1. 3. 01. Applied Geophysics What is it
3.01 Applied Geophysics What is it? Map changes in the physical properties of rocks to determine geological structure and lithology, locate minerals and hydrocarbons, investigate environmental hazards, archaeological investigations…
Jo Morgan
Room 2.38b

2. Books 1. Principles of Geophysics, N. H. Sleep and K
Books 1. Principles of Geophysics, N.H. Sleep and K. Fujita, Blackwell (Numerate) 2. The Solid Earth, by C.M.R. Fowler, Cambridge University Press, 1990 (most useful) 3. Principles of applied geophysics D.S. Parasnis Chapman and Hall 4. Introduction to Geophysical prospecting, Dobrin and Savitt MCGraw Hill (Numerate) 5. Applied Geophysics Telford, Geldhart, Sheriff and Keys Cambridge University Press 6. An introduction to geophysical processing, Kearey and Brooks, Blackwell (Non-numerate)

3. Coursework = NONE Week 1 Overview and Gravity 1 Week 2 Gravity 2
Week 3 Seismology
Week 4 Refraction
Week 5 Electrical methods
Week 6 Magnetics
Week 7 Other methods and exam review
Week 8 Set exercises
Coursework = NONE
Weekly problems, solutions provided before the end of the week
ALL handouts, problem solutions, ppts, typical exam questions will be placed on the ESE website

4. Examination Answer any 2 of 4 questions At least 1 question will contain a numerical calculation At least 1 part of a question will be based on the set problems 1 question will be in a similar style to question 1 in the example exam questions on the ESE web site Formulae will be supplied – but you need to know how to use them

5. Lecture format Lecture today Overview 40-45 minute lecture
15-20 minute break
1 hour practical
Lecture today
Overview
Gravity introduction
Field method
Data reduction
Gravity anomaly for a mass m
Gravity anomaly for simple shaped bodies
Problems 1-3

6. Overview Overview What physical properties do we measure? Density
Magnetic properties
Resistivity, capacitance
Velocity
To work – the target body must have sufficiently different physical properties to the surrounding rock
Overview

7. Gravity data from the Indian ocean
The sea floor topography is relatively flat, but gravity imaging highlights the fracture zones as the sediments infilling these fractures are lower in density than the oceanic crust.

8. Seismic data is used to visualise subsurface structure – in this case a channel in the North Sea
Veritas)

9. Ground penetrating radar image
The large oval shaped structure is thought to be a garden pond that was probably used for domesticating eels. The rectangular anomalies are believed to be military buildings on the villa premises.

10. Resistivity data Dry sand Sand partially filled with oil
Water filled sand

11. Overview Gravity 1 In gravity surveys we measure g
g varies with elevation, latitude,
topography, tides, instrument
drift and near-surface density
We make a number of corrections
to produce a gravity anomaly that
only reflects near-surface density
Salt domes, sedimentary basins, mine shafts = gravity low
Metalifferous ore bodies, anticlines = gravity high
Overview

12. Newton's law: g = GM/R2
Igneous and metamorphic rocks are usually denser than sedimentary
Most rocks will have a range of densities, and density is often related to porosity
Overview

13. Removes all effects except the near-surface density
Newton's law: g = GM/R2
average g ~ 9.81 ms-2,
g at poles ~9.83 ms-2
g at equator ~ 9.78 ms-2
g decreases as you climb a hill
Gravity anomaly = observed g - expected g
Overview
Removes all effects except the near-surface density

14. Overview Accurate gravity surveying is very slow
Gravity anomalies are very
small compared to the main field
Usually measured in mgal or gu
1mgal = 1 x 10-5 ms-2
1 gu = 1 x 10-6 ms-2
Accurate gravity surveying is very slow
Level gravimeter carefully
Measure height accurately
20 mins per reading
Return to base every 1-2 hours
Station spacing depends on size of anomalous body
Newton's law: g = GM/R2
Overview

15. Drift correction
Corrects g relative to a base station and removes instrument drift and tidal effects
Δg = gs-gb
gs is the measured gravity at the survey point, gb is the measured gravity at the base station at the same time. Δg is the drift corrected gravity anomaly at the survey point, measured relative to the base station.
Newton's law: g = GM/R2
Overview

16. Other corrections
Newton's law: g = GM/R2
LC ~ ±0.81sin2 gu per 100 m
FAC ~ ±3.086h (gu)
BC ~ ± h (gu)
Eotvos and terrain
Free air gravity anomaly = gs – gb ± LC ± FAC (+ Eotvos and terrain corrections if necessary)
Bouguer gravity anomaly = gs – gb ± LC ± FAC ± BC (+ Eotvos and terrain corrections if necessary)

17. Get isostatic anomalies at foreland basins, oceanic ridges
Isostasy
Newton's law: g = GM/R2
Get isostatic anomalies at
foreland basins, oceanic ridges
and post-glacial basins
and for all small scale features
(these are not isostatically
compensated)
Isostatic anomaly = observed Bouguer anomaly - expected Bouguer anomaly

18. Free air anomaly

19. Blue = gravity low Red = gravity high
Bouguer anomaly
Newton's law: g = GM/R2
Overview
Blue = gravity low Red = gravity high

20. Strong regional dip, deflected by oil-filled anticline, Oklahoma
Newton's law: g = GM/R2
Overview

21. Buried lead-zinc ore-body detected with gravity data
Newton's law: g = GM/R2
Overview

22. Overview )gr = Gm/r2 )g = )gz = Gmz/(x2 + z2)3/2
Gravity anomaly at a point at surface produced by a point mass:
Newton's law: g = GM/R2
)gr = Gm/r2
Overview
Gravity anomaly measured by gravimeter
)g = )gz = Gmz/(x2 + z2)3/2

23. Gravity anomaly due to a spherical body
where b is the radius of the sphere
The maximum depth of the body (zmax) is = 1.3 x1/2

24. Problem 1 Stat. Time Dist. (m) Elev. (m) Reading Base reading
Drift corr’d anom. (gu) LC
(gu)
FAC BC
Free air
anom
Boug.
anom. BS
0805
2934.2 1
0835 20
10.37
2931.3
-12.10
-0.16
32.00
-11.73
19.74
8.01 2
0844 40
12.62
2930.6 3
0855 60
15.32
2930.4 4
0903 80
19.40
2927.2
0918
2934.9