey conducted over 85.5 km². Swarm surveys can also be adapted to efficiently conduct infill surveys and replace multi‐vessel undershoots. The efficiency of the survey can further be increased when the autonomous marine vehicles are towing short streamers with multiple receivers. Synthetic tests show that the seismic images for swarm surveys are comparable to those from streamer surveys, while little variation in image quality is found when reducing the number of autonomous marine vehicles but equipping them with a short streamer with multiple receivers.     

深水拖曳式大功率时频发射与多链缆多分量电磁探测系统

海洋可控源电磁探测方法是目前海洋油气资源勘探和海底地质构造调查中不可或缺的物探手段之一.本文研究的深水拖曳式大功率时频电磁发射与多链缆多分量电磁探测系统,是深水双船拖曳式可控源海洋电磁勘探系统的关键组成部分.发射系统利用深拖缆实现了电能在复杂海洋环境下长距离、大功率和低损耗传输,基于大功率变流拓扑结构攻克了2000 A量级、0.01~16 Hz可控的时频域大电流冲击脉冲发射;接收系统建立了主节点与最多5个从节点的数据链接,单个从节点记录到了三轴正交的电磁场分量以及运动姿态信息.海洋试验表明,深水双船拖曳式海洋可控源电磁勘探系统借助发射系统与接收系统之间的时间同步、时频观测、可变收发距和观测装置等技术优势,显著提升了数据质量和有效的数据数量,为后续处理和反演提供了硬件支撑.经过海上试验,整套系统的技术性能得到多次验证,未来有望在深水油气、天然气水合物和海底硫化物等资源勘查以及海底地质构造调查领域获得广泛应用.