Interpretation of ground VLF-EM data in terms of inclined sheet-like conductor models

The response of two-dimensional, inclined, sheet-like conductors with, low conductance values to plane wave electromagnetic fields in the very low frequency (VLF) range has been evaluated by using a numerical technique. The conductance values of the conductors considered are appropriate for those produced by water and/or clay-filled fracture and shear zones in the Precambrian crystalline rocks of the Canadian Shield. The surrounding host rock was assumed to be, resistive with resistivities in the 1–10 k.m range to reflect the high resistivities over the shield areas. No overburden was assumed in this analysis.The results of the computations are presented in the form of characteristic interpretation diagrams to interpret ground VLF data in the field, where facilities for direct numerical modelling may not be available. A method for interpreting ground VLF data using such characteristic diagrams has been proposed in this paper which requires a prior knowledge of the host rock resistivity and the inclination of the conductor. These two parameters may be derived from a VLF resistivity survey and from appropriate filtering of the VLF tilt angle response. The interpretation method was applied to a ground VLF anomaly obtained at a research site near Atikokan in NW Ontario, which yielded an interpretation compatible with information from geological mapping.Geological Survey of Canada Contribution No. 51888.     

Airborne Geophysics: Application to a Ground-Water Study in Botswana

Summary
A ground-water study carried out in the Serowe area of eastern Botswana between 1985–1988 has provided the opportunity to evaluate the role of a multiparameter low-level airborne geophysical survey in a hydrogeological investigation. The survey included magnetic, VLF (very low frequency), and coaxial EM (electromagnetic) measurements. In total, 7,500 line kilometers were flown over an area of 3,300 km2 with a nominal ground clearance of 20 m and a line spacing of 400 m.
The main aquifer, the Ntane Sandstone Formation (Karoo age), is confined between mudstones below and basalt above, and is broken into a series of graben and horst structures by numerous E-W striking faults. All bedrock, however, is completely masked by a 20–60 m thick overburden of sands, calcretes, silcretes, and sandstones known collectively as the Kalahari beds.
Airborne magnetic and VLF geophysical surveys have been used to penetrate this masking cover. Images and stacked profiles obtained from the survey revealed structural and geological features of major hydrogeological significance. This provided the information necessary for the formulation of a conceptual model.
The results helped guide the subsequent exploration drilling program in an efficient and effective manner, cutting down the need for extensive ground surveys. The investigation confirmed the availability of a 35,000 m3/day resource, sustainable for a 25-year period. Highest yields were obtained from fracture zones associated with VLF anomalies. Potential wellfields were identified in confined sections of the aquifer, with production boreholes to be sited, where possible, on fractures associated with VLF anomalies.     

Integrated Geophysical Studies in the East-Indian Geothermal Province

Integrated geophysical surveys using vertical electrical sounding (VES), very low frequency (VLF) EM, radiation counting, total magnetic field and self-potential (SP) measurements are carried out to characterize the geothermal area around a hot spring in the Nayagarh district, Orissa, India that lies in the East Indian geothermal province. The study was performed to delineate the fracture pattern, contaminated groundwater movement and possible heating source. VES interpretations suggest a three- to four-layer structure in the area. Resistivity survey near the hot spring suggests that weathered and fractured formations constitute the main aquifer system and extend to 60 m depth. Current flow measured at various electrode separations normalized by the applied voltage suggests that fractures extend to a greater depth. Detailed VLF study shows that fractures extend beyond 70 m depth. VLF anomaly has also very good correlation with the total magnetic field measured along the same profiles. Study results suggest that a gridded pattern of VLF survey could map the underground conductive fracture zones that can identify the movement of contaminated groundwater flow. Therefore, precautionary measures can be taken to check further contamination by delineating subsurface conducting structures. Self potential (SP) measured over the hot spring does not show a large anomaly in favor of the presence of a sulphide mineral body. A small positive (5–15mV) SP anomaly is measured which may be streaming potential due to subsurface fluid flow. A high radiation is measured about four kilometers from the hot spring, suggesting possible radiogenic heating. However, the exact nature of the heating source and its depth is not known in the area. Deep resistivity followed by a magneto-telluric survey could reveal the deeper structures.     

Geophysical techniques to aquifer locating and monitoring for industrial zones in North Hanoi, Vietnam

Geophysical methods were applied for hydrogeological targets in many countries including Vietnam. This paper presents results of using complex geophysical techniques as well as 2D electrical resistivity imaging (ERI), vertical electrical sounding (VES), very low frequency (VLF), and seismic refraction for geological structure investigation for locating the aquifers and assessing the hydrogeological conditions for groundwater potential in industrial zones of North Hanoi, Vietnam. The locations of two aquifers are determined by their depth and thickness on the basis of resistivity and seismic velocity values which were proved by stratifications of three boreholes to 40–60 m of depth on the study area. There are connections from surface water to shallow aquifer by hydraulic windows, as follows from VLF data. The deeper aquifer can be considered as a potential groundwater supply, but the water level is descending in time, as shown by hydrological monitoring. However, with careful use and by reducing sources of pollution, groundwater can continue to be an important natural resource for future.     

黄土覆盖区钼矿综合地球物理找矿技术组合:以沙坡岭钼矿为例

覆盖区寻找隐伏矿成为当前找矿工作的热点问题,为了能在地表地质信息少的黄土覆盖区,寻找隐伏的有经济价值的矿床,本文以沙坡岭钼矿为例,综合应用r能谱、高精度磁法、高密度电法三种地球物理方法进行找矿勘探,辅以偏提取化探法对地球物理异常进行约束,结合地质资料进行解译,预测了有利的成矿区域,通过钻探得到了有效验证,初步建立了黄土覆盖区钼矿的综合地球物理找矿模式,为下一步的综合勘探打下了基础.     

基于多元信息约束的隐伏矽卡岩型铁矿床的定位——以云南北衙金矿区红泥塘矿段为例

矽卡岩型矿床是云南北衙金多金属矿区的一种主要矿床类型,受岩体、构造及层位等控制,其有利找矿空间为中酸性岩体与碳酸盐岩的接触带.以岩石物性为纽带,实现目标地质体与地球物理场的转换,提取有效信息.接触带中低电阻率、岩体的低密度性、铁矿的高磁性和高极化率等都是矽卡型铁矿床的特有地球物理属性.通过地质地球物理等多元信息的层层约束,解析不同级次的成矿要素问题,实现对隐伏矽卡岩型铁矿床的定位,建立地球物理勘探模式,并在矿区外围开展方法有效性的验证,取得了良好的勘探效果,提出一套适应于北衙地区及类似成矿地质背景下的隐伏矽卡岩型铁矿床定位预测方法.     

Assessing the contribution of electrical resistivity tomography (ERT) and self-potential (SP) methods for a water well drilling program in fractured/karstified limestones

ERT and SP investigations were conducted in carbonate rocks of the Dinant Synclinorium (Walloon Region of Belgium) to find suitable locations for new water wells in zones with little hydrogeological data. Since boreholes information needed to be representative of the area, large fractured zones were searched for the drillings. Large ERT profiles (320 to 640 m) allowed us to image the resistivity distribution of the first 60 m of the subsurface and to detect and characterize (in terms of direction, width and depth) fractured zones expected to be less resistive. Data errors, depth of investigation (DOI) indexes and sensitivity models were analyzed in order to avoid a misinterpretation of the resulting images. Self-potential measurements were performed along electrical profiles to complement our electrical results. Some negative anomalies possibly related to preferential flow pathways were detected. A drilling campaign was conducted according to geophysical results. ‘Ground truth’ geological data as well as pumping tests information gave us a way to assess the contribution of geophysics to a drilling program. We noticed that all the wells placed in low resistivity zones associated with SP anomalies provide very high yields and inversely, wells drilled in resistive zones or outside SP anomalies are limited in terms of capacity. An apparent coupling coefficient between SP signals and differences in hydraulic heads was also estimated in order to image the water table.     

MAGNETIC WAVETILT MEASUREMENTS FOR GEOLOGICAL FRACTURE MAPPING1

The measurement of wavetilt is diagnostic for determining the electrical characteristics of the upper layers of the ground at VLF and LF frequency ranges. Theoretical and field studies have indicated that electric wavetilt using the transverse magnetic (TM) waves detects lateral inhomogeneities virtually instantly as abrupt changes in electrical properties are encountered. Theoretical studies have also indicated that magnetic wavetilt measurements using transverse electric (TE) waves are superior to electric wavetilts for such purposes. An experimental survey was conducted at two locations near Atikokan, Ontario, to verify the theoretical predictions. The survey area, forming a part of a large granitic pluton, was mapped earlier by various geophysical techniques, including the ground VLF-EM method, to detect weak conductors formed either by the presence of fractures in the bedrock filled with water and/or clay, or overburden filling bedrock depressions. A small, multi-turn, horizontal loop was used during the survey as the transmitter to generate TE waves at eleven frequencies from 10.7 to 58.5 kHz. The magnetic wavetilt measurements detected all previously known conductors at the two locations. In addition, the survey detected several weak conductors that were missed by the VLF survey. Thus, the survey indicated the usefulness of magnetic wavetilt results for detection of weak conductors at shallow depths, which may have application in engineering geophysical surveys. The multi-frequency wavetilt data also provided some indications of the depth and depth extent of such conductors.     

Two-dimensional, regularised inversion of VLF data

Very low frequency electromagnetic (EM) methods using VLF transmitters have found many applications in subsurface geophysical investigations. Surface measurements involving both the vertical component of the magnetic field (VLF-EM or VLF-Z) and of the apparent resistivity (VLF-R) are increasingly common. Although extensive VLF data sets have been successfully used for mapping purposes, modelling and interpretation techniques which asess the third (i.e. depth) dimension appear limited.Given a profile of VLF-R measurements the main purpose of the present study is to demonstrate an automatic method for the construction of a resistivity cross-section. The technique used is one of a new generation of regularised inversion methods. These techniques attempt to overcome the problem of equivalence/non-uniqueness in EM sounding data by constructing the resistivity distribution with the minimum amount of structure that fits the data.VLF data represent a special case of plane-wave EM sounding in that they conform, in practice, to a single-frequency technique. This fact imposes a limitation in the amount of vertical resolution that we can expect using such data. In the case of two-dimensional modelling and inversion, resolution through the cross-section is a resultant attribute from both vertical and lateral resistivity gradients within the subsurface. In order to provide insight into the practical application of regularised inversion techniques to VLF data, both synthetic and field examples are considered. Both sets of examples are primarily concerned with VLF data applied to near-surface fault mapping where the main aim is to assess the location, dip and depth extent of conductive subsurface features.     

An airborne tensor VLF system. From concept to realization1

Radio signals from very low frequency (VLF) transmitters distributed world-wide have been used for several decades to study the lateral variations of the electrical conductivity in the upper few hundred metres of the earth's crust. Traditionally, in airborne applications, the total magnetic fields from one or two transmitters are measured to form the basis for construction of maps that primarily show those conductive structures that are parallel or subparallel to the direction to the transmitters. The tensor VLF technique described in this paper makes use of all signals available in a predefined frequency band to construct transfer functions relating the vertical magnetic field and the two horizontal magnetic field components. These transfer functions are uniquely determined for a particular measuring site and contain information about the lateral conductivity variations in all directions. First experiences with real field data, acquired during a test survey in Sweden, show that maps of the so-called peaker, the spatial divergence of the transfer functions, give an image of the conducting structures. Most of the structures can be correlated to small valleys filled with conducting sediments or valleys underlain by conductive fracture zones in the crystalline rocks.     

Shallow karst exploration using MT-VLF and DC resistivity methods1

A geophysical test was carried out over a well-located and fairly embedded karstic conduit. The MT-VLF method was selected because of its high resolution and its ability to provide a resistivity parameter sensitive to water and Clay. This method was used together with DC resistivity methods which allow vertical adjustment of the VLF data and show consistency between the investigation and target depths. After correcting the deformations due to the polarization of the primary field, the MT-VLF data show clearly, in the Central part of the site, that the conduit does not coincide with an anomaly axis but coincides with the boundary between a conductive area and a resistive area. 2D modelling confirms that direct detection of the conduit is not feasible and that the conduit is located close to a conductive zone corresponding to a completely Clay-filled fractured zone. This situation was observed on the whole site and the conduit seemed systematically to thread its way between the conductive zones to join the outlet. The distribution of the conductive fractured zones and the direction of the hydraulic gradient were two important elements in predicting the location of the conduits. A 3D approach would increase the probability of finding the conduits in such a case.     

Integrated geological and geophysical exploration for concealed ores beneath cover in the Chaihulanzi goldfield, northern China

An integrated geological and geophysical investigation was carried out in the near‐mine areas of the Chaihulanzi gold‐field, Inner Mongolia, northern China, to determine the south‐eastern strike extension of the principal mineralized shear system, in the search for new resources. In this case study, surface geophysical surveys played an important role in defining the mineralized structures beneath cover. A re‐investigation of the mine geology by observing the underground exposures of mineralization and its hosting structures, coupled with re‐evaluation and re‐examination of previous exploration data, revealed that the mineralization style at the Chaihulanzi gold‐field is not the skarn‐type as previously assumed, but is structurally controlled by an oblique‐sinistral shear structure. Moreover, a south‐eastward shallow‐plunging mineralized corridor within the principal mineralized shear system was also identified, implying that the ore‐forming fluid‐flow might move through the mineralizing system, from the south‐east at depth to the north‐west. These new geological findings imply that the mineralized shear system should extend much farther south‐eastwards along strike beneath cover, and thus a conceptual target area was proposed. A detailed geophysical survey program, involving the application of VLF‐EM, Stratagem EH4, CSAMT and gradient IP measurements, was carried out to test the validity of the conceptual target. Orientation surveys on the geologically known Line‐18 traverse revealed that VLF‐EM and Stratagem EH4 were the most effective methods of detecting the unseen mineralized system, whereas CSAMT could only provide low‐resolution data, and IP proved to be unsuitable in this environment due to the ubiquitous presence of graphite‐bearing schists in and around the mineralized system. A follow‐up systematic VLF‐EM survey highlighted the principal mineralized shear system as a linear conductive belt, tracing the principal mineralized system for a further 750 m of strike length beneath cover from the previous exploration limit (Line 18). Stratagem EH4 soundings over six parallel traverses perpendicular to the mineralized trend revealed that the principal mineralized shear structure extends for more than 500 m in the dip direction, a strong indication of the presence of potential deep mineralization under the surveyed area. Detailed modelling of the Stratagem EH4 sounding images provided well‐defined targets for test drilling. Subsequent test drilling on some of these targets returned encouraging results as several core‐intercepts of economic gold mineralization were encountered. This led to a more extensive drilling and underground prospecting program, focusing on deep mineralization south‐eastwards along the mineralized trend.     

综合物探方法在滑坡地质勘探中的应用研究

综合物探方法在滑坡等地质灾害的勘探中发挥着重要的技术作用。这里以河南省某地滑坡勘探为例,利用高密度电阻率法和地质雷达法等综合物探方法,分析两种物探方法的技术原理、联合解释,并结合地质钻探、地质调查等资料,查明该场地滑坡体的分布情况。结果表明,综合物探方法能有效地减少物探的多解性,提高成果解释精度,其综合物探解释成果与地质情况较为吻合,显示出综合物探方法在滑坡等地质灾害勘探中的重要作用。最后,对综合物探方法及综合解释进行了简要的总结和展望。     

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.     

Resolution of airborne VLF data

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.     

甚低频电磁法龙头山银铅锌矿床勘查中的应用

随着找矿、勘探与开发生产实践的不断深入,表露矿与浅部矿渐趋减少,找矿难度逐渐增大,找到一种简捷、迅速的勘查、探矿方式,迫在眉睫.根据实践证明,甚低频电磁法是较为理想的选择.在运用甚低频电磁法时,通过对所测数据进行Fraser滤波等技术处理,并且结合矿区地质研究控矿规律及矿体赋存规律,从而有效地圈定掩盖区异常地质体及其产状和展布方向,预测矿体空间赋存部位,为深部找矿提供依据.     

部分区域约束下的交叉梯度多重地球物理数据联合反演

交叉梯度联合反演方法通过对多种地球物理模型实现结构耦合,在岩石物性关系不确定的情况下,既能提高反演结果的可靠性,又能减少反演的多解性,还能减少不同方法解释结果之间的矛盾.当不同的模型观测数据覆盖范围不一致时,交叉梯度联合反演通常需要取出重叠区域数据进行联合反演,并且建模时还要扩展一些模型范围.本文首先提出并实现了部分区域约束下的交叉梯度多重地球物理数据联合反演算法;接着进行了算法的模型试算;最后,我们将该反演算法用于本溪-集安深部地质调查重磁电综合地质地球物理解释中.结果表明：该算法不但能在重叠区域内很好地恢复结构相似的模型,而且在非重叠区域与重叠区域的边界处仍然可以得到平滑变化的模型;在本溪-集安10号剖面所获得的结构上相似的电阻率、密度及磁化率模型较好地反映了该区的深部地质结构,对于确定深部地质体的性质提供了有力的证据.     

A PORTABLE LOCAL LOOP VLF TRANSMITTER FOR GEOLOGICAL FRACTURE MAPPING*

A portable low-power Very Low Frequency (VLF) transmitter using a large square loop antenna has been designed, assembled and tested by the Geological Survey of Canada (GSC) for geological studies of fracture patterns in igneous rock masses. Standard laboratory equipment, consisting of a signal generator, a 1100-W power amplifier and several high-power tuning capacitors, was used for the purpose. Field tests at the Chalk River facilities of Atomic Energy of Canada Limited have demonstrated a remarkable similarity between survey results obtained using the VLF signals from the local loop transmitter and from distant US Navy VLF transmitters. The local loop was used to simulate the fields from navy stations NAA in Cutler, Maine and NSS in Annapolis, Maryland. Conductor axes, mapped by using these navy stations, and by using the loop antenna yielded almost identical results. A survey was also done in the same area with the local loop placed in such a manner that the direction of the VLF field was at 45° to the field directions from NAA and NSS. In this case, the same conductor axes were located with only minor shifts in position, indicating that conductors whose axes lie within 45° of the direction of the primary horizontal magnetic field are mapped. Thus, it is probably sufficient to have two sources with orthogonal VLF fields to map all VLF conductors in an area. Since in most areas at least one navy VLF station can be used, the local loop transmitter can be used to generate a signal at right angles to the direction from the navy transmitter to allow a more complete VLF survey coverage.     

Using geophysical methods to characterize an abandoned uranium mining site,Portugal

A major plan for Portugal Mainland is being envisaged to use old open pits from abandoned uranium mining sites as “Waste Containment Deposits”. These areas will store mining waste from other adjacent mines. The old mining sites classification to this kind of usage is carried out accordingly to its location, accessibility, geological and hydrogeological conditions. Mining waste deposition in the open pits may however cause environmental problems related with geological and hydrogeological features that must be predicted and prevented before a particular site is chosen. Therefore, the identification of faults and conductive zones that may promote groundwater circulation and the spread of contaminated waters is of great importance, since the surrounding area is highly populated. The possible negative environmental impacts of the presence of such potential waste disposal sites are therefore being assessed using geophysical methods and geological outcrop studies in several geological and hydrogeological critical areas. The abandoned Quinta do Bispo uranium mine is one of such places. This old open pit, chosen as one of the sites to be used in the near future as a “Waste Containment Deposit” (accordingly to the above mentioned criteria), needs to be characterized at depth to prevent any possible negative environmental impacts. Thus, the acquisition, processing and interpretation of electromagnetic, electrical and both seismic refraction and reflection have been carried out. 2D schematic models have been constructed, showing alteration and faults zones at depth. These fault zones control groundwater circulation and therefore, future water circulation problems with negative environmental impact may be predicted and prevented.     

Application of Geophysical Techniques in Identifying UNE Signatures at Semipalatinsk Test Site (for OSI Purposes)

This article describes geological and geophysical studies of an underground nuclear explosion area in one of the boreholes at the Semipalatinsk test site in Kazakhstan. During these studies, the typical elements of mechanical impact of the underground explosion on the host medium—fracturing of rock, spall zones, faults, cracks, etc., were observed. This information supplements to the database of underground nuclear explosion phenomenology and can be applied in fulfilling on-site inspection tasks under the Comprehensive Nuclear-Test-Ban Treaty.