共查询到20条相似文献，搜索用时 31 毫秒
1.
R.D. OGILVY 《Geophysical Prospecting》1986,34(5):769-788
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. 相似文献
2.
The interpretation of VLF-EM surveys in terms of buried conductors can be assisted by the application of a linear filter to the observed in-phase component of the vertical magnetic field. One such scheme is examined critically by using the calculated response from a variety of synthetic models to compute theoretical current density pseudosections. The results confirm that this filter technique provides a useful complementary tool for studying the third (i.e. depth) dimension. For single, steeply dipping plates (> 45°) diagnostic information may be derived concerning the depth, size, lateral location, and direction of dip, since the current density maxima seems always to occur within the conductor or at least within one data interval. However, there are some limitations which do not appear to be widely recognized. For single plates the angle of dip cannot be resolved by the current density transformation. Pattern distortions can occur where targets are in close proximity, where the cross-sectional form of the conductor is complex or where the dip is shallow (< 45°). In these latter cases the current density maxima may not occur within the conductive structure and therefore cannot be used to infer depth of burial and/or conductor shape with the same degree of confidence. 相似文献
3.
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. 相似文献
4.
In the quantitative data interpretation for HLEM induction prospecting, a vertical half-plane model in an insulating medium is widely employed. For this assumption to be valid, the steeply dipping massive sulphide dykes must have large strike lengths and depth extents, but small thickness. We report investigations, using the laboratory scale-modelling method, on the response variation of large vertical conductors as the thickness is varied. We conclude that a steeply dipping large dyke can be approximated by a half-plane model only if its thickness is less than half the skin depth. An inductively thick conductor produces larger amplitudes and relatively higher quadrature compared to a thin conductor, even if both have the same induction number. 相似文献
5.
In a weathered environment estimates of depth and conductance of metallic sulphide dykes from conventional anomaly index diagrams for a vertical half-plane in air have to be corrected, besides the usual corrections, for: 1. moderate conductivity of the host rocks, and 2. finiteness of strike length S and depth extent D. Model experiments have been carried out to evaluate the response variation of a vertical planar conductor with varying depth extent and strike length for both insulating and conductive surroundings. The results indicate: 1. A conductor with finite depth extent (D/L < 2.5) or strike length (S/L < 5.0) in an insulating medium yields a lower estimate of conductance (mineralization) and a greater depth. 2. A moderately-conductive host rock enhances the anomaly and rotates the phase so that the conductor appears to be more resistive (less mineralized) and shallower. The results have practical significance since in weathered surroundings a highly-mineralized body of finite size could be missed, or misjudged, because of low estimates of conductivity and depth. 相似文献
6.
M. PODDAR 《Geophysical Prospecting》1982,30(1):86-100
A survey using Crone pulse electromagnetic (PEM) equipment was conducted near the village of Gani in Andhra Pradesh, India. The anomalies obtained had a shape typical for an inclined sheet conductor. From the multichannel TEM response, the multifrequency Slingram response was calculated. A comparison of the Slingram response of the Gani conductors with the free-air model response of a thin sheet conductor shows phase rotation and attenuation of the anomaly vector, thereby confirming the presence of a conductive overburden. Interpretation of the Gani results using free-air phasor diagrams causes an overestimation of depth and conductance of the target conductor unless data obtained at a sufficiently low frequency are used for the interpretation. For areas like Gani this frequency can be as high as 336 Hz. 相似文献
7.
The results of a physical scale model study of the conventional mode of operation of fixed loop electromagnetic systems and an alternative mode called the tx-parallel mode in which traverses are run parallel to the long axis of the rectangular transmitter loop are presented. The results show that over thick or dipping conductors, the tx-parallel configuration provides coupling with the target which is comparable with that provided by the conventional configuration. In addition, the tx-parallel configuration is shown to provide more consistent indications of the direction and magnitude of conductor dip. Over wide conductors, where separate conventional surveys are needed to define the opposite edges of such conductors, it is shown that only a single tx-parallel survey is needed to locate both edges of the conductor. The tx-parallel results were found to allow better resolution of the individual anomalies due to closely spaced parallel conductors. The tx-parallel response of identical sheet conductors of opposite dip indicated that the response of the separate sheets could be recognized even when the two sheets were placed at zero separation. This was found to be due in part to spatial displacement of the individual current vortices within each conductor owing to their mutual repulsion. 相似文献
8.
O. OLSSON 《Geophysical Prospecting》1980,28(3):415-434
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. 相似文献
9.
本文利用有限差分法对垂直走滑断层和垂直倾滑断层两种剪切破裂源在不同埋深和不同破裂尺度(指垂直于地面方向上断层尺度)情况下的近场理论地震图进行了讨论,并得到了地表不同震中距处的加速度付氏谱。通过对上述两种源所产生的近场效应的对比分析,本文得到了以下初步结果: (1) 在埋深相同,断层面宽度相同的情况下,垂直走滑断层源对近场地表的影响比垂直倾滑断层源大。(2) 在垂直倾滑断层源的埋深、破裂尺度相同的情况下,在地表同一震中距处产生的水平位移要比垂直位移大。(3) 两种破裂源在地表同一震中距处产生的位移的最大振幅随破裂尺度的增大而增大,且近似为线性关系;在极震区内不同震中距处产生的位移的最大振幅随震中距的增大而减小,且随震中距的变化率在减小;在地表不同震中距处产生的加速度的付氏谱的主峰周期随震中距的增大而变长,随破裂的增大,付氏谱中主峰周期也加长,且长周期成份愈多;随着破裂源埋深的增加,地表不同震中距处产生的位移的最大振幅变小,即等震线变疏。(4) 走滑断层源产生的加速度付氏谱的主峰周期比倾滑断层源长。 相似文献
10.
PETER FURNESS 《Geophysical Prospecting》1993,41(1):113-130
Furness , P. 1993. Gradient Array Profiles over thin resistive veins. Geophysical Prospecting 41 , 113–130. Gradient array geoelectric surveys are commonly used to explore for buried sheet-like targets. It is demonstrated that under certain circumstances the responses of such features can become practically equivalent for a range of models differing considerably in resistivity and thickness. This is the dual of the familiar equivalence phenomenon associated with thin beds in geoelectric sounding operations. Hence the geoelectric response profiles over thin dipping resistive veins are conveniently studied by considering equivalent forms with vanishing thickness and infinite resistivity but with the same resistivity-thickness product. The effects of varying the dip, depth of burial, depth extent and resistivity–thickness product of such features on the gradient array response profiles are investigated by means of a series of numerical experiments. These demonstrate several effects of significance to survey design and data interpretation. In particular it is found that while the resistivity–thickness product influences the amplitude of the response profiles it has little effect on their form. A similar behaviour is found to be exhibited by veins with extended depth extent when the dip is varied. A complete quantitative interpretation of gradient array profiles over thin dipping resistive veins is thus only possible for veins of limited depth extent. For veins with an extended depth dimension, the data is ambiguous (in a practical sense) in terms of the dip and the resistivity–thickness product. Here, the interpretation can only be expected to deduce the horizontal and vertical locations of the vein apex. 相似文献
11.
《Journal of Applied Geophysics》2006,58(2):158-175
The interpretation of airborne VLF data represents an important aspect of geophysical mapping of the upper few hundred meters of the Earth's crust, especially in areas with crystalline rocks. We have examined the ability of the single frequency VLF method to provide quantitative subsurface resistivity information using two generic models and standard airborne parameters with a flight altitude of 70 m and a frequency of 16 kHz. The models are long thin conductor (10 m thick, 10 Ω m resistivity and 1 km long) and a wider buried conductive dike (100 Ω m resistivity and 500 m wide). Using standard regularized inversion it turned out that for both models the conductivity of the conductors are underestimated and the vertical resolution is rather poor. The lateral positions of the minimum of the resistivity distributions coincide well with the true positions of the shallow conductors. For deeper conductors the position of the minimum resistivity moves from the edges of the conductor into the conductor. The depth to the minimum of the resistivity anomalies correlates well with the true depth to the top of the conductors although the latter is always smaller than the former.Interpretation of field airborne data collected at 70 m flight height resolved both small scale and large scale near surface conductors (conductance ∼1 S). Deeper conductors show up in the VLF data as very long wavelength anomalies that are particularly powerful in delineating the lateral boundaries of the conductors. Many of the VLF anomalies in the Stockholm area are dominated by these deep conductor responses with some near surface conductors superimposed. The deep conductors often follow topographic lows coinciding with metasediments. We interpret the frequent absence of near surface responses at 70 m flight height as a result of weak coupling between the primary VLF wave and the small scale (in all three dimensions) near-surface conductors.Radio magnetotelluric (RMT) ground measurements were carried out along a short profile coinciding with part of an airborne profile. Using data at 9 frequencies (14–250 kHz) small scale conductors in the upper few tens of meters, not identified from the airborne data, could be well resolved. Large scale deeper conductors could be identified by both methods at nearly the same positions. 相似文献
12.
One of the significant problems of modern deep magnetotellurics is the recognition of anisotropy in the crustal and mantle
conductive zones. In the paper we perform numerical experiment comparing several 2D models of crustal and mantle isotropic
and anisotropic prismatic conductors. Anisotropy is modeled by alternating horizontal or vertical thin layers of different
resistivities (the vertical layers are parallel to the prism strike). Using these models, we examine conditions under which
the magnetotelluric and magnetovariational response functions distinguish between isotropy and anisotropy. The resolution
of MT and MV studies depends on the sediments conductance, lithosphere resistance and deep conductor width. Calculations show
that the most favorable conditions for anisotropy studies are observed in the active regions characterized by small sediments
conductance (10–20 S) and moderate lithosphere resistance (108 Ohm·m2). However, in the stable regions, where sediments conductance exceeds 50–100 S and the lithosphere resistance comes up to
109 Ohm·m2, the crustal and mantle anisotropic and isotropic conductors manifest themselves in the equivalent magnetotelluric and magnetovariational
functions, which cannot distinguish between anisotropy and isotropy and admit both the interpretations. 相似文献
13.
The response of a rectangular plate to the Crone PEM system has been investigated for different values of depth, dip and conductance parameters. The conductance aperture diagram is presented to estimate the conductance and depth to the top of a vertical sheet. Information regarding dip and depth may be obtained using the appropriate nomogram. 相似文献
14.
The magnetometric resistivity (MMR) method uses a sensitive magnetometer to measure the low-level, low-frequency magnetic fields associated with the galvanic current flow between a pair of electrodes. While the MMR anomalies of simple structures such as dikes and vertical contacts have been determined analytically, there is a lack of systematic information on the expected responses from simple three-dimensional bodies. We determine the characteristic anomalies associated with square, plate-like conductors, which are excellent models of many base metal mineral deposits. The anomalies of plates of finite size are determined numerically using an integral equation method. A plate is subdivided into many sections and the current flow within each section is solved by equating the electrical field within each section to the tangential electrical field just outside it. When the plate size is small in relation to either the depth or the transmitter spacing, the shape and amplitude of the anomaly produced is closely approximated by a current dipole model of the same length and depth. At the other extreme, a large plate is represented by a half-plane. The dipole and half-plane models are used to bracket the behaviour of plates of finite size. The form of a plate anomaly is principally dependent on the shape, depth and orientation of the plate. A large, dipping plate near the surface produces a skewed anomaly highly indicative of its dip, but the amount of skew rapidly diminishes with increased depth or decreased size. Changes in plate conductivity affect the amplitude of the anomaly, but have little effect on anomaly shape. A current channelling parameter, determined from the conductivity contrast, can thus be used to scale the amplitude of an anomaly whose basic shape has been determined from geometrical considerations. The separation into geometrical and electrical factors greatly simplifies both the interpretation and modelling of MMR anomalies, particularly in situations with multiple plates. An empirical formula, using this separation, predicts the anomaly of two or more parallel plates with different conductances. In addition, the relation between the resolution of two vertical, parallel plates of equal conductance and their separation is determined. The ability of the integral equation method to model plate-like structures is demonstrated with the interpretation of an MMR anomaly in a survey conducted at Cork Tree Well in Western Australia. The buried conductor, a mineralized graphitic zone, is modelled with a vertical, bent plate. The depth to the top of the plate, and the plate conductance, is adjusted to fit the anomaly amplitude as closely as possible. From the modelling it would appear that this zone is not solely responsible for the observed anomaly. 相似文献
15.
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. 相似文献
16.
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. 相似文献
17.
18.
The Normalized Full Gradient (NFG) method was proposed in the mid–1960s and was generally used for the downward continuation of the potential field data. The method eliminates the side oscillations which appeared on the continuation curves when passing through anomalous body depth. In this study, the NFG method was applied to Slingram electromagnetic anomalies to obtain the depth of the anomalous body. Some experiments were performed on the theoretical Slingram model anomalies in a free space environment using a perfectly conductive thin tabular conductor with an infinite depth extent. The theoretical Slingram responses were obtained for different depths, dip angles and coil separations, and it was observed from NFG fields of the theoretical anomalies that the NFG sections yield the depth information of top of the conductor at low harmonic numbers. The NFG sections consisted of two main local maxima located at both sides of the central negative Slingram anomalies. It is concluded that these two maxima also locate the maximum anomaly gradient points, which indicates the depth of the anomaly target directly. For both theoretical and field data, the depth of the maximum value on the NFG sections corresponds to the depth of the upper edge of the anomalous conductor. The NFG method was applied to the in-phase component and correct depth estimates were obtained even for the horizontal tabular conductor. Depth values could be estimated with a relatively small error percentage when the conductive model was near-vertical and/or the conductor depth was larger. 相似文献
19.
The induced polarization response for a 2D horizontal cylinder embedded in a half-space is calculated for a uniform electric source. Response curves, in the form of apparent charge-ability taking into account the effect of the air-earth interface, exhibit a sharp decrease in amplitude with an increase in depth of burial of the target. The resistivity contrast between the cylinder and the host plays a dominant role in determining the IP response, i.e. the amplitude decreases considerably with the increase in resistivity contrast. The decrease is due to the defocusing effect caused by the resistive cylinder. The current lines tend to deviate away from the cylindrical target. In the case of a highly conducting cylinder, apparent defocusing takes place as current lines are confined to the surface of the conducting cylinder. An increase in chargeability contrast is reflected as a steady rise in the response. The peak response at the centre is reduced by about half the magnitude when the air–earth interface is not considered. The variation of response along the profile, though noticeable, is not as high as that obtained at the centre. 相似文献
20.
The suggested interpretation technique is based on discrete linear filtering of VLF data. The output of the described filtering results is expressed in terms of an equivalent current density at a specified depth that would cause the measured magnetic field. The most practical six-point filter gives an accuracy of 8%. The filter is an extension of the commonly used Fraser filter to process VLF dip-angle data. Filtering the same data set for various depths gives an idea about the change of current density with depth. Areas with high current-density correspond to good conductors. The conductor dip can also be determined. The use of the technique is illustrated on theoretical and field examples. In all cases a good correlation with original models and other types of geophysical measurements was obtained. As shown in the last example, the filtering technique is also applicable in interpretation of other electromagnetic methods. 相似文献