Gold - sulphide mineralization in ultramafic-mafic-granite complex of Jashpur,Bastar craton,central India: Evidences from geophysical studies

Time Domain Induced Polarization (TDIP) investigation in conjunction with high resolution electrical resistivity tomography surveys carried out at seven sites along a 5.2 km line in Jashpur and Raigarh districts of Chhattisgarh State yielded promising results leading to the demarcation of mineralized zones. These sites are confined to ultramafic-mafic-felsic intrusive complexes representing gabbro-pyroxenite-granite sequences characterized by gold - sulphide mineralization. Two dimensional (2D) geophysical datasets yielded interesting results and a good correlation is found between 2D inverted resistivity tomography and time domain induced polarization (IP) models. Both high resistivity-high chargeability as well as low resistivity-high chargeability conditions have been inferred from these datasets. This observation is consistent with conductive metallic mineralization. Interpretation of the Pharsabahar-Pandripani site, reveals a chargeability magnitude of 9-14 mV/V, which represents a strong metallic conductor associated with gold-sulphide ore body. The results at Samarkachar site are equally significant, showing a good correlation with the presence of a low resistivity and high chargeability zone. Hence, the present study suggests a new scope for application of resistivity and IP tomography surveys for mineral prospecting in favourable geological settings.     

3D behaviour of contamination in landfill sites using 2D resistivity/IP imaging: case studies in Portugal

Aiming at defining a valid spatial contamination model, resistivity and induced polarization (IP) measurements were used to investigate contamination plumes in the vicinity of two municipal landfills (Ovar and ílhavo). Previous geophysical surveys and underground water samples confirmed the contamination. However 2D resistivity/IP surveys enabled in obtaining a more accurate spatial model. The Ovar survey consisted of two profiles with nine Wenner soundings each; the ílhavo survey was carried out along two individual lines using a Wenner standard pseudo-section. In both situations, negative IP values were found associated with positive IP values, which can be explained mainly by 2D or 3D geometric effects caused by the presence of the conductive plumes. The data were modelled using a 2D inversion program (RES2DINV) and the resulting resistivity and chargeability distributions were displayed as pseudo-sections. The resistivity and chargeability pseudo-sections define the contamination plumes and the sedimentary structure. These case studies illustrate the advantages of 2D resistivity/IP surveys for the mapping of shape and dimension of contamination associated with landfills.     

考虑激发极化效应的瞬变电磁一维Occam反演

大地的感应激发极化效应有时会在瞬变电磁晚期响应上产生负值变号现象。常规实电阻率瞬变电磁反演由于没有考虑激发极化效应,对于观测数据负值部分的解释一直存在问题,这影响了反演解释的真实性和可信度。对此,本文首先将Cole-Cole复电阻率模型加入正演计算,并验证了计算的准确性。然后在阻尼最小二乘法的基础上加入Occam平滑约束来构建反演方程,能够同时反演出零频电阻率、充电率、时间常数以及频率相关系数,实现了一维瞬变电磁复电阻率反演算法。最后,建立具有不同程度激发极化效应的典型层状理论模型进行反演试算与结果分析,结果表明,在激发极化效果明显的低阻高极化地层中,复电阻率反演效果更好。与实电阻率反演结果的对比说明,瞬变电磁复电阻率反演既可以达到实电阻率的常规反演效果,也能解决实电阻率无法实现的负值拟合问题。     

时域激电法单极-偶极装置在铅-锌矿勘查中的应用效果

文章描述了测区地质概况和岩(矿)石电性特征,结合图示简述了测深装置原理、布极方式、探测深度等.通过时间域激发极化法电阻率和充电率特征,根据中间梯度法推测了断裂和主要岩性分界,浸染状铅-锌金属硫化物在平面上的走向和矿化范围,依据单极-偶极测深能够有效地确定含铅锌浸染状金属硫化物矿体空间位置,结合点源梯度剖面上充电率异常特征,推测了极化体产状.钻探验证在断层中见铅锌矿化,取得了较好的地质效果.     

Case study: a 3D resistivity and induced polarization imaging from downstream a waste disposal site in Brazil

A contaminated site from a downstream municipal solid waste disposal site in Brazil was investigated by using a 3D resistivity and induced polarization (IP) imaging technique. This investigation purpose was to detect and delineate contamination plume produced by wastes. The area was selected based on previous geophysical investigations, and chemical analyses carried out in the site, indicating the presence of a contamination plume in the area. Resistivity model has successfully imaged waste presence (ρ < 20 Ωm), water table depth, and groundwater flow direction. A conductive anomaly (ρ < 20 Ωm) outside wastes placement was interpreted as a contamination plume. Chargeability model was also able to imaging waste presence (m > 31 mV/V), water table depth, and groundwater flow direction. A higher chargeability zone (m > 31 mV/V) outside wastes placement and following conductive anomaly was interpreted as a contamination plume. Normalized chargeability (MN = m/ρ) confirmed polarizable zone, which could be an effect of a salinity increase (contamination plume), and the clay presence in the environment.     

Inversion of IP-affected TEM responses of a two-layer earth

A numerical experiment has been applied to explore the potentialities and the limitations of the inversion of IP-affected TEM responses of a two-layer earth with a polarizable layer above (Model 1) or below (Model 2) a nonpolarizable layer. The IP effects have been incorporated into the models via a Cole–Cole complex frequency-dependent conductivity. One of us computed synthetic in-loop and coincident-loop transient responses with added Gaussian noise, and the other performed single and joint inversion of the two sets of pseudo-experimental data. Model 1 turns out to be advantageous over Model 2 in TEM applications and gives a good fit of the Cole–Cole parameters (chargeability, IP time constant, and exponent) even in the absence of a priori information. In the case of Model 2, the lack of a priori information causes problems as to recognize which layer is polarizable, and the fit of the Cole–Cole parameters is generally worse. The layer thicknesses and resistivities are rather accurate in both groups of models, irrespective of whether a priori information is available. As the upper layer increases in thickness (H₁), the fit of its parameters ever improves in both models while the parameters of the lower layer, on the contrary, contain a greater error. Joint inversion of in-loop and coincident-loop transients improves the fit in most cases.Relative rms error (σ_rel) does not depend on the upper layer thickness for Model 1 but decreases as H₁ increases in the case of Model 2. The error in joint inversion is times that in single inversion, which means that additional criteria other than σ_rel may be useful to estimate the inversion quality.     

地球物理学方法在区分多年冻土区咸水层和淡水融化层中的应用

Direct current resistivity and ground penetrating radar surveys were employed to obtain the value of the resistivity and dielectric constant in the brine near the Barrow, Alaska. The geophysical surveys were undertaken together with the permafrost drilling program for the measuring of the ground temperature regime and for the core sampling. The sampled cores were measured for their physical and chemical properties in the laboratory under different temperature conditions (-60 to 20 ℃). Laboratory results support field observations and led to the development of a technique for distinguishing freshwater taliks and brine layers in permafrost. These methods were also employed in freshwater taliks near Council,Alaska. The electrical resistivity is a powerful and sensitive parameter for brine detection. However, the resistivity is a less sensitive indicator of the soil type or water content under highly saline conditions.High frequency dielectric constant is an ideal second parameter for the indication of the soil type, liquid water content and other physical properties. The imaginary part of the dielectric constant and resistivity have a significant dependence upon salinity, i.e. upon freezing temperature. The ground temperature regime and the freezing point of the brine layer are important parameters for studying the electric properties of permafrost terrain.     

三氯乙烯污染土壤的复电阻率特征和频谱参数研究

复电阻率法(complex resistivity, CR)由于其对孔隙水化学性质、固-液界面和流体含量的敏感性已成为具有良好前景的污染物识别工具。本研究选取三氯乙烯(trichloroethylene, TCE)作为典型有机污染物,利用频谱激电仪对TCE污染土壤进行了复电阻率测量,讨论孔隙水盐度、水饱和度和土壤类型对污染土壤的复电阻率特征和频谱参数影响。试验结果表明:污染土样的复电阻率均随水饱和度的降低和孔隙水盐度的减小而增大;土壤黏土含量影响孔隙水连续性,当孔隙水出现不连续状态时,实部和虚部出现大幅度变化。总充电率M随着孔隙水盐度的增加而增加,随水饱和度的降低先减后增。黏土颗粒重排导致的比表面积减小使得总充电率M减小,双电层极化加强使总充电率M升高;平均弛豫时间常数随孔隙水盐度的升高而降低,二者呈对数相关,随水饱和度变化的主要原因为可极化孔隙尺寸的变化。本研究探讨了地球物理参数与水文地质参数间的关系,为SIP方法在实际有机污染场地调查与评估提供了理论依据。     

内蒙古陈台屯斑岩型铜矿地质、地球物理特征 及找矿方向

内蒙古陈台屯铜矿区是一个以斑岩型铜矿为主要目标的勘查区,位于大兴安岭中段,成矿时代为中侏罗世。目前在矿区发现了斑岩型铜矿化和脉状铜矿化两种矿化类型。斑岩型铜矿化主要形成于陈台斑岩体顶部与二叠系大石寨组安山质火山岩的内、外接触带上,围岩蚀变作用强烈,并具有明显的分带特征,自下而上、由内到外可分为钾化带、黄铁-绢英岩化带、泥化带和青磐岩化带,斑岩型铜矿化主要集中于黄铁-绢英岩化带中。脉状铜矿主要充填于万宝组沉积地层和大石寨组火山岩的裂隙中,围岩蚀变仅发育黄铁-绢英岩化和弱青磐岩化。通过对矿区开展激电中梯测量和CSAMT电阻率测深并施工钻探验证,发现高极化率异常与斑岩型铜矿体和脉状铜矿体均具有良好的空间对应关系,可以作为下一步铜矿勘查的重要目标;低电阻率异常既可以由斑岩型铜矿化作用所引起,又可以由孔隙度较高的万宝组砂岩所引起,因此首先需要区分异常形成的原因,进而用低电阻率异常指导找矿,但高电阻率异常通常代表了致密的、不含矿的地质体。     

Interpretation of lithogeochemical and geophysical data to identify the buried mineralized area in Cu-Au porphyry of Dalli-Northern Hill

Geochemical and statistical analyses of 165 soil samples led to Cu-Au anomaly separation in Dalli-Northern Hill by using fractal geometry and U-spatial statistic. There was a good correlation between the anomaly areas and outcrops of quartz diorite porphyry (QDP) in the study area. Based on the interpretation of soil data and anomaly areas determination, one trench was suggested in NE-SW direction for follow-up exploration. Based on the rock sample analyses from the trenches in QDP rocks, the ratio of $ \frac{{{\mathrm{Ba}} \times {\mathrm{K}}}}{{{\mathrm{Ca}} \times {\mathrm{Zn}}}} $ clearly separated the mineralization area and considered as an index ratio for follow-up exploration stage. Dalli-Northern Hill anomaly includes pyrite/chalcopyrite/bornite, which is dominantly abundant in the ore body. Three IP profiles (IP04, IP05, and IP06) confirmed the conductivity of the sulfide zone. The background chargeability in the study area was about 5.4 mV/V and the average of maximum apparent chargeability in the study area was 44.4 mV/V, which could be related to the higher intensity of Cu-Fe sulfide minerals. The contact of andesite and quartz diorite has shown the strongest chargeability (71 mV/V) and high magnetic anomalies in the study area. DDH03 and DDH04 boreholes mostly confirmed that the IP/magnetic/geochemical anomalies were related to the contact of intrusive rocks and the wall rock of andesite. The combination of geochemical information from soil and rock together with geophysical data (induced polarization/resistivity) led to the subsurface geological cross sections and location of the mineralized zone enriched of sulfide. The mineralized zone in Dalli-Northern Hill distinct could be identified by potassic to weak sericitic and trace chloritic alteration, high values of the $ \frac{{{\mathrm{Ba}} \times {\mathrm{K}}}}{{{\mathrm{Ca}} \times {\mathrm{Zn}}}} $ ratio accompanied with high gold and copper values and enrichment from magnetite minerals. The high magnetic susceptibility, low resistivity, and high chargeability are considered as geophysical properties of the mineralized zones.     

求地下极化体真柯尔-柯尔参数的研究

对求地下极化体真柯尔-柯尔参数的方法进行了研究。研究表明:诺模图校正法只在已知大地的电阻率分布并且极化体的电阻率与围岩接近时才可以使用;在大地电阻率分布未知的情况下,用反演单个测点视谱的方法不可能求得极化体的真柯尔-柯尔参数;对一些简单的大地模型可以用联合反演2个或多个测点视谱的方法求极化体的真柯尔-柯尔参数;先求大地模型的电阻率分布,再对单个测点的视谱进行反演也可能是1种求真柯尔-柯尔参数的合适的方法。     

利用共轭梯度方法的激发极化三维快速反演

利用共轭梯度方法实现了激发极化(IP)三维快速反演。首先,利用共轭梯度方法反演电位数据,得到地下的三维电阻率模型,(由于避免了直接求偏导数矩阵,因此反演计算速度很快。)然后,以此电阻率模型为地下电导率分布,再反演IP数据得到三维极化率分布理论模型。试算结果表明其效果较好。      

Integrated geophysical and geological studies for mineral prospecting in Betul-Chhindwara belt (BCB), Central India

Integrated study combining high resolution electrical resistivity tomography and time domain induced polarization was carried out in Betul-Chhindwara belt Madhya Pradesh, Central India in order to evaluate and delineate the polymetallic sulphide mineralization, its nature, type of deposit and depth. On interpretation of the models results clear cut anomalies revealed showing chargeability ～2 to 54 mV/V up to a maximum depth of 131m. This range of chargeability signify signature of metallic conductor. Nevertheless the basement rock is clearly mapped, showing substantial resistivity contrast. In addition detailed analysis of the integrated results from geology, geochemistry and Scanning Electron Microscope–Energy Dispersive X-ray Spectroscopy resulted from in situ rock samples shows good correlation with resistivity and IP results. This integrated study confirms the presence of conducting sulphide mineral ore body and the results and findings need test drilling at the geophysical anomalous site(s) to confirm the depth persistence and evaluate the metallic conductor.     

激发极化法在内蒙古阿荣旗矿产远景调查中的应用

激发极化法是寻找硫化物金属矿床的一种有效方法,被广泛应用于矿产勘查中.在内蒙古阿荣旗矿产远景调查中,运用时间域激发极化法,通过激电中梯扫面测量后,绘制视电阻率与视极化率等值线平面图,迅速地圈定激电异常.依据地质与化探调查结果,选择其中3个激电异常区做激电测深.通过对激电测深数据反演,得到极化率与电阻率断面图,确定了潜在矿（化）体的形态.     

时域激电单极-偶极、偶极-单极排列测深法二维地电断面成像

介绍了不对称测深法装置的测深原理;在吉林某地进行了激电测深勘探,结果显示:正向单极-偶极和反向偶极-单极采集的视电阻率、视充电率原始数据在拟断面图的分布特征,和大地电磁测深原始数据受到的静态效应类似,视电阻率、视充电率受局部地形和电磁噪声的影响,在拟断面图中呈假的带状异常。正向单极-偶极、反向偶极-单极装置获取的视电阻率、视充电率数据也可分别进行二维反演,但联合单极-偶极/偶极-单极装置测深二维反演地质结果最好,具有采集的数据量大、激电信号强 、穿透深度大、勘探精度高等优点,根据视电阻率、视充电率二维反演在地电断面成像技术,能够准确确定电性异常体的空间分布,为钻探验证电异常提供准确的地球物理依据。     

Magnetic measurements in electrical prospecting by resistivity methods

The electrical resistivity and induced polarization (IP) methods are widely used in geological mapping, prospecting and exploration of mineral deposits, engineering geology, hydrogeology, archaeology, and geotechnical and environmental applications. Historically, these methods have formed the basis of the electrical prospecting technique. In these methods, a DC or low-frequency AC electrical current is introduced into the earth through a grounded transmitter line. The measured quantity is the electric field. However, if the earth’s resistivity or chargeability changes horizontally, this change gives rise to an anomalous magnetic field, which is studied by the magnetometric resistivity (MMR) and magnetic induced polarization (MIP) methods, respectively. Along with advantages, some shortcomings are inherent in the MMR and MIP techniques. Apparently, the main drawback of these methods is that the magnetic fields of both the transmitter line wire and ground electrodes on the surface are several orders of magnitude greater than the anomalous magnetic field response. This introduces a significant “noise” to magnetic-resistivity data. We investigate the potential of using a circular electric dipole (CED) in magnetometric resistivity techniques. It has been found that the application of a CED, instead of a conventional transmitter line, dramatically enhances the signal-to-noise ratio.     

层状极化介质激发极化效应特征研究

这里研究层状极化介质激发极化效应特征。首先,在忽略电磁效应的情况下,用柯尔～柯尔模型表征极化介质复电阻率,计算三层不同地电断面复电阻率的振幅和相位随极距变化的曲线;然后,分析极化层不同充电率和不同时间常数、不同激发频率和极化层不同埋深,对测深曲线的影响规律,并提出了具有实际指导意义的极化层埋深与极距的对应关系。这些工作都能够为野外开展复电阻率测量和资料解释起到参考作用。     

高密度激发极化法测量在峪耳崖金矿区的应用

运用美国生产的高密度电法仪器对峪耳崖金矿区16~28线进行了测量。该仪器可同时测得视电阻率和视充电率2种数据,并可自动成图和地形改正。通过高密度电法工作圈定了地表较浅部的矿化异常区,除主要矿化异常赋存于岩体接触带外,在灰岩中也发现高极化异常区,表明在灰岩地层内也存在金属硫化物矿化现象,这为下一步找矿工作提供了新的思路和方向。通过钻探结果确定该矿区异常区有2种类型,即高阻高充电异常和低阻高充电异常,主要分布在岩性的接触带附近,尤其是内接触带。     

火炉岭脚-棚坞多金属矿区地球物理特征

火炉岭脚-棚坞多金属矿区开展的电法综合勘探工作和矿区内视极化率、视电阻率异常特征综合分析表明,该异常体为高极化高阻体,并推断出其产状变化和分布范围;同时结合矿区的区域地质情况以及已实施的槽探和钻孔资料,判定出该矿化赋存于岩体接触带内,且呈浸染状矿化。由于推断区内矿化体的产状和岩体接触面产状不一致,且矿化体的产状自东向西变化较大,因此认为该区多金属成矿不仅受到区内接触带蚀变的影响,更主要受到断裂构造的控制。     

激发极化法在四川红层地区水资源勘察中的应用

阐述了激发极化法的基本原理和技术方法,利用激发极化法的极化率、电阻率、衰减度半衰时、充电率等极化参数在四川红层地区寻找地下水,并探讨了红层地区的含水层位,以及含水层位的确定。该方法受地形影响小,曲线反映异常直观,能有效解决红层地区的找水任务。