Gravity and Magnetic Surveying Lecture 4 Gravity and Magnetic Surveying

Terrain corrections Bouguer correction assumes and infinite slab. Effect of hills or valley cause Gravity to be less. To correct both must be added.

Terrain corrections can be computed using transparent template, called a Hammer Chart, which is placed over a topographic map. Chart is centred on gravity station and topography read off at centre of each segment.

Terrain chart for topographic effects

Global Free Air Gravity Map See topographic effect Andes, Himalayas, Deep ocean basins

Matlab Program for Cylinder x=[-10:.1:10]; %depth a(1)=1; %radius a(2)=0.5; %density drho=1e3; x1=x; G=6.67e-11; R=sqrt(x1.^2+a(1)^2); dg=2*G*drho*pi*a(2)^2*a(1)./R.^2; f=dg*1e5; figure(1) plot(x,f,'r') axis([-10,10,-2,2])

Program for Talwani Polygon close all xs=[-10:.1:10]; sumzee=0.0; for i=1:length(xd)-1; x=xd(i)-xs;xp=xd(i+1)-xs; z=zd(i); zp=zd(i+1); th=atan2(z,x); thp=atan2(zp,xp); phi=atan2((zp-z),(xp-x)); if z-zp==0, zp=0.0001;end a=xp+zp*((xp-x)./(z-zp)); term1=(th-thp); num=cos(th).*(tan(th)-tan(phi)); den=cos(thp).*(tan(thp)-tan(phi)); term2=tan(phi).*log(num./den); den zee=a.*sin(phi).*cos(phi).*(term1+term2); sumzee=zee+sumzee; end G=6.672e-11; rho=1e3;

Gravity across the US and its relationship with tectonic features. Thick crust or thin lithosphere = low gravity because of less mass.

Magnetic Field Measurements and Interpretation

Roberts and Jachens, USGS, 1999

The dipole Can be caused by a bar magnet or a loop of current carrying wire

Dipole (two poles) Note direction of field Earth’s field is mainly a dipole, but not only. North pole is south magnetic pole, but that may flip

Geomagnetic Field Dynamo in outer core generates the geomagnetic field.

Geographic north is determined by the spin axis. Magnetic north (actually a south magnetic pole) is Determined by currents in the core. It is presently near Greenland The geomagnetic field. Declination is angle field makes with geographic north. Inclination is the angle the field makes with The horizontal (dip). D=13.5o and I=60o in LA

Location North Magnetic Pole Year Latitude ( °N) Longitude ( °W) 2001 81.3 110.8 2002 81.6 111.6 2003 82.0 112.4 2004 82.3 113.4 2005 82.7 114.4

Principle of the geodynamo A dynamo makes electrical current by moving a coil relative to a magnetic field (e.g. that from a magnet). One way to make a magnetic field is to generate an electric current. The geodynamo Is thought to be caused by electric currents (a billion amperes) in the liquid iron core. Motion of the iron as the core convects regenerates the current.

Change in declination of compass at the geomagnetic observatory London

Contours of geomagnetic field Contours are in units of 5000 nT (nanoteslas) and range from 25,000-65000 nT

Non-Dipole Geomagnetic Field This pattern changes in time drifting hundreds of km across the globe/year. Thought to be due to eddy currents in the outer core it shows that these Motions are a million times faster than plate tectonics; Why?

Magnetism in rocks records the Earth’s magnetic field at time of cooling Hot (above Curie Point) magnetism is random. Cool (below Curie Point) magnetism locks in to direction of the magnetic field at that time. It is then stable w.r.t. future Reversals. Blocking temperature is temperature below the Curie Point that magnetism locks in.