Equipment for carrying out quantitative geoelectromagnetic model experiments

Summary The paper describes equipment designed to study the effect of a conducting hostrock/overburden on the electromagnetic anomaly of sulphide ore bodies embedded therein. The model was constructed strictly according to the theory of electromagnetic similitude so as to constitute a direct reading reproduction of the field vectors. The experiments were conducted at a fixed, crystal-controlled frequency of 100 kHz using mainly a graphite sheet immersed in a dilute hydrochloric acid solution of predetermined conductivity to simulate vein and manto type of ore deposits surrounded by a partially conducting zone. Both the inphase and quadrature components of the anomaly were measured in terms of the primary field after elimination of the regional anomaly by means of a measuring bridge and a compensator. A comparison of the anomaly profiles obtained over the ore model when it is held in air with those obtained for corresponding situations inside a conducting solution shows a general enhancement of response [1–3].     

VLF ANOMALIES FROM A PERFECTLY CONDUCTING HALF PLANE BELOW AN OVERBURDEN*

A theoretical solution to the electromagnetic problem of a perfectly conducting half plane below a conducting overburden has been obtained. The VLF anomalies have been computed for different overburden conductivity and thickness and also for different dip angles of the half plane. In the computations the contribution to the secondary magnetic field from the electric Hertz potential has been neglected. The anomaly curves which are displayed as EM 16 readings, show a fairly complicated behaviour. This is mainly due to the phase shift and attenuation of the field caused by the conductivity of the overburden and the host rock. From the anomaly curves it is possible to define the apparent depth to the top of the conductor as the distance between the peak value and the cross-over of the real component. The apparent depth is usually larger than the actual depth, but it is possible to determine the actual depth to the conductor from the relation between the peak-to-peak anomaly and the apparent depth. When the peak-to-peak anomaly is fairly large, it is also possible to make estimates of the dip angle. However, a complete set of master curves will be a necessary tool for interpretation of VLF data when there is need to obtain more accurate estimates of the half plane parameters. In a specific case the theoretical calculations are shown to be in good agreement with measured data.     

ENHANCEMENT OF ELECTROMAGNETIC ANOMALIES BY A CONDUCTING OVERBURDEN*

The effect of a conducting overburden on the electromagnetic response of sulphide ore-bodies has been studied with the help of quantitative electromagnetic model experiments. These experiments were conducted at a fixed, crystal controlled, frequency of 100 kHz using a number of transmitter-receiver configurations, though the results discussed here mostly pertain to a horizontal coplanar system. An analysis of the anomaly profiles—after accounting for the regional anomaly—indicates a general enhancement of the response. This is attributed (a) to the concentration of the current lines in the host-rock effected by the embedded target and (b) to the change in the phase and space orientation of the field vectors, brought about by the conducting overburden. The enhancement is most pronounced in the case of non-symmetrical bodies such as veins and ribbon-like structures represented by sheet models, but is also observed to a lesser degree for isometric structures. These results are expected to pave the way for a more accurate interpretation of the induction prospecting data.     

COMPUTATION OF VLF RESPONSE OVER HALF-PLANE AND WEDGE MODELS*

A theoretical solution is presented to the problem where a VLF anomaly is generated by a conducting half-plane or a perfectly conducting wedge below a stratified overburden. The solution is obtained by the use of a scattering matrix for plane-wave eigenfunctions. VLF anomalies have been computed for different values of the conductance and dip of the half-plane. The phase of the VLF anomaly due to a conducting half-plane depends on the conductance and the distance to the half-plane. Close to the half-plane the tilt angle and ellipticity are of opposite sign for a perfect conductor, but the ellipticity will change sign for a poor conductor. The VLF anomaly for a perfectly conducting wedge is essentially determined by the position of the upper surface of the wedge, i.e. the anomaly will closely resemble the anomaly of a perfectly conducting half-plane in the same position as the upper surface of the wedge.     

I.P. AND RESISTIVITY MODEL STUDIES OVER THIN DYKES*

Induced polarization and resistivity model studies over thin dykes for varying resistivity contrasts, depth of burial, and dip angles show striking parallelism of the surface apparent resistivity contours with the boundary of the body. This effect may be utilized for the estimation of strike length of the body. Results show that intermediate values of the electrode spacing is satisfactory for detection under widely varying conditions of resistivity contrast and depth. The percentage frequency effect (P.F.E.) and the metal factor (M.F.) responses are found to be more sensitive to the variation in the depth of burial than the resistivity responses. Pseudosections for P.F.E. and M.F. are concentrated much closer to the body than the resistivity pseudo-sections.     

Numerical modelling of airborne electromagnetic anomalies originating from low-conductivity 3D bodies1

Responses of a multifrequency, multicoil airborne electromagnetic (AEM) system were modelled numerically for 3D electrical conductors embedded in a resistive bedrock and overlain by an overburden of low to moderate conductivity. The results cover a horizontal coplanar coil configuration and two frequencies, 7837 Hz and 51 250 Hz. The models studied are single or multiple, poor conductors (conductance lower than 0.1 S) embedded in a host rock of high but finite resistivity (5000 Ωm) and overlain by a layer of overburden with finite thickness and low to moderate conductivity (conductance up to 2 S). On the basis of the modelling results, limits of detectability for poor conductors have been studied for the various model structures. The results indicate that the anomaly from a steeply dipping, plate-like conductor will decrease significantly when the conductor is embedded in a weakly conductive host rock and is overlain by a conductive overburden. However, an anomaly is obtained, and its magnitude can even increase with increasing overburden conductivity or frequency. The plate anomaly remains practically constant when only the overburden thickness is varied. Changes in overburden conductivity will cause the plate-anomaly values to change markedly. If the plate conductance is less than that of the overburden, a local anomaly opposite in sign to the normal type of anomaly will be recorded. Another major consequence is that conductors interpreted with free-space models will be heavily overestimated in depth or underestimated in conductance, if in reality induction and current channelling in the host rock and overburden make even a slight contribution to the anomalous EM field. The lateral resolution for the horizontal coplanar coil system was found to be about 1.7 times the sensor altitude. Similarly, the lateral extension of a horizontal conductive ribbon, required to reach the semi-infinite (half-space) behaviour, is more than three times the sensor altitude. Finally, screening of a steeply dipping plate, caused by a small, conductive horizontal ribbon, is much more severe than screening of the same plate by an extensive horizontal layer.     

NUMERICAL MODELLING OF A NEW MULTI-FREQUENCY ELECTROMAGNETIC SYSTEM*

The amplitude of the horizontal magnetic field in the ground between two parallel wires, both carrying an alternating current in the same direction, is likely to have a saddle point if the separation between the wires is small and the frequency is low. The amplitude has a maximum in the vertical direction and a minimum in the horizontal. Rectangular geological structures in the ground which are centered between the wires have a varying effect on the magnetic fields at the surface. In general, the vertical magnetic field “crosses over” at the center of the structure. A shallow and flat lying conductor displays a broad flat type of profile when the horizontal magnetic field between the wires is measured. Changing the structure to a narrower but more conducting one at depth will provide a more pointed but still broad profile. The phase of the horizontal field is also increased. When the structure is a thin vertical dyke, the amplitude of the horizontal magnetic field anomaly due to the dyke rapidly decreases as the depth of the dyke is increased. The phase of the horizontal field is less sensitive to changes in depth of the dyke but is more sensitive to the conductivity ratio of the dyke and the half-space. The amplitude of the vertical magnetic field anomaly due to the dyke is only slightly influenced by conductivity contrast or the depth of the dyke. The phase of the vertical magnetic field, however, is strongly influenced by the conductivity contrast, particularly if the conductivity frequency product is greater than hundred. In essence, the field behaves like that of the conventional vertical loop source, but the fields are uniform over much larger areas. This suggests the possibility of using dip angle measurements for rapid reconnaissance.     

INDUCTION ANOMALY DUE TO AN ELONGATED COVERED ORE-ZONE EXCITED BY A LONG CURRENT CARRYING CABLE*

A detailed analysis of the electromagnetic anomaly due to an elongated mineralized zone of low resistivity in presence of a fixed-transmitter (a long cable carrying an, alternating current Ie^Iωt) is presented where (i) the host rock is not highly resistive and might contain some disseminated mineralization, (ii) the ore-body has an inhomogeneous conductivity, (iii) there is a contrast in magnetic properties of the ore-body and the host rocks, and (iv) source is close to the conducting system i.e., anomalous zone is lying shallow. The numerical computation of the generalized analytical expressions have been made. The selective screening behaviour of the cover is delineated and suitable frequency ranges for maximum detectability of a covered conductor have been obtained. A paradoxical decrease of the in-phase component of the response function with the increase of the core conductivity has been found for large conductivities of the cover. For moderate values of response parameter the inhomogeneity in the conductivity is found to significantly affect the induced field. The present analysis of the various features of the secondary field will aid to the existing interpretation sensitivity of the induction prospecting data for porphyry conducting ore deposits with zonal wall-rock alteration and sulphide distributions.     

NUMERICAL VLF MODELING*

The VLF response of laterally inhomogeneous and anisotropic models is calculated numerically using the finite element method. Some results are presented for a slab model in terms both of the polarization parameters, i.e., the tilt angle and ellipticity of the magnetic polarization ellipse, and the amplitude ratio |H_z/H_x|. On the basis of both the ellipticity and the tilt angle, it is possible to discriminate between a poor conductor and a good one. The direction of the dip can be determined from the anomaly profiles of all diagnostic parameters. The effect of the conductive overburden is most noticeable on the ellipticity profile: one observes attenuation for a poor conductor and “negative attenuation” for a good conductor. The anomaly profiles for anisotropic cases are consistent with the ones of the isotropic cases.     

SOME APPLICATIONS OF IP-TECHNIQUE FOR DIFFERENT GEOPHYSICAL PROSPECTING PURPOSES *

The usefulness of IP-technique for different geophysical purposes is discussed on the basis of field examples. IP and resistivity results are not always correlated, and the IP-information may be unique in prospecting for very low grade sulphide mineralizations. The IP-technique seems to be useful sometimes in prospecting for disseminated Ni-Cu- mineralizations. A massive sulphide mineralization could be localized using IP. The disturbances from different kinds of geological objects other than prospecting targets generally are smaller in IP-work than in resistivity work. It was possible to discriminate between apparent resistivity anomalies caused by shear zones and by sulphide mineralizations by means of IP-measurements. The influence of magnetite on IP- measurements is briefly discussed. The relation between ground magnetic measurements ΔZ and the IP-parameter is in many areas weak and does not seriously influence the interpretation of the IP-measurements. A high correlation between ΔZ and IP-measurements has been found on a titaniferous iron ore. This ore body gives strong IP-anomalies indicating that it is a good IP-target. Finally, the application of small moving arrays in very detailed IP-measurements is found useful for detailed mapping of a mineralized zone under a thin overburden and for determination of the dip of mineralized zones.     

THEORETICAL TRANSIENT EM RESPONSE CURVES OVER A THIN DIPPING DYKE IN FREE SPACE–SEPARATED INLINE LOOP CONFIGURATION*

Theoretically exact type curves are presented to show the effect on response shape of changing dip, strike, transmitter-receiver separation, depth of burial and target conductance. Relatively small dip variations produce a marked asymmetry in response shape. For steeply dipping conductors, strike can be determined from the cross-over width of the anomaly. The curves illustrate the dynamic character of the transient process, and suggest how the spatial disposition and quality of large sheet conductors might be inferred from the extent of peak-amplitude displacement with time, and rate of eddy-current diffusion. A generalized interpretation scheme is proposed based on dimensionless response characteristics which will facilitate the rapid determination of conductor dip, conductance and depth of burial for any time regime.     

INTERPRETATION AND DEPTH OF INVESTIGATION OF GRADIENT MEASUREMENTS IN DIRECT CURRENT GEOELECTRICS*

Gradient measurements in a homogeneous electrical primary field can easily be interpreted for simple models. The simplified solution (conducting or resistant body in a homogeneous space in a homogeneous electrical field) is often sufficiently accurate, as comparisons with the exact solution (body of finite resistivity in a homogeneous half-space in a quasihomogeneous electrical field) show. The exact geometry of the body cannot be determined by gradient measurements; the same anomaly of apparent resistivity can be caused by different bodies. In particular, the similarity between a sphere and a cube of the same volume is very high. There is a distinct influence of the resistivity of the overburden: the higher this resistivity is, the stronger is the effect caused by a buried body. If a deviation of 10% of the apparent resistivity is assumed as the lower boundary at which a buried body can be detected by gradient measurements, the depth of investigation for a three-dimensional body is approximately equal to its width; in the two-dimensional case the thickness of the overburden can be twice the width. If the overburden has a resistivity which is higher than the resistivity of the substratum, these depths are greater. The greatest possible depth is approximately three times the width of the body.     

二维激电中梯数据磁类比拟合反演研究

本文通过柯尔-柯尔模型研究地下不同几何参数的极化椭球体在地表的中梯双频激电响应,从而了解不同产状极化体产生极化率的特征,便于后期交互建模。由理论推导可知,任何具有激发极化特征的地质体在地表产生的激电异常都可近似为一个地下电偶极子的三维空间积分,这个积分形式与磁场正演计算公式一致,这意味着我们可以用解释磁法异常的方法解释激电异常,即磁类比法。只要根据先验信息实时改变模型,使激电正演得到的曲线与实测激电曲线逼近,当拟合差小于均方误差时,我们将最后一次模型近似看作是真实模型,由此获得地下极化体的顶埋深、倾角和几何大小等空间参数。实测数据测试表明,这种磁类比人机交互方法加入矿体的一些已知信息后,其拟合反演结果比自动反演更具合理性。      

ANALYSIS OF SOME MULTI-FREQUENCY,MULTI-SEPARATION ELECTROMAGNETIC SURVEYS*

In the interpretation of anomalies obtained with moving source-receiver dipole electromagnetic surveys, use is frequently made of the so-called phasor or vector diagrams. Most such diagrams are based on model experiments or theoretical calculations on bodies of simple geometry. If the anomaly at some given coil-separation and frequency is known it is possible to predict the anomaly at some other separation and frequency. While the results of such predictions often agree admirably in the laboratory experiments, wide, systematic discrepancies are observed in full-scale field work. Some of the discrepancies can be explained by various phenomenological arguments and in this respect the effect of overburden conductivity appears to be more serious than is generally supposed. The effect of phase rotation due to the overburden is discussed in detail. The paper is mainly a cautionary note based on some full-scale data of multi-frequency, multi-separation surveys against the blind use of vector diagrams, but it also discusses the extent of the extra information and interpretation aid provided by such surveys.     

三维地电模型数值模拟中视电阻率真假异常特征分析

本文概要介绍了三维数值模拟中双网格系统的剖分方法,并通过几种典型模型的有限单元法三维数值模拟计算（网格单元电导率分块均匀、电位三线性变化）,充分揭示了对称四极电阻率测深法三维地电模型视电阻率真假异常的分布规律,分析了产生真假异常的原因,并研究了真异常的形态与引起真异常的地质体位置的关系。     

SCALE MODEL ELECTROMAGNETIC RESPONSE TO INLINE AND BROADSIDE SYSTEMS AT SKEW TRAVERSES OF A DIPPING HALF PLANE EMBEDDED IN A CONDUCTING HOST ROCK*

Laboratory scale model experiments have been performed to obtain the electromagnetic response of a finitely conducting half plane embedded in resistive/conductive surrounding and excited by an oscillating magnetic dipole. Inphase and quadrature profiles are presented for two horizontal coplanar transmitter-receiver systems (inline and broadside) for normal and skew traverses and for different dips of the conductor. It is observed that the broadside system is more diagnostic in delineating the strike and dip of the conductor and is more sensitive to the conducting host rock. The broadside profile over a vertical or dipping half plane is characterized, when traversing perpendicular to strike, by two positive peaks flanking a zero response when the coils are over the top edge of the conductor. For skew traverses a negative peak replaces the zero response. An increasing asymmetry in the anomalies is caused by changing the dip of the conductor from the vertical in both the systems, but it is more pronounced for the broadside system. The quadrature response in the broadside system changes in a characteristic way when the target is surrounded by a conducting host rock. The comparative results of the two systems may, therefore, be useful in the induction prospecting for ore deposits approximated by a half plane, especially in delineating the strike, dip, and effect of conductive host rock.     

三维井地磁测联合约束反演

井中磁测在铁矿等磁性金属矿床勘查中具有举足轻重的作用.针对磁性金属矿床深部资源勘查中存在的有效信号弱、矿体形态复杂等技术难题,本文利用井中三分量磁测资料和地面磁测资料直接反演地下空间磁化率的分布情况,充分发挥纵、横向分辨率高的优势,通过磁化率的变化来确定矿（化）体的范围.分析了磁化率和磁场H_ax、H_ay、Z_a、ΔT各参量之间的关系,导出了磁化率反演方程;将钻孔编录、磁化率测井等结果作为约束条件构建了目标函数.以组合倾斜板状体模型为例,验证了方法的有效性,并在青海野马泉铁多金属矿区开展应用试验,试验结果与已有地质资料基本一致.该方法有效抑制了因反演数据源单一、缺乏约束条件而造成反演结果的多解性,实现了磁测资料的精细反演解释,为查明地下矿体空间位置、形态及规模,提高找矿效果,提供了一种新的解决方案.     

深海6000 m拖曳式瞬变电磁系统及其应用

针对我国西南印度洋合同区热液硫化物矿快速、有效以及便捷地质勘探装备的要求,研发了由甲板控制系统、万米光电复合缆、仪器舱拖体和天线拖体组成的深海6000 m拖曳式瞬变电磁系统.为了便于拖体布放和快速发现异常,选择重叠回线收发装置类型;采用理论数值计算确定了在拖曳高度不大于50 m的前提下,观测1～100ms窗范围内的二次场响应,可以发现近海底深海热液硫化物矿堆;另外,通过不同拖延深度海上试验,研究了拖曳深度对瞬变电磁的影响规律及仪器性能.大洋第30航次第二航段在西南印度洋脊热液区,应用该系统发现了明显的瞬变电磁异常.印证了系统的有效性.     

Analogue model measurements for a horizontal magnetic dipole embedded within a conducting medium

The validity of an analogue model method employing a horizontal magnetic dipole source situated within a conducting layer for the cases of a poorly conducting model earth and a highly conducting model ocean is studied by comparing model magnetic field measurements with theoretical calculations. The model is then used to study one example for each case; the response of a conducting cylinder simulating an ore body embedded in the earth, and the response of a conducting wedge simulating a shelving ocean.     

Electromagnetic response of a covered inhomogeneous permeable conductor in time domain

Summary An initial value induction problem has been solved to examine the response of a massive sulphide ore body under conditions of common occurence. The generalized expression for the induced field due to a step-function dipolar field in the presence of a non-uniformly conducting permeable sphere surrounded by a finitely resistive overburden has been derived. Two cases, when the concentric shell is (i) insulated from and (ii) in galvanic contact with the inner core, have been considered. The analytical results are in computable form to yield the decay-characteristics for application in geoelectromagnetics. Particular cases of the problem treated by the other investigators are discussed.N.G.R.I. Contribution No. 71-234