2nd Year : Geophysical Methods in Geology


Trinity Term 2000

Week 2 :

Tues. 9-10 Lecture 1 : Gravity [ABW].

Gravitational acceleration and potential. Units. International gravity formula. The geoid. Free-air and Bouguer corrections.

Frid. 10-11 Lecture 2 : Gravity [ABW].

Absolute and relative gravity measurements. Survey design. Rock density. Gravity anomalies over sedimentary basins, granite bodies, mountain ranges. Local and regional gravity anomalies. "Trailing errors "

Frid. 11-1 Practical 1 : [ABW]

Interpretation of Bouguer gravity anomalies over the Pennines

Week 3:

Tues. 9-10 Lecture 3 : Gravity [ABW].

Direct and indirect methods of interpretation. Derivative methods. Static and process oriented modelling techniques.

Frid. 10-11 Lecture 1 : Magnetics [EM].

Earth's magnetic field, instruments, data corrections, survey design. Basics of potential field theory, units, structure of Earth's magnetic field. International Geomagnetic Reference Field. Time variations, external and internal sources. Origin of the Earth's field, Induced and remanent magnetism, susceptibility.

Frid. 11-1 Practical 2 : [EM]

Calculation of magnetic anomalies over a series of simple sub-surface bodies

May 19/21 First Weekend Field Trip for half of the class. Undertake gravity, magnetic and seismic measurements over the Beacon Hill pericline in the eastern Mendip Hills [ABW, GW, CM, PJ].

Click here for a picture of the first week-end at Tedbury Camp

The weather was not ideal......

....but, the andesite geology was good.

Groups A, B, C and D original gravity data

Group A FINAL reduced gravity data

Group C FINAL reduced gravity data

Group D FINAL reduced gravity data

Week 4:

Tues. 9-10 Lecture 2 : Magnetics [EM].

Magnetic surveying, instruments, data corrections, survey design. Continental magnetic anomalies, local and regional anomalies.

Frid. 10-11 Lecture 3 : Magnetics [EM].

Modelling of magnetic anomalies, and anomaly interpretation

Frid. 11-1 Practical 3 : [ABW] + [EM]

Write-up and finish Practicals 1 and 2

Pennines 1 answer sheet

Pennines 2 answer sheet

May 26/28 Second Weekend Field Trip for half of the class. Undertake gravity, magnetic and seismic measurements over the Beacon Hill pericline in the eastern Mendip Hills [ABW, DH, GA, PJ].

Click here for a picture of the second week-end at Tedbury Camp

......50% of the A team at Tedbury (note the sedimentary structures (slumps??) in the Carboniferous Limestone)

......at the face of the northern limb of the Beacon Hill pericline at Fairy Cave Quarry.

Groups A, C and D original gravity data

Group D FINAL reduced gravity data

Group B Magnetics data

Group C Magnetics data

Seismic reflection profile data acquired during Week-ends 1 and 2

Map showing seismic profiles in the field to the NW of the intersection between the Frome Road and Fosse Way - Groups A1, B1 and D1

Map showing seismic profiles in the field south of Frome Road to the SW of the Waggon & Horses pub - Groups C1, B2, C2 and D2

Line 22-1

Line 20-2

Line 21-1

Line 27-1

Line 27-2

Week 5:

Tues. 9-10 Lecture 1 : Seismic refraction [SD].

Instruments and survey design. Travel-time distance graphs. Interpretation of layer thickness and velocities. Inverse methods. Examples of oceanic and continental velocity and crustal structure

Frid. 10-11 Lecture 2 : Seismic reflection [ABW].

Instruments and survey design. Shot gathers and Common Depth Point (CDP) stacks. Velocity analysis. Normal Moveout. Migration. Examples of seismic reflection profiles of sedmentary basins and mountain belts

Frid. 11-1 Practical 4 : [ABW]

Interpretation of the BIRPS "SWAT 4" seismic reflection profile of the Celtic Sea basin.

Week 6:

Tues. 9-10 Practical 5 :

Mendips field gravity data processing and reduction [ABW]

Frid. 10-1 Practical 6 :

Mendips field gravity data processing and reduction [ABW]

Gravity anomaly interpretation and modelling [ABW]

Week 7:

Tues. 9-10 Practical 7 :

Mendips field magnetics data processing and reduction [EM]

Frid. 10-1 Practical 8:

Mendips field magnetics data processing and reduction [EM]

Magnetic anomaly interpretation and modelling [ABW]

Calculated magnetic anomalies over E-W trending dipping dykes

Week 8:

Tues. 9-10 Practical 9:

Mendips field seismic data processing and reduction [ABW]

Frid. 10-1 Practical 10:

Mendips field seismic data processing and reduction [ABW]

Reading :

"An introduction to geophysical exploration" by P. Kearey and M. Brooks. Blackwell Science. 254p
Highly recommended. Chapters 2-7 are particularly relevant to the course.

"An introduction to seismic interpretation" by R. McQuillin, M. Bacon and Barclay : Graham & Trotman.
A good supplmentary text for seismics.

Field Trip Report - Guidelines

Selected journal references

Lacoste-Romberg land gravimeter
SmartSeis seismic reflection and refraction
GARMIN 12XL GPS Receiver

Gravity formula and keywords
Seismic reflection formula and keywords

Last years field trip

Gravity and topography profile centered on the Waggon and Horses - obtained during the 1998 reconnaiscance trip

Oxford Marine Geophysics (Gravity, Magnetics, Seismic) Group
Oxford Paleomagnetics Group

Society of Exploration Geophysicists (SEG)
SEG Gravity and Magnetics Committee - includes useful glossary of potential field terms
Example of Gravity and Magnetics consultancy - includes a "primer on gravity and magnetics

Department of Earth Sciences Home Page

Tony Watts - Last modified July 10th 2000