Isomagnetic Map Research Articles

Integrated Geophysical Mapping of Near-surface Structures and Utilities in Ile-Ife, Nigeria

Magnetic, self-potential (SP) and Ground-penetrating radar (GPR) methods were employed to investigate the near-surface conditions beneath a road in Obafemi Awolowo University Campus, Ile-Ife with the aim of mapping previously suspected dyke, fractures, buried utilities in the area. Fifty stations were established for the magnetic and SP methods along five and four traverses respectively while GPR data was acquired along three traverses, all parallel to the general strike of the rocks in the area (NW-SE). The magnetic and SP readings obtained were interpreted qualitatively and quantitatively using least-squares to identify the location/trends and determine source parameters of features of interest. Also, GPR sections were analyzed using basic radar processing tools. The magnetic field readings ranged from -310nT to 220nT while the SP readings with range -126mV to 55mV. The anomaly source was delineated on low closures on the iso-magnetic map while the inflexion points on the SP profiles lie above the target source. Results of quantitative interpretation of magnetic and SP anomalies revealed a spherical, sub-vertical source with depth to top values of 1.6m to 3.85m. The radargrams revealed the positions and depths of utilities such as pipes and cables, shallow-dipping fractures, a road culvert and partially weathered bedrock layers overlying fresh basement in the area. The penetration depth of the GPR was 14 m and the depth to top of the basement ranged from 4 m to 6 m. The techniques employed mapped the subsurface structural/cultural features in the study area and determined their positions and depth extents. crust than the other deeper layers. As a result increased understanding of the dynamics of this part of the Earth is paramount (1). In this paper, the primary aim was to map the shallow portions of the earth within a defined area in Ile-Ife; up to a depth of about 15 m, and confirm the presence of previously suspected fractures, fault, dyke and man-made utilities. Three geophysical methods namely Magnetic, Self-potential (SP) and Ground Penetrating Radar (GPR) were employed in the present study. The magnetic method involves the measurement of variations in total magnetic field of the earth, caused by local differences in the magnetization of the subsurface rocks and soils. It has a wide range of applications and has been used in locating buried metalliferous man-made objects such as cables and pipelines, location of boundaries between rocks, faults, and dykes (2). The SP method is a passive geophysical technique that responds to naturally-occurring potential differences generated mainly by electrochemical, electrokinetic, and thermoelectric sources in the subsurface and has many applications ranging from mineral exploration, hydrogeological, environmental, geotechnical and engineering investigations. There is recorded success in the use of the SP method in shallow subsurface mapping of simple geometrical bodies (3) and sheet-like bodies (4). The GPR technique (which is a novel technique in Nigeria) detects electrical discontinuities due to features such as bedding planes, fault planes etc. in the shallow subsurface (usually less than 50m) by the generation, transmission, propagation, reflection and reception of discrete pulses of high-frequency electromagnetic energy with high-resolution imagery (5). GPR is effective for subsurface fracture and fracture zone imaging (6), soil and rock stratigraphy mapping (7) and underground utility mapping (8). Consequently, this integrated study was done to verify the presence of the geologic structures and cultural utilities in the area, and also to demonstrate the efficacy of the GPR technique in shallow subsurface mapping in a basement complex terrain of southwestern Nigeria.

Magnetocardiography in Early Detection of Electromagnetic Abnormality in Ischemic Heart Disease

Ischemic heart disease (IHD) is one of the leading causes of death in the general population. Ischemia induces changes in the electrophysiologic properties of the myocardium that sometimes cannot be detected with rest ECG, which has a relatively low sensitivity. Magnetocardiography (MCG) which records the magnetic fields generated by the heart, is reported more sensitive for measuring myocardial electric activity. Many analyzing methods using MCG for the diagnosis of IHD are reported. Those methods can be summarized as follows: 1. magnetic measurement of ST and TQ segment shift, 2. spatial dispersion of the magnetocardiographically determined QT intervals (QT dispersion), 3. ST current angle rotation of magnetic field (isomagnetic map) at rest or in exercise magnetocardiography, 4. analyzing current arrow map during ventricular repolarization, and 5. analyzing the integral values of reporalization In this review we will given an overview of the current status of MCG methods for the diagnosis of IHD.

Magnetocardiography in Early Detection of Electromagnetic Abnormality in Ischemic Heart Disease

Ischemic heart disease (IHD) is one of the leading causes of death in the general population. Ischemia induces changes in the electrophysiologic properties of the myocardium that sometimes cannot be detected with rest ECG, which has a relatively low sensitivity. Magnetocardiography (MCG) which records the magnetic fields generated by the heart, is reported more sensitive for measuring myocardial electric activity. Many analyzing methods using MCG for the diagnosis of IHD are reported. Those methods can be summarized as follows: magnetic measurement of ST and TQ segment shift, spatial dispersion of the magnetocardiographically determined QT intervals (QT dispersion), ST current angle rotation of magnetic field (isomagnetic map) at rest or in exercise magnetocardiography, analyzing current arrow map during ventricular repolarization, and analyzing the integral values of reporalization In this review we will given an overview of the current status of MCG methods for the diagnosis of IHD.

Visualization of atrial excitation by magnetocardiogram.

We tried to visualize normal atrial excitatory process by magnetocardiogram (MCG) measured from both anterior chest and back using our newest multi-channel SQUID system. Twenty normal subjects were studied. After measuring the normal (B(z)) component of the magnetic field, we constructed an isomagnetic and vector arrow map from spatial derivatives of the normal (B(z)) component in the tangential direction. By the MCG measurement from the anterior chest, current arrows were recognized in the right upper portion which were directed to the left lower from the beginning of the P wave to the P-wave peak. By the measurement from the back, current arrows were able to be visualized in the right to middle upper portion which were directed to the left or left lower just before the P-wave peak. We conclude that we successfully recognized the right atrial excitation and its spread to the left atrium and observed the time course of normal atrial excitatory process by the MCG measurement from not only anterior chest but also back using 64-channel SQUID system.

左室負荷疾患における心磁図 Ｉｓｏｍａｇｎｅｔｉｃ ｍａｐおよびｓｉｎｇｌｅ ｍｏｖｉｎｇ ｄｉｐｏｌｅ法による診断

左室負荷疾患における心磁図 Ｉｓｏｍａｇｎｅｔｉｃ ｍａｐおよびｓｉｎｇｌｅ ｍｏｖｉｎｇ ｄｉｐｏｌｅ法による診断

Magnetocardiographic studies of ventricular repolarization in old inferior myocardial infarction.

Isomagnetic maps of 50 normal subjects (control group) and 23 patients with old inferior myocardial infarction (IMI group) were recorded in order to analyse T wave abnormalities in inferior myocardial infarction. The T wave of the magnetocardiogram (MCG) in the control group showed negative deflections in the left upper portion and positive deflections in the right lower portion, thus resulting in a T vector directed leftward and inferiorly. The T wave of the IMI group was flat or positive in the left upper portion and flat or negative in the right lower portion, suggesting a T vector directed superiorly. In addition, opposing dipoles were observed in 36.4% of the IMI group; i.e. one directed superiorly, presumably due to inferior myocardial ischaemia, and the other directed inferiorly due to normal repolarization. Localized T vector abnormalities could be detected by the MCG in some cases, in which coronary T waves of the standard electrocardiogram had returned to normal. Furthermore, multiple dipoles were more frequently observed in the isomagnetic map than in the isopotential map (5 vs. 15; P less than 0.01). These results suggest that the MCG is helpful in diagnosing myocardial ischaemia when this is not detectable on the electrocardiogram.

Isomagnetic maps in right ventricular overloading

Isomagnetic maps were recorded in normal subjects and in patients with systolic overloading of the right ventricle. The isomagnetic maps examined in this study indicated the instantaneous current source of the heart by applying the "corkscrew rule." The magnetic field recorded by a second derivative gradiometer detected clearly the cardiac current source from the right ventricle, which is located close to the anterior chest wall, and improved diagnostic sensitivity. Moreover, the isomagnetic map showed multiple dipoles, which are difficult to detect in the electrocardiogram or isopotential map. These results suggest that the magnetocardiogram provides useful information on current sources to supplement information obtained by the conventional electrocardiogram.

The P wave in the magnetocardiogram

The P wave of the magnetocardiogram (MCG) was investigated in normal subjects and in patients with right and left atrial overloading. In normal subjects, the MCG P wave was positive at right lower sternal sites and negative in the other portions. The current source deduced from the MCG and the isomagnetic map was directed inferiorly and to the left throughout the entire phase of atrial activation. In patients with right atrial overloading, the direction of the current source deduced from the MCG was similar to that of normal subjects, but its amplitude was significantly greater. In left atrial overloadings, a negative-positive biphasic P wave was seen more frequently than in normal subjects at left parasternal sites. In the late phase of atrial activation, the MCG could detect the two dipoles, i.e. one directed inferiorly and other directed to the left. These dipoles might correspond to right and left atrial activities, respectively. These results show that the MCG might add useful information on current source to the conventional electrocardiogram.

医療における人体磁界測定の有効性 心磁図の効力

Action current of the heart muscle produces electric field on the body surface and also weak magnetic field around the thorax. Magnetocardiogram (MCG) is a record of this magnetic field by means of a very sensitive magnetometer. We usually record MCG at 36 points of the anterior chest wall by means of a SQUID with the transducer coil for the second gradient of the magnetic field. Electrocardiography has been used widely for the diagnosis of the cardiovascular diseases, but its limitations have also been known. MCG can not be expected to provide different kinds of informations from ECG, because both methods handle such informations related to the electromotive forces of the heart. But there may be some possibility that the MCG can give some diagnostic informations which can not be detected by ECG, because the magnetism is somewhat different from electricity in physical properties. For example diagnostic sensitivity of the right atrial overloading is 13.6% in ECG and 56.6% in MCG, showing higher diagnostic ability of MCG. As for the diagnosis of the right ventricular hypertrophy, diagnostic sensitivity is 31.3% in ECG and 73.3% in MCG, also showing the higher sensitivity in the latter than the former. Sensitivity for the diagnosis of the myocardial damage (abnormal ventricular repolarization) in hypertensive subjects was 20.0% in ECG and 42.5% in MCG. Sensitivity is especially higher in MCG in slight to moderate cases of hypertension. Multiple dipoles were detected in 7.5% in isopotential map and 27.5% in isomagnetic map at the peak of the T wave in lead II of the standard leads. The reason of the higher sensitivity of MCG for the diagnosis of the abnormal ventricular repolarization is thought to be due to the superiority of MCG in detecting multiple dipoles. At present, liquid helium is necessary to record MCG, and the multi-elements MCG system are not available with a reasonable price. It is expected that the MCG may become to be used widely in clinical medicine as a supplementary diagnostic method to the ECG, if these weak points are overcome.

Magnetocardiographic P waves in normal subjects and patients with mitral stenosis.

The P wave of the magnetocardiogram (MCG) was investigated in normal subjects and patients with mitral stenosis to determine its characteristics in normal conditions and left atrial overloading (LAO) and to analyze atrial activation by a magnetic field. In normal subjects, the MCG P wave was negative in left parasternal sites and positive in right lower sternal sites. The current source deduced from the MCG pattern and isomagnetic map was directed inferiorly and to the left through the entire phase of atrial activation, suggesting that in most normal cases the P wave reflects right atrial activity. In patients with mitral stenosis, a negative-positive biphasic P wave was seen more frequently than in normal subjects in left parasternal sites (p less than 0.005). In the late phase of atrial activation, the current source deduced from the isomagnetic map was shifted superiorly and to the left, suggesting an increased leftward force due to LAO. The MCG was similar in sensitivity to the ECG, for diagnosis of LAO, but in a few cases LAO could be detected from the MCG but not the ECG. These findings suggest that the MCG is clinically useful for diagnosis of LAO.

Analysis of current dipole studied by isomagnetic map and vector arrow map

Analysis of current dipole studied by isomagnetic map and vector arrow map

Magnetic anomalies associated with the southeastern continental margin of South Africa

A total magnetic intensity, iso-magnetic map is presented and discussed. Between East London and Durban large east-west trending anomalies are known on land and can be traced onto the continental shelf but not beyond the slope. Elsewhere the continental shelf is characterized by a remarkably quiet magnetic field. A feature of the map is the linear anomaly, named the Cape Slope Anomaly, which is parallel to the continental margin and coincides approximately with the 68° small circle about the early pole of opening for the South Atlantic as given by Le Pichon and Hayes (1971). The anomaly is traced between 30°54′S, 30°48′E and 37°45′S, 20°31′E and is interpreted as occurring over the truncated edge of a semi-infinite, sub-horizontal, remanently magnetized plate in oceanic crust beyond the continental margin. Between 37°03′S, 21°49′E and 37°41′S, 21°12′E the Slope Anomaly occurs over a ridge named the Agulhas Ridge. A continuous seismic reflection profile over the ridge shows acoustic basement occurring under a cover of sediments. A two dimensional model study indicates that the basement materials may belong to the body causing the anomaly with the exception of the basement material that forms the landward peak of the ridge, which is non-magnetic.