Resistivity Log Data Research Articles

The Evaluation Groundwater Quality in Daqing Oilfield by Using Logging Data

In order to research the groundwater quality of the Taikang Formation in Tertiary in west of Daqing Oilfield, resistivity logging data of 140 wells in the study area were used to analyze the relations between the groundwater quality and sedimentary sand body and buried depth. The south area of Hongweixing well area and the east of Xishuiyuan well area, including Ranghulu, Qianjincun, Dulitun and Nanshuiyuan well area are favorable for high quality groundwater. The shallower buried water quality is better than the deeper buried water quality of the Taikang Formation in Tertiary. The research provides the evidences to evaluate the groundwater quality in west Daqing Oilfield and determine specific well location, improving the efficiency of exploring the underground drinking water.

Logging Evaluation Method and its Application for Measuring the Total Organic Carbon Content in Shale

The acquisition of the total organic carbon (TOC) content mainly relies on the geochemical analysis and logging data. Due to geochemical analysis is restricted by coring and experimental analysis, so it is difficult to get the continuous TOC data. Logging evaluation method for measuring TOC is very important for shale gas exploration. This paper presents a logging evaluation method that the shale is segmented according to sedimentary structures. Sedimentary structures were recognized by core, thin section and scanning electron microscope. Taking Wufeng-Longmaxi Formation, Silurian, Muai Syncline Belt, south of Sichuan Basin as research object, the shale is divided into three kinds: massive mudstone, unobvious laminated mudstone, and laminated mudstone. TOC within each mudstone are calculated using GR, resistivity and AC logging data, and an ideal result is achieved. This method is more efficient, faster and the vertical resolution is higher than △logR method.

Gas hydrate saturation at Site C0002, IODP Expeditions 314 and 315, in the Kumano Basin, Nankai trough

AbstractThe degree of gas hydrate saturation at Integrated Ocean Drilling Program (IODP) Site C0002 in the Kumano Basin, Nankai Trough, was estimated from logging‐while‐drilling logs and core samples obtained during IODP Expeditions 314 and 315. Sediment porosity data necessary for the calculation of saturation were obtained from both core samples and density logs. Two forms of the Archie equation (‘quick‐look’ and ‘standard’) were used to calculate gas hydrate saturation from two types of electrical resistivity log data (ring resistivity and bit resistivity), and a three‐phase Biot‐type equation was used to calculate gas hydrate saturation from P‐wave velocity log data. The gas hydrate saturation baseline calculated from both resistivity logs ranges from 0% to 35%, and that calculated from the P‐wave velocity log ranges from 0% to 30%. High levels of gas hydrate saturation (>60%) are present as spikes in the ring resistivity log and correspond to the presence of gas hydrate concentrations within sandy layers. At several depths, saturation values obtained from P‐wave velocity data are lower than those obtained from bit resistivity data; this discrepancy is related to the presence of free gas at these depths. Previous research has suggested that gas from deep levels in the Kumano Basin has migrated up‐dip towards the southern and seaward edge of the basin near Site C0002. The high saturation values and presence of free gas at site C0002 suggest that a large gas flux is flowing to the southern and seaward edge of the basin from a deeper and/or more landward part of the Kumano Basin, with the southern edge of the Kumano Basin (the location of site C0002) being the main area of fluid accumulation.

Improvements in crosshole GPR full-waveform inversion and application on data measured at the Boise Hydrogeophysics Research Site

Abstract Crosshole ground penetrating radar (GPR) tomography has been widely used and has the potential to improve the obtained subsurface models due to its high spatial resolution compared to other methods. Recent advances in full-waveform inversion of crosshole GPR data show that higher resolution images can be obtained compared to conventional ray-based GPR inversion because it can exploit all information present in the observed data. Since the first application of full-waveform inversion on synthetic and experimental GPR data, the algorithm has been significantly improved by extending the scalar to a vectorial approach, and changing the stepped permittivity and conductivity update into a simultaneous update. Here, we introduce new normalized gradients that do not depend on the number of sources and receivers which enable a comparison of the gradients and step lengths for different crosshole survey layouts. An experimental data set acquired at the Boise Hydrogeophysics Research Site is inverted using different source–receiver setups and the obtained permittivity and conductivity images, remaining gradients and final misfits are compared for the different versions of the full-waveform inversion. Moreover, different versions of the full-waveform inversion are applied to obtain an overview of all improvements. Most improvements result in a reducing final misfit between the measured and synthetic data and a reducing remaining gradient at the final iteration. Regions with relatively high remaining gradient amplitudes indicate less reliable inversion results. Comparison of the final full-waveform inversion results with Neutron–Neutron porosity log data and capacitive resistivity log data show considerably higher spatial frequencies for the logging data compared to the full-waveform inversion results. To enable a better comparison, we estimated a simple wavenumber filter and the full-waveform inversion results show an improved fit with the logging data. This work shows the potential of full-waveform inversion as an advanced method that can provide high resolution images to improve hydrological models.

The Prediction and Evaluation of the Groundwater Quality of Taikang Formation in Tertiary in West of Daqing Oilfield by Using Resistivity Logging Data

In order to research the groundwater quality of the Taikang Formation in Tertiary in west of Daqing Oilfield, resistivity logging data of 140 wells in the study area were analyzed to forecast the groundwater total dissolved solids (TDS) distribution of the plane. When the resistivity logging value (Rt) of the Taikang Formation in Tertiary is above 45Ω.m and its corresponding value of Formation water TDS is below 1000 mg/L, the quality of groundwater accords with life standards for drinking. The south area of Hongweixing well area and the east of Xishuiyuan well area, including Ranghulu, Qianjincun, Dulitun and Nanshuiyuan well area are favorable for high quality groundwater. The research provides the evidences to evaluate the groundwater quality in west Daqing Oilfield and determine specific well location, improving the efficiency of exploring the underground drinking water.

Fracture analysis and determination of in-situ stress direction from resistivity and acoustic image logs and core data in the Wenchuan Earthquake Fault Scientific Drilling Borehole-2 (50–1370 m)

After the Wenchuan Earthquake on May 12th, 2008, the Wenchuan Earthquake Fault Scientific Drilling Project (WFSD) was initiated in order to investigate the structure of the fault zones and the mechanism of the earthquake. The WFSD contains four boreholes (WFSD-1, WFSD-2, WFSD-3 and WFSD-4) lying at the maximum displacement locations along the Yingxiu-Beichuan fault zone and the Guanxian-Anxian fault zone, and WFSD-2 is the second borehole and is still being drilled. Core samples, resistivity and acoustic image logging data were acquired from 50 to 1370m. The natural fractures, borehole breakouts, drilling-induced fractures and drilling-enhanced natural fractures were identified from the cores and the image logs and were statistically analyzed. The strikes of the natural fractures systematically vary and can be sorted into four groups according to depth: (1) above 637m, mainly striking ENE–WSW; (2) in the interval of 637–932.6m, striking NNE–SSW; (3) in the interval of 932.6–1200m, directed ENE–WSW then to WNW–ESE, while striking NE–SW from 1030m to 1150m; (4) from 1200m to 1370m, maintaining a strike of WNW–ESE. The natural fractures from 50m to 637m seem to be reverse faults which strike approximately parallelly to the main fault. Two sets of conjugate fractures around 1002.4m indicating subvertical maximum principal paleo-stress direction may be a subordinate structure of the main fault caused by a local stress field, and it reveals the complex stress field of Yingxiu-Beichuan fault zone when the fractures formed. A total of 12 BOs, 2 sets of DIFs and one set of DEFs with an overall length of 30.4m were interpreted from 960m to 1370m in WFSD-2. The average SHmax orientation interpreted for WFSD-2 (960–1370m) is 120.7°–300.7°N (i.e. WNW–ESE) with the standard deviation of 9.2° and it is consistent with the stress status of Yingxiu-Beichuan fault zone which is one of the main fault zones in the 2008 Wenchuan Earthquake.

The Underground Water Quality Evaluation from Electrical Logging Data in Sandstone District: A Case Study in Daqing Oil Field

As an important yardstick of water quality evaluation, total dissolved solids (TDS) is strongly correlated with apparent resistivity and static spontaneous potential (SSP) parameters from electrical logging curves.A lot of electrical logging data accumated in the hydrogeologic survey during last decades can be used to predict the underground water TDS. Based on electrical characteristic analysis of sandstone reservoir, taking the influfential factors of the Archie’s Equation and creation mechanism of spontaneous potential (SP) as starting points, the theoretical relationship between the underground water TDS and formation resistivity (Rt) without considering the infuence of the porosity is modeled in this paper, and also the relationship between the underground water TDS and SSP in the same section of drilling well. Utilizing the apparent resistivity logging and static potential logging data, and employing the numerical fitting method, the relationship between groundwater TDS and conductivity is analyzed and determined, finally, the electrical parameters of determining the water quality and the regional evaluation of water quality are expounded in Daqing oil field,songliao plain Quaternary sandstone aquifer.

Wavelet Analysis of Geophysical Well-log Data of Bombay Offshore Basin, India

Geophysical well-log (bore-hole) data facilitate understanding of the physical properties of the subsurface formations as a function of depth measured in a well. In the present study, the wavelet transformation technique was applied to the well-log data of three wells in the Bombay High oil field, India, in order to identify depths to the tops of oil and/or gas formation zones (pay zones). Continuous wavelet transformation (CWT) was performed on gamma-ray, resistivity, neutron porosity and velocity log data sets in order to determine the space-localization of the oil and/or gas formation zones. The choice of a mother wavelet is important and largely depends on the data under investigation. We have applied a variety of wavelets to the different log data sets to not only identify the depths to the tops of formation zones, but also to determine the optimum wavelet that best characterizes the pay zones. On examination of scalogram plots of each log corresponding to each wavelet for their better resolution in identifying the formation boundaries, we have found that the scalograms corresponding to the Gaus1 wavelet appeared to give the best resolution in identifying the depths of pay zones in all the well-log data sets of all three wells. To further validate the above observation, a histogram analysis of CWT coefficients is made. This showed that, of all the wavelets considered for the present study, Gaus1 wavelet is the most appropriate and optimum for determining the space-localization of pay zones in all the well-log data sets considered in the present study. The depths of pay zones estimated from scalogram plots of logs agree well with those provided by the Oil and Natural Gas Corporation Ltd., India.

전기비저항 검층으로 얻은 전기비저항과 터널 현장 암반등급의 상관관계에 관한 연구

터널 시공 중 암반등급은 지보패턴의 결정에 큰 영향을 주는 요소이다. 따라서 터널 설계 시에 터널이 시공되는 노선의 암반등급을 파악하기 위해서 시추조사를 수행한다. 시추구간 사이의 미시추 구간에서는 전기비저항 탐사, 탄성파 탐사 등과 같은 지구물리탐사를 통해 암반등급을 파악한다. 따라서 전기비저항과 암반등급의 상관관계를 찾기 위한 연구가 다수 수행되었다. 그러나 대부분의 연구는 실내시험 또는 전기비저항 탐사로부터 얻어진 전기비저항과 암반등급의 상관관계에 대한 연구이다. 따라서 본 연구에서는 현장의 전기비저항 검층 시험으로부터 얻어진 전기비저항과 RMR의 상관관계를 분석하였다. 현장 데이터 분석 결과, 전기비저항과 RMR은 약 90%이상의 매우 높은 상관관계가 있는 것으로 나타났다. 또한 RMR 평가 요소들과 전기비저항의 상관관계 분석을 수행한 결과, 전기비저항과 불연속면상태는 약 80%이상의 양호한 상관관계가 나타났으나 지하수상태는 약 57%이하의 낮은 상관관계가 나타났다. Rock mass rating (RMR) is the key factor when designing the appropriate support pattern of tunnel projects. Borehole drilling is usually performed along the tunnel route in order to determine the rock mass rating to be used for tunnel design. The rock mass rating at the non-boring region between boreholes is usually assessed through geophysical surveys such as electrical prospecting, seismic prospecting, etc. Many studies were carried out to find out the correlation between electrical resistivity and rock mass rating. However, most researches were aimed at obtaining the relationship between the two parameters utilizing experimental results obtained from laboratory tests or electrical prospectings. In this paper, efforts were made to analyze and obtain relationships between the electrical resistivity obtained from in-situ electrical resistivity logging data and the rock mass rating. Correlation studies using field data showed that the electrical resistivity is highly correlated with the rock mass rating with the determination coefficient more than 90%. The correlation analysis was also carried out between RMR classification parameters and the electrical resistivity. It was shown that the correlation between the condition of discontinuities and the electrical resistivity was very high with the determination coefficient more than 80%; that between the groundwater condition and the electrical resistivity was very low with the determination coefficient less than 57%.

Resistivity logs of the Chinese Continental Scientific Drilling main hole: Implication for the crustal electrical structure of Dabie-Sulu Terrane, Central-Eastern China

To understand the electrical structure of the Sulu (苏鲁) terrane, we analyzed the resistivity logs of the Chinese Continental Scientific Drilling main hole (CCSD-MH) and obtained the statistical characteristics of the resistivity log of ultrahigh-pressure (UHP) metamorphic rocks. To check the logs’ quality, we compared the resistivity log data with the core test data. The two datasets show a good coherence. On the whole, the resistivity of the UHP metamorphic rocks from the CCSD-MH is very high (mostly higher than 1 000 Ω·m). From orthogneiss to paragneiss, amphibolite, retrograded eclogite, rutile eclogite, phengite eclogite, and ultramafic rock, the resistivity decreases gradually. The fracture zone, enrichment of metal minerals, and serpentinization of peridotite can probably cause the decrease of resistivity. Comparing the electrical structure of the crust of Dabie (大别) terrane and Sulu terrane, we can conclude that (1) the electrical structure of the crust of Dabie terrane and Sulu terrane shows a very big difference, (2) the high-resistivity zone of the north Sulu orogen extended to a very great depth (60 km), but the high-resistivity zone of Dabie orogen is over the depth of 25 km.

Analytical theory relating the depth of the sulfate‐methane transition to gas hydrate distribution and saturation

We develop a theory that relates gas hydrate saturation in marine sediments to the depth of the sulfate‐methane transition (SMT) zone below the seafloor using steady state, analytical expressions. These expressions are valid for systems in which all methane transported into the gas hydrate stability zone (GHSZ) comes from deeper external sources (i.e., advective systems). This advective constraint causes anaerobic oxidation of methane to be the only sulfate sink, allowing us to link SMT depth to net methane flux. We also develop analytical expressions that define the gas hydrate saturation profile based on SMT depth and site‐specific parameters such as sedimentation rate, methane solubility, and porosity. We evaluate our analytical model at four drill sites along the Cascadia Margin where methane sources from depth dominate. With our model, we calculate average gas hydrate saturations across GHSZ and the top occurrence of gas hydrate at these sites as 0.4% and 120 mbsf (Site 889), 1.9% and 70 mbsf (Site U1325), 4.7% and 40 mbsf (Site U1326), and 0% (Site U1329), mbsf being meters below seafloor. These values compare favorably with average saturations and top occurrences computed from resistivity log and chloride data. The analytical expressions thus provide a fast and convenient method to calculate gas hydrate saturation and first‐order occurrence at a given geologic setting where vertically upward advection dominates the methane flux.

An Improved Model to Simulate Mud (Drilling Fluid) Dispersion through Porous Media

An improved model of mud dispersion has been introduced in this work. The advantages of this model consist of a new analytical correlation for dispersivity by using resistivity log data and using a new aspect of capacitance dispersion model. Mathematical formulations were expressed, solved by numerical model taking advantage of actual log and formation data. Achieved results yielded reasonable values and trends which can be used to predict the drilling fluid concentration near wellbore region and interpret the well log data. In comparison with the previous models (Civan and Engler, 1994; Donaldson and Chernoglazov, 1987), this model uses more reasonable data and assumptions making it closer to real conditions.

Estimation of stress magnitude and physical properties for coal seam of Rangamati area, Raniganj coalfield, India

Well log analysis has been carried out to evaluate resistivity and density log data of about seven wells distributed over 3.5 sq. km area of Rangamati, Raniganj coalfield, India. Geological logs of these boreholes have recorded occurrence of four major consistent coal seams (A, B, C and D) at different depths, varying between 52 to 200 m in the area. Cleat volume/porosity ranges from 1% to about 3% in this area. Vertical stress magnitude ranges from 0.55 to 4.59 MPa from 52 to 202 m and increases with depth. The gradient of vertical stress magnitude decreases within coal seams compared to the stress gradient at roof and floor of the same seams. A methodology is proposed for estimation of permeability for a macro-cleat system of coal from well log derived porosity and from known cleat spacing from Rangamati area. Permeability value ranges from 0.5 md at a depth of 193–200 m to 18 md at a shallower depth of 45–53 m while vertical stress decreases from 4.566 MPa to 1.254 MPa respectively. Permeability values of four coal seams have been correlated with effective horizontal stress to infer the maximum horizontal stress orientation.

Exploration for gas hydrates in the deepwater, northern Gulf of Mexico: Part II. Model validation by drilling

This study examines the accuracy of the predictions of gas hydrate saturations made based on five-step analysis of 3D seismic data prior to 2005 drilling, logging, conventional coring, and pressure core sampling through the gas hydrate stability zone at two focus sites in the northern Gulf of Mexico. These predictions are detailed in Part I (2008). Here we conduct a detailed analysis of the gas hydrate saturation using both resistivity and P-wave velocity log data and analyze the pre-drilling predictions, which were made almost exclusively on the basis of seismic data, with no local logging control. Well log measurements, core data analysis, and pressure core-degas experiments all indicated general agreement with the pre-cruise analysis regarding the location and approximate concentration of gas hydrates in the sediments. We find that seismic predictions are generally consistent with log-based estimates after upscaling to seismic frequencies. We recalibrated the pre-drill model based on the new field data so that a refined version of the model could be used for future work.

Low-amplitude BSRs and gas hydrate concentration on the northern margin of the South China Sea

The passive northern continental margin of the South China Sea is rich in gas hydrates, as inferred from the occurrence of bottom-simulating reflectors (BSR) and from well logging data at Ocean Drilling Program (ODP) drill sites. Nonetheless, BSRs on new 2D multichannel seismic reflection data from the area around the Dongsha Islands (the Dongsha Rise) are not ubiquitous. They are confined to complex diapiric structures and active fault zones located between the Dongsha Rise and the surrounding depressions, implying that gas hydrate occurrence is likewise limited to these areas. Most of the BSRs have low amplitude and are therefore not clearly recognizable. Acoustic impedance provides information on rock properties and has been used to estimate gas hydrate concentration. Gas hydrate-bearing sediments have acoustic impedance that is higher than that of the surrounding sediments devoid of hydrates. Based on well logging data, the relationship between acoustic impedance and porosity can be obtained by a linear regression, and the degree of gas hydrate saturation can be determined using Archie’s equation. By applying these methods to multichannel seismic data and well logging data from the northern South China Sea, the gas hydrate concentration is found to be 3–25% of the pore space at ODP Site 1148 depending on sub-surface depth, and is estimated to be less than values of 5% estimated along seismic profile 0101. Our results suggest that saturation of gas hydrate in the northern South China Sea is higher than that estimated from well resistivity log data in the gas hydrate stability zone, but that free gas is scarce beneath this zone. It is probably the scarcity of free gas that is responsible for the low amplitudes of the BSRs.

A Hybrid Method for Electromagnetic Propagated Resistivity Logging Data Inversion

Using an inverse technique for the array electromagnetic propagated resistivity logging (EPRL) data, a fine interpretation can be obtained about the resistivity distribution of an invaded profile. Generally, the Gauss-Newton algorithm (GN) is an efficient technique for the inverse problems; however, as a gradient-type optimization method, its accuracy and convergence depend strongly on the initial value. Even though this problem can be avoided by using a differential evolutionary algorithm (DE) as a global search optimization, it is computationally less efficient. In this paper, a hybrid inversion method of differential evolution has been developed to remove the strong dependence of the accuracy and convergence on the initial value. In this new method, an additional operation, which is designed with GN, is performed only to the best individual with a delay in the evolution processes of DE. Hence, the GN operation is used for the improvement of the convergence speed without leading to any decrease of the robustness of DE. The hybrid method is then extended to apply the inversion of EPRL data. Our results demonstrate its speed, steadiness, and efficiency of this hybrid method

Velocity-depth trends in Mesozoic and Cenozoic sediments from the Norwegian Shelf: Reply

Sonic velocity, density, and resistivity log data from 60 wells on the Norwegian Shelf have been used to investigate velocity-depth trends in sedimentary rocks as a function of sediment composition, porosity, pore-pressure, burial-history, and compaction processes. A first-order linear velocity-depth trend line has been estimated from published velocity data. The data analyzed in this study, however, show significant variations from this trend line, indicating that no general velocity-depth function can be used when performing more accurate analyses like depth conversion of seismic data, pore-pressure prediction, or basin modeling. Lower Tertiary smectitic sediments from the northern North Sea and Haltenbanken are characterized by relatively low velocities compared to the overlying Pliocene and Pleistocene sediments, causing a distinct velocity inversion. A significant velocity increase at a burial depth corresponding to 70–100C was found and may reflect the alteration of smectite to illite and the initial precipitation of quartz cement in both sandstones and shales. Overpressured Jurassic sediments from Haltenbanken have lower velocities than equivalent hydrostatically pressured sequences but no significant porosity difference. The reduced velocities may be a direct response to lower effective stresses and, thus, reduced elastic compaction. Low velocities in source rocks are mainly attributed to the relatively soft kerogen and resulting velocity anisotropy. The high velocity/depth ratio of Barents Sea sediments (after correcting for Tertiary exhumation) is explained by the burial history of the area, the subsequent thermal exposure of the sediments over time, and thus, the amount of quartz cementation.

Study on the Two‐Dimension Full‐Parameter Inverse Method of the Electromagnetic Propagation Resistivity Logging

AbstractThe multiple propagation resistivity (MPR) tool of the Baker‐Hughes INTEQ is the widely used electromagnetic propagation resistivity logging tool. It has four phase difference curves and four amplitude ratio curves. With it as an example, the application of the two‐dimension inverse method to the electromagnetic propagation resistivity logging data in vertical well is presented. Because the sensitivity of logging response to layer's boundary is much higher than that to other formation parameters, so how to accurately pick up layer's boundary is important. With the method described in this paper, we begin by picking the layer's boundaries from the highest resolution curve with the characteristic recognition method, and perform vertical one‐dimension inversion for the shallow and deep detection phase difference curves. Utilizing this result to construct the initial model for two‐dimension inversion, we carried out the full‐parameter inversion of MPR data by using an improved damping regularized Gauss‐Newton optimization algorithm. The satisfying inversion result is obtained for the two typical 23‐layer models. The inversion result can significantly improve the quality of MPR interpretation. In addition, it is found that the inversion results from only five curves are nearly the same to the results from eight curves.

Building 2-D stratigraphic and structure models from well log data and control horizons

An algorithm to build a gridded 2-D seismic velocity (or any other physical property) model from well log data and control horizons is developed. Interpolation of well log data onto a 2-D grid uses inverse distance weighting or linear interpolation, guided by the shape of the control horizons that are predefined from seismic or other 2-D constraints. A key feature of the models is that they may contain layers that are truncated by unconformities or at faults, or that lap out smoothly at their tops and bottoms. Abrupt or smooth terminations are controlled by user flags. Applications are illustrated using resistivity and acoustic impedance log data from the Blake Ridge (offshore Carolina) and using a complex structure produced by overthrust tectonics in western Canada. Geologically reasonable models can be produced only if there are sufficient wells to sample every salient element in the model and sufficient control horizons to define the lateral character of the structures at the required level of detail.

Wellsite quasi-2D inversion of array resistivity logging data

New generation galvanic array tools can provide high-resolution resistivity logging data in conductive borehole environments. Fast and accurate interpretation of these array data is required for timely decisions on well completions. Due to the high nonlinearity between the galvanic data and the earth formation, inversion is necessary in interpreting resistivity logging data. A typical array tool data set, however, consists of several tens of measurements at each logging depth, acquired at a dense sampling of 4 cm. Such a large amount of data makes traditional inversion-based interpretation very slow and thus it is impossible to deliver the results to customers at the well site.In this paper, we develop a quasi-2D wellsite deliverable inversion algorithm that utilises trained back-propagation (BP) neural networks as forward modelling engines. We first reduce the 2D data into quasi-1D data via numerical focusing. We further simplify the quasi-1D data by performing borehole correction. The numerically focused and borehole effect corrected data are then inverted to provide information about the 2D resistivity structure. Synthetic tool responses are first generated over a set of cylindrically 1D earth models covering a large range of resistivity contrasts and invasion lengths. The earth model parameters are input to the neural network as the stimuli while the associated responses are used as the desired neural network output. The neural network is then trained to model the tool response. After validation, the trained neural network is applied in an inversion algorithm as a forward modelling engine that combines the computation of the potentials and numerical focusing into a single step and allows the inversion to be performed at real time. We tested our inversion algorithm on both synthetic and field data. The numerical examples show that this fast inverse algorithm is a useful tool in providing information about the formation resistivity at wellsite.