Magnetic Survey for exploration and its interpretation.pptx

A magnetic survey is a geophysical exploration method used to detect and map variations in the Earth's magnetic field caused by the magnetic properties of subsurface materials. This technique is widely applied in mineral exploration, petroleum exploration, and archaeological studies. Here's a general overview with case study and problems to be solved.

1. Magnetic Surveying Omer M. Ahmed [email protected]

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

11. Cross-over points A major problem in aerial surveys is determining the locations of the stations, which can be done using GPS or, alternatively, using aerial photographs.

12. To aid in interpretation, it is essential to gather as much geological information and previous well data as possible. Just like gravity, we can use… Direct interpretation. Indirect interpretation (and automatic inversion). Preliminary interpretations of contour maps or profiles. Interpretation

13. In this type, the area is divided into parts according to the value of the anomaly (low or high). There are two types of quantitative interpretation Half width Common tangents Direct ( Qualitative) Interpretation

14. Common example of the quantitative interpretation

15. It is always true that the width of a magnetic anomaly is the same order of magnitude as the depth of the body. Just don't expect too much precision! 1. Half width method d= ½ X

16. 2. Common tangents X 15 5 10 -10 -5 -15 5.1 5.05 5 4.9 4.95 A B C E F= nT Distance (M) X10 -4 d=1.3X

17. Indirect Interpretation Same approach as in gravimetry (improvement of a initial model): • Automatic inversion useful since anomalies are complex. • Model built using a series of dipoles (sum of positive and negative poles).

18. Calculated data ….. Observed data Example of Modeling Problem?

19. PRACTICAL

20. Problem 1 A magnetic survey in an area along a line provided the following data: Make the correction then calculate the magnetic anomaly from the given table of reading for every point or station ( R readings in nT ), consider IGRF=38500 γ .

21. ∆T=Time1─BS. Time ∆T=Time2─BS. Time …. ∆R=Reading1 ─ BS. Reading ∆R=Reading2 ─ BS. Reading …. Read of Drift=∆R/∆T(Last reading) Drift= Read of Drift X ∆T (for each) ∆ Rc =∆R─ Drift Magnetic Anomaly = Observed Magnetic (∆ Rc ) - IGRF – Time Variation Used equations

22. ∆T ∆T=Time1─BS. Time ∆T=Time2─BS. Time ….

23. ∆R=Reading1 ─ BS. Reading ∆R=Reading2 ─ BS. Reading …. ∆R

24. Read of Drift=∆R/∆T(Last reading) Drift= Read of Drift X ∆T (for each) Drift Read of Drift = -0.08696

25. ∆ Rc =∆R─ Drift ∆ Rc

26. R/ST. Time (T) ∆T Reading (R) ∆R Drift ∆ Rc Time Variation Magnetic Anomaly B.S 05:20 38811 50 -38550 1.1 05:40 20 38100 -711 -1.739 -709.261 112 -39321.261 2.1 05:45 25 38130 -681 -2.174 -678.826 130 -39308.826 3.1 05:50 30 38000 -811 -2.609 -808.391 140 -39448.391 4.1 05:55 35 38100 -711 -3.043 -707.957 120 -39327.957 5.1 06:00 40 38140 -671 -3.478 -667.522 100 -39267.522 6.1 06:05 45 38020 -791 -3.913 -787.087 90 -39377.087 7.1 06:10 50 37915 -896 -4.348 -891.652 80 -39473.652 8.1 06:15 55 37890 -921 -4.783 -916.217 70 -39486.217 9.1 06:20 60 37999 -812 -5.217 -806.783 120 -39426.873 10.1 06:25 65 38115 -696 -5.652 -690.348 15 -39205.348 11.1 06:30 70 38224 -587 -6.087 -580.913 20 -39100.913 12.1 06:35 75 38315 -496 -6.22 -489.78 115 -39104.78 13.1 06:40 80 38490 -321 -6.957 -314.043 110 -38924.043 14.1 06:45 85 38956 145 -7.391 152.391 90 -38437.609 15.1 06:50 90 39109 298 -7.826 305.826 80 -38274.174 B.S 07:15 115 38801 -10 -10 50 -38550

27. Home Work A magnetic survey in a mineralized zone along a line provided the following data Interval of measurement is 500m. Length of the profile is about 12 km. Plot x-y graph of the anomaly and obtain the depth by half width . Take IGRF=38500 γ

28. Problem 2

29. A magnetic survey in a mineralized zone along a line provided the following data Interval of measurement is 500m. Length of the profile is about 12 km. Plot x-y graph of the anomaly and obtain the depth to the cavity by half width 1500 s1o 1520 1550 1595 1650 720 1640 1580 1540 1510 1500 500 1000 1500 3500 10 4500 5500 Plot the magnetic anomaly distance versus magnetic values and estimate the depth to the source by half width half maxima method V. Calculate the magnctic latitude from the given table of inclination. Plot the relation between inclination versus latitude. Use the formula tan(inclination)2 tan(latitude) Mag. Inclination in degrees 10 25 30 35 10 12 13 15 16 80 85 Problem 3

30. Advantages • Simple • Fast • Cost effective • No artificial source required • Good qualitative tool for mapping

31. • Very sensitive to non-unicity in the modelling solutions • Mainly qualitative • Very sensitive to metallic fences, rails (difficult to use in urbanized regions) Drawbacks

32. Telford W. M., Geldart L. P., and Sheriff R. E., 1990. Applied geophysics. Cambridge University Press 1990, 2 nd edition 770p. Philip Kearey , Michael Brooks, and Ian Hill 2002. An Introduction to Geophysical Exploration. Blackwell Science Ltd., 3 rd edition 262p. Reference