Magnetotelluric survey for exploration of a volcanic-rock reservoir in the Yurihara oil and gas field,Japan*

The Yurihara oil and gas field is located on the southern edge of Akita Prefecture, northeastern Japan. In this area, drilling, surface geological surveys and many seismic surveys have been used to investigate the geological structure. Wells drilled into the Nishikurosawa Basalt Group (NBG) of Miocene age found oil and gas reservoirs at depths of 1.5–2 km. Oil and gas are now being produced commercially and further exploration is required in the surrounding areas. However, since the neighbouring areas are covered with young volcanic products from the Chokai volcano, and have a rough topography, the subsurface distribution of the NBG must be investigated using other methods in addition to seismic reflection. According to the well data, the resistivity of the NBG is comparatively higher than that of the overlying sedimentary formations, and therefore the magnetotelluric (MT) method is expected to be useful for the estimation of the distribution of the NBG. An MT survey was conducted along three survey lines in this area. Each line trended east–west, perpendicular to the regional geological strike, and was composed of about 25 measurement sites. Induction vectors evaluated from the magnetic field show that this area has a two-dimensional structure. The evaluated resistivity sections are in agreement with the log data. In conclusion, we were able to detect resistive layers (the NBG) below conductive layers. The results indicate that the NBG becomes gradually less resistive from north to south. In the centre of the northern line, an uplifted resistive area is interpreted as corresponding to the reservoir. By comparison with a seismic section, we prove the effectiveness of the integration of seismic and MT surveys for the investigation of the morphology and internal structure of the NBG. On other survey lines, the resistive uplifted zones are interpreted as possible prospective areas.     

海岸效应对近海地区大地电磁测深数据畸变作用研究

在近海地区采集的大地电磁测深数据通常受到海岸效应的影响,使得大地电磁测深数据发生畸变,因而很难利用大地电磁测深资料较为可靠地获得地下深部的电性结构.本文通过正演模拟方法,分析和总结海水深度变化和海底地形变化对近海地区大地电磁测深数据的畸变影响.当测区与海岸线的距离小于目标频率的大地电磁场趋肤深度时,高导海洋的存在会严重影响测区内电磁场的分布.由于海岸效应的影响,大地电磁测深视电阻率曲线和相位曲线均会发生不同程度的畸变,在低频部分,这种畸变作用尤为明显.大地电磁测深一维Occam反演方法和二维非线性共轭梯度反演方法,对近海地区浅部地层具有较好的反演效果.随着海水深度的增加和海底地形的复杂变化,两种反演方法均会出现不同程度的假异常,为地质解释工作造成了影响.近渤海地区的实测大地电磁测深数据在低频部分可能受到海岸效应的影响而导致视电阻率曲线的严重畸变.     

考虑电流型畸变的大地电磁三维反演

大地电磁测深(MT)的观测数据易受到由近地表小尺度非均匀体或地形起伏引起的电流型畸变干扰,消除或压制这种干扰对获取可靠的深部电性结构至关重要.当区域结构为二维时,电流型畸变可采用张量分解等方法予以消除或压制.当区域结构为三维时,畸变问题更加复杂和严重,传统张量分解方法往往效果不佳或无效,严重地制约了MT三维反演技术的实用性.对此,本文提出一种考虑电流型畸变的MT三维反演算法,将完整的电流型畸变参数引入到目标函数,并采用非线性共轭梯度法与电阻率参数同时反演,从而达到压制畸变的目的.该算法有两个关键点:一是通过分析实测数据所遭受畸变的分布特征,在目标函数中对其进行有效约束;二是在迭代过程中,通过自适应地调整双正则化因子保障算法的稳定和效率.理论模型测试结果显示,常规三维反演算法不能合理解释数据中的畸变成分,而只能通过引入虚假异常体强制地拟合受畸变数据,从而造成电阻率模型严重失真.与之相比,本文算法能够在反演中自动求解各测点所受到的畸变,获得更接近真实的电阻率模型.     

Application of the Common Offset Seismic Reflection Method to Urban Active Fault Surveys

The method and principle of common offset seismic surveys as well as the field data gathering and processing technique were introduced briefly. Through two urban active fault survey examples in Fuzhou and Shenyang, the efficiency and limitation of using the common offset seismic reflection technique to carry out urban active fault surveys were probed. The results show that this technique has the properties of high resolving power, better reconstruction of subsurface structures, and real-time analyzing and interpretation of investigation results on site. This method can be used to quickly locate objects under investigation accurately in the areas with thinner Quaternary overburdens and strong bedrock interface fluctuations.     

Mapping of thin sandy aquifers by using high resolution reflection seismics and 2-D electrical tomography

Electrical tomography, which gives good results even in fairly complex geological environments, has given a new lease of life to electrical methods in hydrogeological surveys. Nevertheless, a rapid decline in resolution with increasing depth remains the main problem of the electrical methods. In the Pannonian basin in Croatia, at a test area, combining both electrical tomography and seismic reflection methods provides data that better constrain the lithological and hydrogeological model of the subsurface.Electrical tomography revealed a rather thick packet of sediments with increased resistivity at depths of 40–100 m. Using the electrical forward modelling, the existence of two different hydrogeological models was shown. The first model presupposes a reasonably homogeneous packet of sandy clays or clayey sands, and the other model presupposes the alternation between layers of clays and sands. From the hydrogeological point of view, the second model is perspective, but unfortunately, the use of electrical tomography alone does not allow the ambiguity to be resolved. The separation of these two models became possible using seismic reflection. Three seismic environments were isolated from the seismic profile treated, and the strongest reflections were discovered in the first seismic environment, which covers the depths from 40 to 100 m. It was determined that the second model is more acceptable, because these reflections are caused by lithological changes, that is, the alternations of sands and clays. The interpretation is consistent with exploratory borehole data. The conclusion is that electrical tomography gives data concerning the sediment lithology up to depths of 40 m, but at greater depths combined interpretation of electrical and seismic data constrains the subsurface model better.     

大地电磁测深中相位微分的研究

大地电磁测深研究中,长期以来使用视电阻率和相位作为解释参数,但视电阻率和相位均存在一定的局限性。本文从理论上研究了相位微分的性质,探讨了Bostick电阻率和Cagniard电阻率同相位微分之间的关系,通过实例说明了相位微分的使用方法。结果表明,一维和二维TM模式的相位微分是一个很好的参数,对H型模型和A型模型都能得到较准确的地质体参数。     

Sub‐sample time shift and horizontal displacement measurements using phase‐correlation method in time‐lapse seismic

Hydrocarbon production and fluid injection affect the level of subsurface stress and physical properties of the subsurface, and can cause reservoir‐related issues, such as compaction and subsidence. Monitoring of oil and gas reservoirs is therefore crucial. Time‐lapse seismic is used to monitor reservoirs and provide evidence of saturation and pressure changes within the reservoir. However, relative to background velocities and reflector depths, the time‐lapse changes in velocity and geomechanical properties are typically small between consecutive surveys. These changes can be measured by using apparent displacement between migrated images obtained from recorded data of multiple time‐lapse surveys. Apparent displacement measurements by using the classical cross‐correlation method are poorly resolved. Here, we propose the use of a phase‐correlation method, which has been developed in satellite imaging for sub‐pixel registration of the images, to overcome the limitations of cross‐correlation. Phase correlation provides both vertical and horizontal displacements with a much better resolution. After testing the method on synthetic data, we apply it to a real dataset from the Norne oil field and show that the phase‐correlation method can indeed provide better resolution.     

A SIMPLE FORM OF PRESENTATION OF MAGNETOTELLURIC DATA USING THE BOSTICK TRANSFORM*

A new presentation for magnetotelluric data is proposed to be used together with the traditional representation. It is easily computed by applying the Bostick transform to all field data points. The result is an approximate depth-resistivity diagram which contains an indication of the data quality as well as the homogeneity of the subsurface. This resistivity section should be used in the interpretation of surveys in which accuracy is either not of prime concern or cannot be achieved. Since this mode of presentation is easily understood it can be used to familiarize earth scientists with the use of the MT method in exploration.     

The cone penetration test and 2D imaging resistivity as tools to simulate the distribution of hydrocarbons in soil

The purpose of geophysical electrical surveys is to determine the subsurface resistivity distribution by making measurements on the ground surface. From these measurements, the true resistivity of the subsurface can be estimated. The ground resistivity is related to various geological parameters, such as the mineral and fluid content, porosity and degree of water saturation in the rock. Electrical resistivity surveys have been used for many decades in hydrogeological, mining and geotechnical investigations. More recently, they have been used for environmental surveys. To obtain a more accurate subsurface model than is possible with a simple 1-D model, a more complex model must be used. In a 2-D model, the resistivity values are allowed to vary in one horizontal direction (usually referred to as the x direction) but are assumed to be constant in the other horizontal (the y) direction. A more realistic model would be a fully 3-D model where the resistivity values are allowed to change in all three directions. In this research, a simulation of the cone penetration test and 2D imaging resistivity are used as tools to simulate the distribution of hydrocarbons in soil.     

Integrated geophysical and chemical study of saline water intrusion

Surface geophysical surveys provide an effective way to image the subsurface and the ground water zone without a large number of observation wells. DC resistivity sounding generally identifies the subsurface formations-the aquifer zone as well as the formations saturated with saline/brackish water. However, the method has serious ambiguities in distinguishing the geological formations of similar resistivities such as saline sand and saline clay, or water quality such as fresh or saline, in a low resistivity formation. In order to minimize the ambiguity and ascertain the efficacy of data integration techniques in ground water and saline contamination studies, a combined geophysical survey and periodic chemical analysis of ground water were carried out employing DC resistivity profiling, resistivity sounding, and shallow seismic refraction methods. By constraining resistivity interpretation with inputs from seismic refraction and chemical analysis, the data integration study proved to be a powerful method for identification of the subsurface formations, ground water zones, the subsurface saline/brackish water zones, and the probable mode and cause of saline water intrusion in an inland aquifer. A case study presented here illustrates these principles. Resistivity sounding alone had earlier failed to identify the different formations in the saline environment. Data integration and resistivity interpretation constrained by water quality analysis led to a new concept of minimum resistivity for ground water-bearing zones, which is the optimum value of resistivity of a subsurface formation in an area below which ground water contained in it is saline/brackish and unsuitable for drinking.     

Papua New Guinea MT: looking where seismic is blind1

Hydrocarbon exploration in the Papuan fold belt is made extremely difficult by mountainous terrain, equatorial jungle and thick karstified Miocene limestones at the surface. The high-velocity karstified limestones at or near the surface often render the seismic technique useless for imaging the subsurface. In such areas magnetotellurics (MT) provides a valuable capability for mapping subsurface structure. The main structural interface which can be mapped with MT, due to the large electrical contrast, is the contact between the resistive Darai limestone and the underlying conductive sediments of the Ieru Formation. In some areas the base of the Darai can be mapped with reasonable accuracy by fitting 1D models to the observed MT data. However, in many cases where 2D and 3D effects are severe, 1D interpretations can yield dramatically incorrect results. Numerical and field data examples are presented which demonstrate the severity of the 1D errors and the improvements in accuracy which can be achieved using a 2D inverse solution. Two MT lines over adjacent anticlines, both with well control and seismic data, are used to demonstrate the application of 1D and 2D inversions for structural models. In both cases the seismic data provide no aid in the interpretations. The example over the Hides anticline illustrates a situation where 1D inversion of either TE or TM mode provides essentially the same depth to base of Darai as 2D inversion of both TE and TM. Both models provide base Darai depth estimates which are within 10% of that measured in the well. The example over the Angore anticline illustrates the inadequacy of 1D inversion in structurally complex geology complicated by electrical statics. The TE mode fits a 1D Darai thickness of 800 metres while the TM mode fits a 1D Darai thickness of 3500 metres, bracketing the thickness of 2450 metres observed in the well. The final 2D inversion model provides a depth estimate of 2250 metres. Four MT lines along the Angore anticline have been interpreted using 2D inversion. A high degree of correlation exists between lineaments observed on an airborne radar image and zones of low resistivity within the high-resistivity material interpreted as Darai limestone. These low-resistivity zones are interpreted as fault zones. Three-dimensional modelling has been used to simulate 3D statics in an otherwise 2D earth. These data were used to test the Groom-Bailey (GB) decomposition for possible benefits in reducing static effects and estimating geoelectric strike in the Papua New Guinea (PNG) field data. It has been found that the GB decomposition can provide improved regional 2D strike estimates in 3D contaminated data. However, in situations such as PNG, where the regional 2D strike is well established and hence can be fixed, the GB decomposition provides apparent resistivities identical to those simply rotated to strike.     

A field test of imaging properties of rotational invariants of the magnetotelluric impedance tensor

A part of the Békés Basin (an extensional sub‐basin of the Pannonian Basin, where the basement under thick Pannonian sediments is well known from deep boreholes and from seismic measurements, and where many magnetotelluric (MT) soundings have been carried out for frequencies ranging from 1 to 10^?3 Hz) was selected as a test area to assess the imaging performances of various apparent‐resistivity definitions computed with rotational invariants of either the real part of the complex impedance tensor, or its imaginary part, or both. A comparison (based on earlier 3D numerical studies) has been made between the magnetotelluric images obtained in this way and the depths to the high‐resistivity basement, as known from boreholes and seismic investigations. The correlation coefficient between the series of basement depth values at 39 MT sites and the apparent‐resistivity values was found to be stronger and high correlation appeared at a shorter period when it was computed with apparent resistivities based on the real tensor rather than with apparent resistivities based on the imaginary tensor. In the light of our studies, ρ_{Re Z} and the impedance phase seem to be more informative than any other combination of magnetotelluric interpretation parameters.     

A few case histories of subsurface imaging with EMAP as an aid to seismic processing and interpretation1

The electromagnetic array profiling (EMAP) exploration method can be combined with a direct one-dimensional inversion process for conversion to depth to produce a subsurface resistivity cross-section. This cross-section may then be interpreted in parallel with a seismic cross-section to enhance the prediction of rock type and structure. In complex thrust environments and areas of shallow carbonate rocks, the EMAP method is often used to provide additional data either to help the seismic processor and/or to aid the seismic interpretation. In particular, the electromagnetic (EM) data can be used to build an independent seismic velocity file for depth migration. Three EMAP test areas in the western United States are used to demonstrate such a use of EMAP as an expioration tool. The first shows how a velocity file is estimated from resistivity data for seismic depth migration processing in a complex thrust environment. In the second example, the method is applied in layer-cake geology with high seismic velocity rocks at the earth's surface. The third example is another complex thrust environment, but in this case the velocity file derived from the resistivity data is used for stacking the seismic data.     

Electromagnetic mapping of electrical conductivity beneath the Columbia basalts1

Sedimentary rocks beneath the Columbia River Basalt Group are recognized as having potential for oil and gas production, but the overlying layered basalts effectively mask seismic reflections from the underlying sediments. Four electromagnetic (EM) methods have been applied on profiles crossing Boylston Ridge, a typical east–west trending anticline of the Yakima Fold Belt, in an attempt to map the resistivity interface between the basalts and the sediments and to map variations in structure and resistivity within the sediments. The EM surveys detected strong variations in resistivity within the basalts, and in particular the continuous magnetotelluric array profiling (EMAP) revealed resistivity lows beneath the surface anticlines. These low resistivity zones probably coincide with fracturing in the core of the anticlines and they appear to correlate well with similar zones of low seismic velocity observed on a nearby seismic profile. The controlled-source EM surveys (in-loop transient, long-offset transient, and variable-offset frequency-domain) were designed in anticipation of relatively uniform high resistivity basalts, and were found to have been seriously distorted by the intrabasalt conductors discovered in the field. In particular, the resistivity sections derived from 1D inversions were found to be inconsistent and misleading. The EMAP survey provided the most information about the subsurface resistivity distribution, and was certainly the most cost-effective. However, both controlled-source and EMAP surveys call for accurate 2D or 3D inversion to accommodate the geological objectives of this project.     

Three-dimensional imaging of subsurface structures using resistivity data

We have developed a three-dimensional inverse scheme for carrying out DC resistivity surveys, incorporating complicated topography as well as arbitrary electrode arrays. The algorithm is based on the finite-element approximation to the forward problem, so that the effect of topographic variation on the resistivity data is effectively evaluated and incorporated in the inversion. Furthermore, we have enhanced the resolving power of the inversion using the active constraint balancing method. Numerical verifications show that a correct earth image can be derived even when complicated topographic variation exists. By inverting the real field data acquired at a site for an underground sewage disposal plant, we obtained a reasonable image of the subsurface structures, which correlates well with the surface geology and drill log data.     

Trans‐dimensional Bayesian inversion of controlled‐source electromagnetic data in the German North Sea

This paper presents the first controlled‐source electromagnetic survey carried out in the German North Sea with a recently developed seafloor‐towed electrical dipole–dipole system, i.e., HYDRA II. Controlled‐source electromagnetic data are measured, processed, and inverted in the time domain to estimate an electrical resistivity model of the sub‐seafloor. The controlled‐source electromagnetic survey targeted a shallow, phase‐reversed, seismic reflector, which potentially indicates free gas. To compare the resistivity model to reflection seismic data and draw a combined interpretation, we apply a trans‐dimensional Bayesian inversion that estimates model parameters and uncertainties, and samples probabilistically over the number of layers of the resistivity model. The controlled‐source electromagnetic data errors show time‐varying correlations, and we therefore apply a non‐Toeplitz data covariance matrix in the inversion that is estimated from residual analysis. The geological interpretation drawn from controlled‐source electromagnetic inversion results and borehole and reflection seismic data yield resistivities of ～1 Ωm at the seafloor, which are typical for fine‐grained marine deposits, whereas resistivities below ～20 mbsf increase to 2–4 Ωm and can be related to a transition from fine‐grained (Holocene age) to unsorted, coarse‐grained, and compacted glacial sediments (Pleistocene age). Interface depths from controlled‐source electromagnetic inversion generally match the seismic reflector related to the contrast between the different depositional environments. Resistivities decrease again at greater depths to ～1 Ωm with a minimum resistivity at ～300 mbsf where a seismic reflector (that marks a major flooding surface of late Miocene age) correlates with an increased gamma‐ray count, indicating an increased amount of fine‐grained sediments. We suggest that the grain size may have a major impact on the electrical resistivity of the sediment with lower resistivities for fine‐grained sediments. Concerning the phase‐reversed seismic reflector that was targeted by the survey, controlled‐source electromagnetic inversion results yield no indication for free gas below it as resistivities are generally elevated above the reflector. We suggest that the elevated resistivities are caused by an overall decrease in porosity in the glacial sediments and that the seismic reflector could be caused by an impedance contrast at a thin low‐velocity layer. Controlled‐source electromagnetic interface depths near the reflector are quite uncertain and variable. We conclude that the seismic interface cannot be resolved with the controlled‐source electromagnetic data, but the thickness of the corresponding resistive layer follows the trend of the reflector that is inclined towards the west.     

THE DISPERSION RELATION, OF EARTH''S ELECTROMAGNETIC RESPONSE FUNCTIONS AND THE JOINT INVERSION OF DATA

On the basis of the dispersion relations of MT field, the necessity and applied prospects of the joint inversions using a pair of MT response functions which are correlative with the dispersion relations, are infered. A filter coefficient algorithm is made, with which the corresponding impedance phase data can be estimated using a set of apparent resistivities. The tests for the observed MT data show that when comparing the impedance phase estimated using the dispersion relation with the ob served phase, it can be checked whether the dispersion relation between observed apparent resistivity and phase data is satisfied or not, and that the use of the phase data corrected using the dispersion relation in the joint inversion is advantageous to obtain more confident results. It is shown that joint inversions are more advantageous than single parameter inversions, and that in the most case the joint inversion using the apparent resistivities of impedance real and imaginary parts is more advantageous than the jointinversion using the normal apparent resistivity and impedance phase. The existence of the dipersion relations between the ratio apparent resistivity and corresponding impedance phase of the orthogonal electric and magnetic field horizontal Components in the frequency EM sounding with horizontal electric dipole(FEMS) are discussed, the better effect of the joint inversion using the pair of EM response functions is obtained. The problems on the one-dimensional joint inversion for the MT and FEMS apparent resistivities, for which the observed frequency bands partly overlape each other, are studied. It is shown that this joint inversion is applicable and effective:the joint inversions of the practical data for two kinds of EM methods at two sites give the results well corresponding to the drilling data. The simulated MT inversions for the data of two kinds of EM methods are made, and more confident results also are obtained.     

基于三维正演的音频大地电磁阻抗相位不变量校正技术

复杂地形、地质条件的大地电磁数据解释容易出现假象,采用三维正演技术模拟地形和地表不均匀体的背景响应,对实测数据阻抗相位不变量进行校正,实现更准确的定性分析;对三维异常体模型的合成数据进行一维、二维多参数反演试算,以确定地形剧变区选择反演技术的最佳方案.合成数据的试反演结果显示一维反演水平切片假异常较多,二维反演能压制测向假异常,但不能压制走向的假异常,水平切片多出现测向条带.使用本文提出的阻抗相位不变量校正法扣除地形、地表背景响应,结合一维、二维反演,能使实际资料解释成果更加可靠.     

The inversion of the data of magnetotelluric sounding and electromagnetic frequency sounding with horizontal electric dipole

On the basis of the dispersion relation of magnetotelluric response functions (MTRF), a filter coefficient algorithm has been made, with which the corresponding impedance phase data can be estimated using a set of apparent resistivity data. The tests of theoretical models and observed magnetotelluric (MT) data show that this algorithm is effective. Comparing the impedance phase estimated using dispersion relation with the observed phase, it can be checked whether the dispersion relation between the observed apparent resistivities and phase data was satisfied. The use of phase data corrected using the dispersion relation in the joint inversion for MT impedance is advantageous to obtain more reliable inversion results. The problems on the one-dimensional joint inversion for the (MT) apparent resistivity and the apparent resistivity of the frequency electromagnetic sounding (FEMS) with horizontal electric dipole, whose observed frequency bands are linked up each other, are studied. The observed data of two kinds of electromagnetic (EM) methods at two sites are used to inverse, the comparison with the drilling data show the results are more reliable. To supply the phase data of FEMS using the dispersion relation, for the apparent resistivity-phase data and impedance real part-imaginary part apparent resistivities of two kinds of EM methods the imitated MT joint inversions are made, and more similar results also are obtained. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,15, 91–96, 1993. The projects sponsored by the Chinese Joint Seismological Science Foundation.     

欧洲中部地壳和上地幔的电导率(英文)

根据捷克和斯洛伐克电磁台站的数据 ,得到波西米亚块体地壳和上地幔的电阻率分布模型。估计电阻率时特别使用了短周期和长周期的电磁数据以及用连续谱方法对固定台站数据的分析结果。根据欧洲地磁感应矢量分布绘出了显著的地电不均匀带的地图 ,将波西米亚块体的视电阻率与中欧有关台站的感应标量长度与W1响应做了比较 ,并根据电阻率 /深度图估计了电导率范围 ,该图与替代性理想导体反演相对应