1. Final review sessions - II
Environmental and Exploration Geophysics I
Final review sessions - II
tom.h.wilson
Department of Geology and Geography
West Virginia University
Morgantown, WV
Tom Wilson, Department of Geology and Geography

2. Exam review week – session II Magnetics lab due today - December 9th
Afternoon of the 10th (at the latest!)
Exam, Friday December 17th; 3-5pm
Tom Wilson, Department of Geology and Geography

3. Last problem set handed back today
What is the horizontal gradient of the Earth’s vertical magnetic field (ZE) in an area where the horizontal field (HE) equals 20,000 nT and the Earth’s radius is 6.3 x 108 cm.
We know
Tom Wilson, Department of Geology and Geography

4. Tom Wilson, Department of Geology and Geography
Evaluate the horizontal gradient of the vertical component of the Earth’s magnetic field. or
See discussions of Equation 7.20
nT/m
Tom Wilson, Department of Geology and Geography

5. Can you find it? What is z? What is I?
4. A buried stone wall constructed from volcanic rocks has a susceptibility contrast of 0.001cgs emu with its enclosing sediments. The main field intensity at the site is 55,000nT. Determine the wall's detectability with a typical proton precession magnetometer. Assume the magnetic field produced by the wall can be approximated by a vertically polarized horizontal cylinder. Refer to figure below, and see following formula for Zmax.
Given background noise level of 5nT.
What is z?
What is I?
Tom Wilson, Department of Geology and Geography

6. Tom Wilson, Department of Geology and Geography
Problem 4
Vertically Polarized Horizontal Cylinder
General form
Evaluate and compare
Zmax= 18nT (using z=1.75m)
Zmax= 15.3nT (using z=1.75m)
Tom Wilson, Department of Geology and Geography

7. Detecting abandoned wells
The online group magnetic problems all have problems similar to problem 5 in the problem set returned today.
The anomaly associated with vertical well casing has a shape like that produced by an isolated pole. So its shape is identical to that of the gravity anomaly associated with a sphere or a point mass.
If the magnetic object is more localized and equidimensional in shape it will drop off more suddenly with distance from the anomaly peak.
Tom Wilson, Department of Geology and Geography

8. Tom Wilson, Department of Geology and Geography
0.9
X3/4
1.55
X1/2
2.45
X1/4
Sphere vs. Vertical Cylinder; z = Diagnostic distance *DIM
Diagnostic positions
Multipliers
Sphere
ZSphere
Multipliers Cylinder
ZCylinder
X3/4 =
X1/2 =
X1/4 =
3.18 2
1.37
2.86
3.1
3.35
2.17
1.31
0.81
1.95
2.03
2.00
Tom Wilson, Department of Geology and Geography

9. Tom Wilson, Department of Geology and Geography
Problem 3 practice final: Silo liner or storage tank?
Figure 1: This profile line crosses through the maximum value of a circular shaped anomaly encountered in a 2D grid of magnetic data.
Tom Wilson, Department of Geology and Geography

10. Tom Wilson, Department of Geology and Geography
By now - a familiar approach to solution?
Diagnostic positions
Multipliers
Sphere
ZSphere
Multipliers Cylinder
ZCylinder
X3/4 =
X1/2 =
X1/4 =
4.6
7.7
12.4
3.18 2
1.37
14.6
15.4
17.0
2.17
1.31
0.81
9.98
10.1
10.0
Cylinder – 10 m deep
Tom Wilson, Department of Geology and Geography

11. Tom Wilson, Department of Geology and Geography
gmax
g3/4
g1/2
g1/4
Sphere or cylinder?
Diagnostic positions
Multipliers
Sphere
ZSphere
Multipliers Cylinder
ZCylinder
X3/4 = 1.6 meters
3.18
2.17
X1/2 = 2.5 meters 2
1.31
X1/4 = 3.7 meters
1.37
0.81
5.01
5.0
5.07
3.47
3.28
2.99
Tom Wilson, Department of Geology and Geography

12. Tom Wilson, Department of Geology and Geography
If you look through the magnetic group problems you should find the following:
Anomaly 1: Sphere with depth to center of 4 meters
Anomaly 2: Cylinder with depth to top of 4 meters
Anomaly 3: Cylinder with depth to top of 10 km
Tom Wilson, Department of Geology and Geography

13. Tom Wilson, Department of Geology and Geography
What is R?
Algebraic manipulation
Given that derive an expression for the radius,
where I = kHE. Compute the depth to the top of the casing for the anomaly shown below, and then estimate the radius of the casing assuming k = 0.1 and HE =55000nT. Zmax (62.2nT from graph below) is the maximum vertical component of the anomalous field produced by the vertical casing.
0.3 meters
Tom Wilson, Department of Geology and Geography

14. Tom Wilson, Department of Geology and Geography
From the practice final …
4. You are given the main magnetic field intensity (FE), the declination (d) and the horizontal projection of FE: HE. What is the vertical component of the earth’s magnetic field? What is the inclination of the main magnetic field?
5. You are standing 10 feet from a point directly over the end of abandoned casing string. The top of the abandoned well casing lies 10 feet beneath the surface. The effective pole strength for the casing string is 5 ups. What is the vertical component of the anomalous field at the observation point? State your answer in nanoTeslas.
Tom Wilson, Department of Geology and Geography

15. Tom Wilson, Department of Geology and Geography
Problem 6 – practice final > Tide & Drift.
At 8 am you start your gravity survey at Base Station 5 in your survey area. You set out to establish another base station (Base Station 6) about an hours drive from Base 5. At 8am the acceleration due to gravity at Base Station 5 is 5.3 milliGals relative to the main base station in your survey area. You make it to the new base station and take a measurement 53 minutes after making the Base 5 observation. Your reading at Base Station 6 is 4.3 milliGals. You return to Base 5 but stop for gas along the way. You finally re-measure the Base 5 acceleration at 10 am (120 minutes after your initial measurement). The reading at Base Station 5 dropped 2 milliGals during that 120 minute period. What is the acceleration at Base Station 6 relative to the main base?
Tom Wilson, Department of Geology and Geography

16. Tom Wilson, Department of Geology and Geography
Problem 7. Another problem that can be addressed using simple geometrical analysis
Tom Wilson, Department of Geology and Geography

17. Tom Wilson, Department of Geology and Geography
Repeat of a familiar analytical approach
Diagnostic positions
Multipliers
Sphere
ZSphere
Multipliers Cylinder
ZCylinder
X3/4 =
X1/2 =
X1/4 =
8.8 16 26
3.18 2
1.37
2.17
1.31
0.81
Tom Wilson, Department of Geology and Geography

18. Tom Wilson, Department of Geology and Geography
Some general questions ….
8. What is the theoretical gravity.
9. Write out the equation that defines the theoretical gravity
10. What is the terrain corrected Bouguer anomaly?
11. Given FE, HE, and d solve for YE, XE, ZE and i.
Tom Wilson, Department of Geology and Geography

19. Exam, Friday December 17th; 3-5pm
Questions? ...
Turn in the Magnetics by Friday afternoon (December 10th) at the latest
Exam, Friday December 17th; 3-5pm
Tom Wilson, Department of Geology and Geography