Lecture-4_magnetic surveying and exploration.pptx

Lecture on magnetic surveying

1. Magnetic Surveying

2. When a material is placed in an external magnetic field, it will become magnetized in the direction of the field, and this magnetization will disappear when the material is removed from the field. This phenomenon is called induced magnetization or magnetic polarization, resulting in the alignment of the poles within the material in the direction of the field. As a result of this alignment, there will be magnetic poles at the ends distributed over the surface of the material. Introduction

3. The unit used in geomagnetic surveys is Tesla 1 Tesla = 1 T = 1 N/Am 1 nT = 10 -9 T=1 γ = 10 -5 Oersted   CGS unit: 1 gauss (G)=10 -4 T 1 gamma ( γ)=10 -5 G Introduction

4. The objective of a magnetic survey is to investigate subsurface geology on the basis of the anomalies in the earth's magnetic field resulting from the magnetic properties of the underlying rocks. The magnetic content (susceptibility) of rocks is extremely variable depending on the type of rock and the environment. The Aim of the Magnetic Survey

5. Exploration of fossil fuels (oil, gas, coal). Exploration of ore deposits. Regional and global tectonics. Large scale geological structures, volcanology. Buried conductive objects ( cables, drums, and metallic debris ). Map old waste sites and landfill boundaries. Archaeological investigations. Engineering/construction site investigation. Applications

6. Survey plan and required equipment: The three types of surveys. Location determination. Preparation of maps and previous studies. Determination of the base station. Acquisition and Reduction of Data

7. The survey can be divided into three types: Reconnaissance survey: Limited and quick measurements are carried out, which form the basis for other two types 2 or 3. Regional survey: The distance between stations and lines (profiles) is generally wide, usually 5 km or more. It is used in initial petroleum exploration and tectonic studies. Detailed survey: The area is divided into very closely spaced stations and lines, with distances between stations being a few meters in mineral exploration and archaeological studies, sometimes as close as one meter or less. The detailed survey is applied in all cases. Surveying

8. Early torsion magnetometers used compass needles mounted on horizontal axes (dip needles) to measure vertical fields. These were in use until about 1960, when they began to be replaced by: Proton precession Fluxgate Alkali vapour magnetometers. Instruments used in magnetic measurements

9. When the current is switched off, the protons realign to the direction of the Earth’s field. Proton Precession magnetometers

10. Much the same design as ground based surveys, except larger line spacing. Sedimentary basins (4 km spacing - 1km flight height or greater). Areas of exposed basement (mineral surveys) 200m spacing - 100-500m flight height for rapid environmental surveys, line spacing of 10-50m - 30-50m flight height. Airborne Surveys