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

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)

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

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 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

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

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.

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

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.

Resistivity data Dry sand Sand partially filled with oil Water filled 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

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

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

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

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

Other corrections Newton's law: g = GM/R2 LC ~ ±0.81sin2 gu per 100 m FAC ~ ±3.086h (gu) BC ~ ±0.0004191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)

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

Free air anomaly

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

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

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

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

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

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 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 0918 2934.9