Keisuke Ushijima

Analytic signal approach and its applicability in environmental magnetic investigations

Abstract We investigate the analytic signal method and its applicability in obtaining source locations of compact environmental magnetic objects. Previous investigations have shown that, for two-dimensional magnetic sources, the shape and location of the maxima of the amplitude of the analytic signal (AAS) are independent of the magnetization direction. In this study, we show that the shape of the AAS over magnetic dipole or sphere source is dependent on the direction of magnetization and, consequently, the maxima of the AAS are not always located directly over the dipolar sources. Maximum shift in the horizontal location is obtained for magnetic inclination of 30°. The shifts of the maxima are a function of the source-to-observation distance and they can be up to 30% of the distance. We also present a method of estimating the depths of compact magnetic objects based on the ratio of the AAS of the magnetic anomaly to the AAS of the vertical gradient of the magnetic anomaly. The estimated depths are independent of the magnetization direction. With the help of magnetic anomalies over environmental targets of buried steel drums, we show that the depths can be reliably estimated in most cases. Therefore, the analytic signal approach can be useful in estimating source locations of compact magnetic objects. However, horizontal locations of the targets derived from the maximum values of the AAS must be verified using other techniques.

Interpretation of magnetic data using an enhanced local wavenumber (ELW) method

This paper presents an enhancement of the local-wavenumber method (named ELW for “enhanced local wavenumber”) for interpretation of profile magnetic data. This method uses the traditional and phase-rotated local wavenumbers to produce a linear equation as a function of the model parameters. The equation can be solved to determine the horizontal location and depth of a 2D magnetic body without specifying a priori information about the nature of the sources. Using the obtained source-location parameters, the nature of the source can then be inferred. The method was tested using theoretical simulations with random noise over a dike body. It was able to provide both the location and an index characterizing the nature of the source body. The practical utility of the method is demonstrated using field examples over dikelike bodies from Canada and Egypt.

Linearized least‐squares method for interpretation of potential‐field data from sources of simple geometry

We present a new method for interpreting isolated potential-field (gravity and magnetic) anomaly data. A linear equation, involving a symmetric anomalous field and its horizontal gradient, is derived to provide both the depth and nature of the buried sources. In many currently available methods, either higher order derivatives or postprocessing is necessary to extract both pieces of information; therefore, data must be of very high quality. In contrast, for gravity work with our method, only a first-order horizontal derivative is needed and the traditional data quality is sufficient. Our proposed method is similar to the Euler technique; it uses a shape factor instead of a structural index to characterize the buried sources. The method is tested using theoretical anomaly data with and without random noise. In all cases, the method adequately estimates the location and the approximate shape of the source. The practical utility of the method is demonstrated using gravity and magnetic field examples from the United States and Zimbabwe.

Reservoir monitoring by a 4-D electrical technique

An advanced geoelectrical technique for imaging potential fractures has been developed by Engineering Geophysics Laboratory of Kyushu University. The method, fluid‐flow tomography (FFT), has been applied to monitor fluid‐flow behaviors during massive water/steam injection operations through a borehole. Fluid‐flow behaviors in potential fractures could be continuously traced and visualized as a function of time by the FFT system. Potential fractures can be evaluated by 3-D inversion of self‐potential (SP) anomalies based on a potential theory similar to the hypocenter of the earthquakes. The FFT method has been applied to determine the location of fractures in a geothermal area in Japan.

Automatic Detection of UXO from Airborne Magnetic Data Using a Neural Network

Recent developments in airborne magnetic detection systems have made it possible to detect small ferro-metallic objects. However, airborne magnetic data can be really large and, therefore, there is an increasing need for a fully automatic interpretation technique that could be used to make decisions regarding the nature of the sources in the field in real time. The massively parallel processing advantage of artificial neural networks makes them suitable for hardware implementations; therefore, using these networks in conjunction with a magnetic system has the potential to greatly speed up the detection of ferro-metallic objects. In this paper, we have developed a new method for detection and characterization of unexploded ordnance (UXO) using a Hopfield neural network as applied to airborne magnetic data. The Hopfield network is used to optimize the magnetic moment of a dipole source representing the magnetic object at regular locations. For each location, the Hopfield network reaches its stable energy state. The location of the object corresponds to the location yielding the minimum Hopfield energy. Output results include position in two dimensions (horizontal location and depth), magnetic moment, and effective inclination. Theoretical and actual field examples show that the Hopfield network is accurate and objective tool for the detection of UXO. Moreover, because the Hopfield neural network is a natural analog-to-digital converter, it is ideally suited for incorporation into airborne magnetic instrumentation systems.

2-D inversion of VES data in Saqqara archaeological area, Egypt

The interpretation of actual geophysical field data still has a problem for obtaining a unique solution. In order to investigate the groundwater potentials in Saqqara archaeological area, vertical electrical soundings with Schlumberger array have been carried out. In the interpretation of VES data, 1D resistivity inversion has been performed based on a horizontally layered earth model by El-Qady (1995). However, some results of 1D inversion are not fully satisfied for actual 3D structures such as archaeological tombs. Therefore, we have carried out 2D inversion based on ABIC least squares method for Schlumberger VES data obtained in Saqqara area. Although the results of 2D cross sections were correlated with the previous interpretation, the 2D inversion still shows a rough spatial resistivity distribution, which is the abrupt change in resistivity between two neighboring blocks of the computed region. It is concluded that 3D interpretation is recommended for visualizing ground water distribution with depth in the Saqqara area.

Integrated Geophysical Survey for Site Investigation at a New Dwelling Area, Egypt

An integrated geophysical survey was carried out in a new dwelling area at 15-May town, southeast Cairo, Egypt. The buildings in this area are intensively affected by dangerous cracks that cause structural instability. The survey aimed to image the shallow subsurface structures, including karstic features, and evaluate their extent, as they may cause rock instability and lead to cracking of the residential buildings. Resistivity profiling (2-D), using a dipole-dipole array and ground penetrating radar (GPR) surveys were carried out along seven parallel traverses extending about 150 meters between the buildings blocks. Additional measurements using a Schlumbereger array and very low frequency electromagnetic (VLF-EM) methods were conducted. The acquired data were processed and interpreted integrally to elucidate the shallow structural setting of the site. Integrated interpretation led to the delineation of hazard zones rich with karstic features in the area. Most of these karstic features are associated with vertical and sub-vertical linear features such as faults, fracture zones, and geologic contacts. These features are the main reason of the rock instability that resulted in potentially dangerous cracking of residential buildings.

Subsurface structural mapping of Gebel El-Zeit area, Gulf of Suez, Egypt using aeromagnetic data

The Gebel El-Zeit area is located on the western coast of the Gulf of Suez, Egypt. The areas in/and around the Gulf of Suez are generally important due to their hydrocarbon resources. In this study, we have applied gradient interpretation techniques (Euler deconvolution and analytic signal) to the aeromagnetic data of the Gebel El-Zeit area. The main objective of this study is to identify and delineate the possible subsurface structure of the area that may assist in locating new hydrocarbon prospects. Results of Euler method suggested that, on the eastern and western parts of the area, the basement could be observed on the ground (~50 m over the ground) and became more deeper on the central part to reach depth of 5 km (from the ground level). Results from the analytic signal method indicated that, the depth to the basement has an average value of 156 m on the eastern side and 758 m on the western side. Generally, the area is characterized by a graben structure bounded by major faults striking in the NW-SE direction.

Structural mapping of Quseir area, northern Red Sea, Egypt, using high-resolution aeromagnetic data

Exploration in the northern Red Sea of Egypt has had little success in locating hydrocarbon accumulation. The main exploration problems in this region are the complex block faulting and Miocene salt structures. The complex basement block structure arises from the different ages of faults and the difficulty of determining the exact age relations. In this paper, we present a case study of structural mapping using application of the Euler method to high-resolution aeromagnetic data collected in the Quseir area of the northern Red Sea of Egypt. The results indicated that the area is affected by sets of fault systems, which are mainly trending in the NNW-SSE, NW-SE and NE-SW directions. The results also delineated boundaries of a long and wide magnetic body on the offshore part of the study area. This basement intrusion is most probably related to the Red Sea rift and perhaps associated with structures higher up in the sedimentary section. As a result, the area above this anomaly is highly recommended for further oil exploration. This example illustrates that high-resolution aeromagnetic surveys can help greatly in delineating subsurface structure in the northern Red Sea of Egypt.

Estimation of depth and shape factor from potential-field data over sources of simple geometry

Summary We present a new method for interpretation of potentialfield anomaly data. A linear equation, involving the anomaly and its horizontal gradient, is derived to provide both the depth and nature of the buried sources. The proposed method is similar to the Euler technique; however, it uses a shape factor (q) instead of a structural index (η) to characterize the buried sources. The method is tested using theoretical simulations with different gravity and magnetic models placed at different depths with respect to the observation height. In all cases, the method adequately estimated the locations and the approximate shapes of the sources. Results from field examples will be discussed at the meeting.