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

Books1. 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 Hall4. Introduction to Geophysical prospecting, Dobrin and Savitt MCGraw Hill (Numerate)5. Applied Geophysics Telford, Geldhart, Sheriff and Keys Cambridge University Press6. An introduction to geophysical processing, Kearey and Brooks, Blackwell (Non-numerate)

Week 1 Overview and Gravity 1Week 2 Gravity 2Week 3 Seismology Week 4 RefractionWeek 5 Electrical methodsWeek 6 Magnetics

Week 7 Other methods and exam reviewWeek 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

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

Lecture format40-45 minute lecture15-20 minute break40-45 minute lecture15-20 minute break1 hour practical

Lecture todayOverviewGravity introductionField methodData reductionGravity anomaly for a mass mGravity anomaly for simple shaped bodiesProblems 1-3

Overview

Overview

What physical properties do we measure?

DensityMagnetic propertiesResistivity, capacitanceVelocity

To work – the target body must have sufficiently different physical properties to the surrounding rock

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.

Gravity data from the Indian ocean

8Veritas)

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

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.

Water filled sand

Resistivity data

Sand partially filled with oil

Dry sand

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

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

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

Removes all effects except the near-surface density

Overview

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 carefullyMeasure height accurately20 mins per readingReturn to base every 1-2 hoursStation spacing depends on size of anomalous body

Overview

Drift correctionCorrects 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.

Other corrections

LC ~ ±0.81sin2 gu per 100 m

BC ~ ±0.0004191h (gu)

Eotvos and terrain

FAC ~ ±3.086h (gu)

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)

Isostasy

Isostatic anomaly = observed Bouguer anomaly - expected Bouguer anomaly

Get isostatic anomalies at foreland basins, oceanic ridges and post-glacial basins

and for all small scale features(these are not isostatically compensated)

Free air anomaly

Overview

Bouguer anomaly

Blue = gravity low Red = gravity high

Overview

Strong regional dip, deflected by oil-filled anticline, Oklahoma

Overview

Buried lead-zinc ore-body detected with gravity data

Overview

Gravity anomaly at a point at surface produced by a point mass:

gr = Gm/r2

Gravity anomaly measured by gravimeter

g = gz = Gmz/(x2 + z2)3/2

Gravity anomaly due to a spherical body

2/322

3

3

4

zx

zbGg

where b is the radius of the sphere

The maximum depth of the body (zmax) is = 1.3 x1/2

Problem 1

Stat. Time Dist. (m)

Elev. (m)

Reading Base reading

Drift corr’d anom. (gu)

LC(gu)

FAC(gu)

BC(gu)

Free airanom(gu)

Boug.anom.(gu)

BS 0805 0 0 2934.2 0 0 0

1 0835 20 10.37 2931.3 2934.49 -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

BS 0918 0 0 2934.9 0 0 0