Electromagnetic Log Research Articles

Identification of hydrogeological features using remote sensing and electromagnetic methods in the hard- rock formations of the Cameroon coastal plain (Central Africa): implications for water borehole location

Sustainable access to groundwater in rural areas such as the coastal plain of Cameroon (Sub-Saharan Africa) is an ongoing challenge due to the lack of knowledge on hydrogeological features favorable to the identification of potential groundwater targets. In this study, remote sensing imagery and electromagnetic surveys were used to assess the hydrogeological potential of the Ngwei locality hard-rock. Automatic extraction of lineaments from Landsat-8 images indicated 200 lineaments with a total length of 293 km corresponding to 0.6 km of lineament per square km, suggesting a median groundwater potential. The main directions are NNW-SSE, N-S and NNE-SSW while the secondary directions are NW-SE; NE-SW and E-W. Surface geophysical investigations showed an electromagnetic field of 0 to 0.57 mV. These surface and borehole investigations confirmed basement fracturing and spatial variations in weathering profiles whose aquifer structure and yields (4.4 m3/h on average) are similar to what is commonly observed in the hard rock aquifer. The correlation between borehole productivity and the nearest lineament suggests that these lineaments are not (at least not all) surface traces of regional discontinuities that may act as major groundwater flow paths. This implies that the productivity of the Ngwei aquifers appears to be dependent on the fissured/fractured horizon, so exploration should be based on a contextual rather than general approach.Analysis of the consistency of the results with the field data allowed to propose a preliminary electromagnetic log model and also shows the value of using this approach to assess the availability of potential groundwater zones in hard rock areas. However, the methods used should be supported by further investigations to allow a better understanding of the hydrogeological characteristics of this part of the Central African basement so that no one is left behind in the achievement of the SDGs.

Real-time forward modeling and inversion of logging-while-drilling electromagnetic measurements in horizontal wells

Based on the pseudo-analytical equation of electromagnetic log for layered formation, an optimal boundary match method is proposed to adaptively truncate the encountered formation structures. An efficient integral method is put forward to significantly accelerate the convergence of Sommerfeld integral. By asymptotically approximating and subtracting the first reflection/transmission waves from the scattered field, the new Sommerfeld integral method has addressed difficulties encountered by the traditional digital filtering method, such as low computational precision and limited operating range, and realized the acceleration of the computation speed of logging-while-drilling electromagnetic measurements (LWD EM). By making use of the priori information from the offset/pilot wells and interactively adjusting the formation model, the optimum initial guesses of the inversion model is determined in order to predict the nearby formation boundaries. The gradient optimization algorithm is developed and an interactive inversion system for the LWD EM data from the horizontal wells is established. The inverted results of field data demonstrated that the real-time interactive inversion method is capable of providing the accurate boundaries of layers around the wellbore from the LWD EM, and it will benefit the wellbore trajectory optimization and reservoir interpretation.

Selection of composite material in electromagnetic log sensor

Subject of Research. The paper considers the issues of choosing an insulating material for a deep-sea electromagnetic log sensor. The effect of the insulating material characteristics on the operational factors of the sensor is studied. Method. Matlab program was used to analyze the effect of design parameters on the instrumental error of the sensor. For this purpose a mathematical model of the instrumental error component was built, and its evaluation was carried out. The stress-strain state of the sensor was simulated by the finite element method in the ANSYS Workbench application package. Analysis of insulating material peeling from the case during shrinkage was carried out in the Static Structural module. Fill simulation was performed in the Fluid Flow module. All materials are taken to be isotropic at calculations. Main Results. The effect of the insulating material characteristics on the instrumental error of the electromagnetic log sensor is shown. The design and technological issues of the insulating material choice are described. Recommendations on the choice of design parameters are given to reduce peeling of the material and the occurrence of sensors emptiness formed by filling with composite material. The validity of the recommendations is confirmed by computer simulation and experiments. Practical Relevance. The results obtained can be applied in the modernization, design and construction of new electromagnetic log sensors.

Influence of structural parameters on instrumental error of electromagnetic log sensor

Instrumental error of an electromagnetic log sensor at depths of more than one kilometer is studied. Operating principle of the electromagnetic sensor is shown and a mathematical model is developed. Analysis of sensor-registered speed dependence on external hydrostatic pressure is presented. It is demonstrated that an additional instrumental error occurs in the electromagnetic sensor due to the structure deformation during deep-sea submersion. It has been found that the value of relative error depends on design parameters and operational conditions of the sensor. It is shown that even when deep-sea vehicles travel at a low speed, the error caused by external hydrostatic pressure can make up a significant portion of the instrumental error and should be taken into account during operation. The design parameters of the sensor, affecting its instrumental error are determined. The influence of temperature on this error is studied. The ratio of design parameters at which the instrumental error does not exceed the permissible value is identified.

A Novel Damping Method for Strapdown Inertial Navigation System

There are periodic oscillation errors in the strapdown inertial navigation system (SINS) when it works in pure inertial navigation mode. It seriously restricts the navigation accuracy of the system. In order to improve the accuracy of the system, the traditional idea of the external horizontal damping in the stabilized inertial navigation system is introduced into SINS. We find that they are equivalent through comparing the damping network of the stabilized inertial navigation system with the compass alignment network. According to the equivalence, the external horizontal damping network of SINS is designed. And the difference between the velocity of the system itself and the velocity of the electromagnetic log are utilized to damp the periodic oscillation. At the same time, a horizontal damping algorithm based on the external velocity is proposed on the basis of the external horizontal damping idea. The effectiveness of the algorithm is verified by the static base of the three-axis turntable, the swing base experiment, and the vehicle simulation experiment. The experiment shows that the external damping network designed in this paper can suppress the system Schuler periodic oscillation error and improve the accuracy of the system.

Risk-Based Approach to Evaluate Casing Integrity in Upstream Wells

Downhole casing leaks in oil and gas wells will highly impact the shallow water horizons and this will affect the environment and fresh water resources. Proactive measures and forecasting of this leak will help eliminate the consequences of downhole casing leaks and, in turn, will protect the environment. Additionally, downhole casing leaks may also cause seepage of toxic gases to the fresh water zones and to the surface through the casing annuli. In this paper, we introduced a risk-based methodology to predict the downhole casing leaks in oil and gas wells using advanced casing corrosion logs such as electromagnetic logs. Downhole casing corrosion was observed to assess the remaining well life. Electromagnetic (EM) corrosion logs are the current practice for monitoring the casing corrosion. The corrosion assessment from EM logs is insufficient because these logs cannot read in multiple casings in the well. EM tool gives average reading for the corrosion in the casing at a specific depth and it does not indicate the orientation of the corrosion. EM log does not assess the 360 deg corrosion profile in the casing and it only provides average value and this may lead to wrong decision. All of this makes EM logs uncertain tools to assess the corrosion in the downhole casing. A unified criterion to assess the corrosion in the casing and to decide workover operations or not has been identified to minimize the field challenges related to this issue. A new approach was introduced in this paper to enhance the EM logs to detect the downhole casing corrosion. Corrosion data were collected from different fields (around 500 data points) to build a probabilistic approach to assess the casing failure based on the average metal loss from the EM corrosion log. The failure model was used to set the ranges for the casing failure and the probability of casing failure for different casings. The prediction of probability of failure (PF) will act as proactive maintenance which will help prevent further or future casing leaks.

Computer simulation of electromagnetic log sensor strength characteristics

Computer simulation of electromagnetic log sensor strength characteristics

A novel EM-Log aided gyrocompass alignment for in-motion marine SINS

An EM-Log (Electromagnetic Log) aided in-motion gyrocompass alignment algorithm is proposed to accomplish the fine alignment for marine SINS (strapdown inertial navigation system) on sailing vessel. The EM-Log is used to track the maneuver velocity of the vessel and remove the earth rotation angular velocity and the angular velocity of the navigation frame relative to the earth frame from accelerometer measurements. In terms of SINS damping technology, we change the structure of gyrocompass alignment loop and employ EM-Log measurements to reduce the negative effects of vessel accelerations. The simulation and experiment results show that our proposed method could accomplish fine alignment of in-motion marine SINS, while the conventional one failed.

The use of adaptive network-based fuzzy inference system for marine AHRS

A kind of marine strapdown attitude and heading reference system (AHRS) based on the principle of strapdown inertial navigation system (INS) is discussed here. With an electromagnetic (EM) log aided, the oscillations included in the attitude and heading errors are bounded by damping network. Furthermore, in order to decrease attitude and heading errors aroused by EM log measurements, we introduce an adaptive network-based fuzzy inference system to control the damping ratio automatically in terms of the vessel maneuvers conditions. The results of test demonstrate the validity of proposed method.

Strapdown fiber optic gyrocompass using adaptive network-based fuzzy inference system

This paper aims to propose a new strapdown fiber optic gyrocompass (strapdown FOGC) using adaptive network-based fuzzy inference system (ANFIS). The strapdown FOGC is based on the principle of strapdown inertial navigation system and utilizes electromagnetic log (EM log) and damping equalizer to bound oscillatory errors existing in attitude and heading. As the introduction of damping technique, extra errors are aroused by EM log errors. To decrease the extra errors, ANFIS is utilized to adjust the damping ratio automatically in terms of the ship maneuver conditions. The simulation and trial results indicate that, compared with the conventional gyrocompass scheme, the proposed one can reduce the attitude and heading errors and improve the system performance effectively.

Pressure transient testing for assessment of wellbore integrity in the IEAGHG Weyburn–Midale CO2 Monitoring and Storage Project

This paper presents an overview of a field program undertaken to assess wellbore integrity in a 54-year old vertical well, conducted as a component of the IEAGHG Weyburn–Midale CO2 Monitoring and Storage Project. The well investigated had been drilled to a depth of 1471m (4824ft), cased with 139.7mm (5.5in.) production casing and cemented with a 1-to-1 blend of Portland cement and pozzolan. Cased-hole logging indicated that the top of the cemented annulus was located approximately 500m (1640ft) above total depth of the well. A built-for-purpose tool was used to conduct pressure transient (vertical interference) testing to assess the hydraulic properties of the cemented annulus within the Watrous Formation, which serves as the regional seal in the vicinity of the Weyburn and Midale fields. A modified version of an existing tool was used to mill through casing then drill small cement core samples. This paper presents an overview of the entire testing program, and a detailed account of the analysis methodology used to estimate the equivalent permeability of the cemented annulus from a vertical interference test. The results of this analysis suggest that vertical permeability of the cemented annulus is very low, falling somewhere in the 7–80 microdarcy range. This result is consistent with cased-hole logging results; more specifically, the combined interpretation of sonic and ultrasonic logs, which suggest effective cement emplacement, and the paucity of casing defects identified by an electromagnetic log, which suggests the cement sheath has effectively shielded the casing from exposure to native formation fluids. The results presented here demonstrate that a well constructed using standard practices and materials of the 1950s can serve as an effective barrier against CO2 migration, although similar work should be undertaken in more wells to confirm that this holds across a broad spectrum of conditions.

Correction and damping of medium accuracy INS using electromagnetic log

The paper focuses on the use of electromagnetic log data exclusively for damping of inertial navigation system (INS). The errors of INS sensors are shown to be almost not corrected. Modeling and experimental results are presented.

Verification of EM log data in marine inertial navigation system correction

This paper considers a method for verifying external navigation data for a local level inertial navigation system using optimal filtering algorithms in a closed-loop correction circuit. Special attention is paid to verification of data from an electromagnetic log. Examples of processing measurements containing various errors are presented.

Transfer alignment considering measurement time delay and ship body flexure

This paper deals with the transfer alignment problem of strap-down inertial navigation systems (SDINS), using electro-magnetic (EM) log velocity information and gyrocompass attitude information of the ship. Major error sources for velocity and attitude matching are lever-arm effect, measurement time-delay, and ship-body flexure (flexibility). To reduce these alignment errors, an error compensation method based on delay state augmentation and DCM (direction cosine matrix) partial matching is devised. A linearized error model for a velocity and attitude matching transfer alignment system is devised by first linearizing the nonlinear measurement equation with respect to its time delay, and augmenting the delay state into conventional linear state equations. DCM partial matching is then properly combined with velocity matching to reduce the effects of a ship’s Y-axis flexure. The simulation results show that this method decreases azimuth alignment errors considerably.

HMS Scott ring laser gyro navigator integration

HMS SCOTT is a United Kingdom ocean survey vessel that hosts a state-of-the-art deep ocean mapping system which was designed, developed, and is currently maintained and periodically updated by the Space and Naval Warfare Systems Center, San Diego, (SSC San Diego). A recent update of this system consisted of the replacement of an obsolete and very costly to maintain inertial navigation system. Another reason for updating the ship's inertial system was to provide higher accuracy attitude data, than was previously available with the old inertial system, to the high resolution multi-beam sonar system in order to produce improved bathymetric charts. After conducting a market survey of suitable inertial navigator systems, the Kearfott SEANAV ring laser gyro navigator (RLGN) system, using a monolithic T-24 gyro, was selected to replace the old inertial system. The selection of the SEANAV RLGN was based on its relatively low cost, high reliability, and, particularly, the roll and pitch data accuracies of typically less than 1-arc-minute. This attitude data accuracy was key to enable a significant improvement in the bathymetry data accuracy. Two SEANAV systems were integrated with GPS receivers, a system master time code generator, and electro magnetic (EM) log and Doppler sonar speed sensors to provide the navigation portion of the mapping data. Operational testing of this updated system aboard HMS SCOTT, in November, 2003, showed a marked improvement in the quality of the map product due, in part, to the improved attitude data provided by the SEANAV RLGN system.

Detection of Contaminant Plumes by Borehole Geophysical Logging

AbstractTwo borehole geophysical methods—electromagnetic induction and natural gamma radiation logs—were used to vertically delineate landfill leachate plumes in a glacial aquifer. Geophysical logs of monitoring wells near two land‐fills in a glacial aquifer in west‐central Vermont show that borehole geophysical methods can aid in interpretation of geologic logs and placement of monitoring well screens to sample landfill leachate plumes.Zones of high electrical conductance were delineated from the electromagnetic log in wells near two landfills. Some of these zones were found to correlate with silt and clay units on the basis of drilling and gamma logs. Monitoring wells were screened specifically in zones of high electrical conductivity that did not correlate to a silt or clay unit. Zones of high electrical conductivity that did not correlate to a silt or clay unit were caused by the presence of ground water with a high specific conductance, generally from 1000 to 2370 μS/cm (microsiemens per centimeter at 25 degrees Celsius). Ambient ground water in the study area has a specific conductance of approximately 200 to 400 μS/cm. Landfill leachate plumes were found to be approximately 5 to 20 feet thick and to be near the water table surface.

The diagnosis of synoptic-scale vertical motion in the seasonal thermocline

The form of the vertical motion field on the ocean synoptic scale in the seasonal thermocline can be diagnosed by applying the ω-equation developed in meteorology. The results show a pattern typical of an unstable baroclinic wave. This was achieved by combining hydrographic data from the seasonal thermocline, collected by a CTD in a towed fish, with surface currents measured from ship's drift using an electromagnetic log.

Depositional Environments and Invasion Profiling in Heavy Oil Sands, Eastern Venezuela: A Case Study: ABSTRACT

A comprehensive study of a single well for INTEVEP and Meneven in the Faja Petrolifera del Orinoco--an extensive (570 × 140 km, 350 × 90 mi) east-west trending heavying oil belt located immediately north of the Orinoco River, eastern Venezuela--resulted in an interpretation of an invasion profile dependent on depositional environments. The data base was derived from: (1) 400 ft (121 m) of whole core (67% recovery) and 124 sidewall cores taken from the 1,500-1,900 ft (457-579 m) interval of the upper Tertiary Oficina Formation, which, in this area, overlies the Precambrian metamorphosed igneous basement. (2) A logging suite of 22 conventional, experimental, and prototype wireline logs. In this area, the Oficina Formation is a complex series of stacked, heavy oil-saturated, sand-dominant, fluvially-transported, unconsolidated sediments overlain by lignite- and mud-dominant backswamp and estuarine sediments. Bedding, mineralogy, lithology, and paleontology define fluvial-channel fills, channel-lag deposits, crevasse splays, overbank levees, swamps, estuarine lagoons, and estuarine-channel fills in a delta-plain environment. Reservoir geometry and facies analyses suggest that several short, wide, deep-channeled, meandering rivers flowed generally northward across a tropical-subtropical plain of low relief carrying the bulk of the sediment in bedload traction (channel fills) and a smaller fraction in density suspension (crevasse splays). A unique logging suite permitted the measurement of invasion in this heavy-oil environment. The invasion of the mud filtrate was measured at varying depths of investigation using, from shallowest to deepest depths: (1) gamma ray spectrometry log, (2) microspherically focused log, (3) nuclear magnetism free fluid index, and (4) deep Laterolog. In addition, water trapped in the fine-grained sands was characterized by comparing the porosity from the electromagnetic propagation log to the free fluid index from the nuclear magnetism log. Identification of the depositional facies coupled with these log responses produced an innovative interpretation of the invasion profile. From an oil-bearing gross interval of 262 ft (80 m) 9 sand units were defined according to their primary and secondary (steam processing) production capability. End_of_Article - Last_Page 516------------

内海航行援助システムの試作

The present paper reported on the development of the prototype coastal navigation support system, using personal computer (TEAC, PS-80) which recently became popular. The system has been constructed for the purpose of load reduction in chart works (ship positioning), so navigator will be released from chart works to keep more careful look out for safe ship operations. The system has the following functions; (1) The system indicates target name, bearing and distance on the CRT display. (2) The informations of ship course and speed are given by gyrocompass and electromagnetic log through A/D converter or by keyboard. (3) The system calculates the ship position from more than two target bearings. (4) The system indicates observed position and dead reckoning position with time on the CRT display. (5) The system indicates name, bearing and distance of next course changing point on the CRT display. (6) Using X-Y ploter, the ship positions are drawn on chart.

Adaptive Kalman filter for marine navigation and gravity measurement

A simple adaptive Kalman filter has been developed to give optimal estimation of a ship's course, speed and position giving sufficient accuracy to be useful for the processing of marine gravity measurements. The inputs to the filter are from the ship's electromagnetic log, gyrocompass and radio navaid. The radio-navaid noise is assumed to be zero mean and white, but it is shown that the filter still works satisfactorily when the noise is coloured, with a spectrum similar to that obtained from long-range marine radio navaids beyond the edges of their present useful operating areas. The log and gyrocompass noises are assumed to be white and nonzero mean, and the effects of these nonzero means (due to wind and current in practice) examined. An algorithm is developed to offset the effects of the nonzero means. The sensitivity of the filter to incorrect data statistics is examined and a limited-memory radio-navaid variance estimator incorporated. Finally, the developed filter is used with real ship data obtained from the ocean survey ship, HMS Hecla.