VẬT lý địa CHẤN intruduction geophysics

Introduction to Geophysics What is Geophysics?? Goals • To make you aware and capable of implementing/discussing as many geophysical techniques as possible – Radar Interferometry – Isostasy – Earthquakes – Gravity – Magnetism – Paleomagnetism – Bathymetry – GPS – Seismic Refraction – Time series analysis – Seismic Reflection – Heat Flow – Down-hole logging – Vertical seismic profiles (VSP) – Geophysical analysis of rock samples Geophysics at the Coarsest Scale • What techniques we have to look at a region at the coarsest scale? – Satellite imagery – ie Landsat, SPOT – Satellite altimetry – Satellite gravity – Satellite magnetism – DEMs • Digital elevation models – DEMs from space – SRTM • Shuttle radar topography mission Some Gravity Basics • We will cover gravity in more detail later in the class – but for now here are the important concepts: Newton’s Law of Gravitation states that the force of attraction F between two masses m1 and m2 whose dimensions are small with respect to the distance r between them is given by the equation below, where G is the Gravitational Constant (6.67x10-11 m3kg-1s-2) Gm1m2 F= r2 r m1 m2 Some Gravity Basics For the case of a mass above the earth we can re-write the previous equation using M as the mass of the Earth, m as the mass of the object, and R as the distance between the centers of the objects GM F = m = mg R GM g= R R m M Force is related to mass by acceleration, and the term g is known as the gravitational acceleration On this theoretical Earth gravity would be constant, however, the Earth’s ellipsoidal shape, rotation, irregular surface relief and internal mass distribution cause gravity to vary over its surface Some Gravity Basics The gravitational field can be defined in terms of the gravitational potential, U: GM U= R Gravitational acceleration g is a vector quantity, having both magnitude and direction The gravitational potential U is a scalar, having only magnitude The first derivative of U in any direction is the component of gravity in that direction Equipotential surfaces are those on which the gravitational potential is constant, ie the geoid The ocean surface is an equipotential surface, and defines the geoid Shape of the Earth The geoid is one way of defining the shape of the earth, but we can also use the ellipsoid The earth is not a perfect sphere, it is an oblate spheroid We can approximate the shape of the earth by an ellipsoid •Radius from center to equator = 6378.16 km •Radius from center to pole = 6356.77 km •Polar shortening of part in 298.25 – often referred to as flattening The geoid and the ellipsoid are not coincident as the earth is not homogeneous From http://rst.gsfc.nasa.gov/Intro/Part2_1b.html Shape of the Earth Similarly, a satellite orbiting the earth moves up and down along its orbit as it is affected by the same gravitational forces that produce the geoidal surface From http://rst.gsfc.nasa.gov/Intro/Part2_1b.html Geoid Sandwell et al, bathy workshop Large geoid low over India (~100 m) – mass defficency A ship going from Darwin, Australia, to the southern tip of India is going downhill! Geoid highs over trenches – old and cold subducted slabs Satellite Altimetry Satellites not carry accelerometers Gravity variations can be calculated from changes in the position (shifts in orbital height) of a satellite as it orbits due to variations in gravity •Tracking of radio signals (using Doppler shifts in frequency) from the satellite help to determine these variations •Locating the position of the satellite with satellite laser ranging •Measure the changing height of the surface (sea level with reference to the ellipsoid) with radar or laser altimetry The presence of extra mass on the seafloor is to cause a deviation of gravitational attraction such that water above the seamount “bunches up” Sandwell et al, bathy workshop Satellite Altimetry The satellite uses a radar pulse to measure the distance to the sea surface Repeating this pulse every 0.001 seconds allows the noise levels to be reduced (waves etc) The difference between this distance and that to the theoretical ocean surface is the geoid anomaly Note that each height value represents an average of observations taken during second when the satellite moves about km over the ground Height precision is on the order of cm Sandwell et al, bathy workshop Gravity from Space Deriving the gravity field from space is only good for looking at coarse detail, mainly in unexplored regions From http://rst.gsfc.nasa.gov/Intro/Part2_1b.html As the geiod (gravitational potential) and the gravity field are related, the gravity field can be calculated from a map of the geoid This is one of the first methods used to get a detailed image of the seafloor Geoid and gravity maps of part of the Gulf of Mexico Gravity from Space From http://rst.gsfc.nasa.gov/Intro/Part2_1b.html Tonga-Kermadec Trench, Louisville Seamount Chain Southern Oceans around Antarctica Bathymetry Sandwell et al, bathy workshop The obvious way to collect bathymetry data for the oceans is using acoustic methods along sparse ship tracks The above map shows survey ship tracks in the South Pacific at the scale of the continental US Making a map of the US using only data from along these tracks would obviously be very ineffective A systematic survey of the oceans by ships would take more than 200 years of survey time at a cost of billions of U.S dollars Bathymetry from Space From http://topex.ucsd.edu/marine_topo/text/topo.html In contrast A complete satellite survey can be made in five years for under $100M This has not yet been done, but we are part of the way there Gravity is correlated with bathymetry at short wavelength • By examining the correlation between bathymetry along sparse ship tracks and the corresponding satellite gravity values, a function can be derived to convert satellite gravity data to bathymetry where there are no ship tracks • This function varies depending on the geology • Highly sedimented • No sediment • etc Bathymetry from Space From http://topex.ucsd.edu/marine_topo/text/topo.html Topography There are a number of sources of topography – many are the domain of geographers, ie: •Maps (USGS, etc) •Conventional Digital elevation models (DEM) •Many of these are derived from contour maps (digitized) •Popular GIS programs can display and analyze these Geophysicists get involved when the technology gets a little more complicated For example: •Imaging radar systems (for example the SIR-A, -B, and –C flown on the Space Shuttle) typically use variations in the signal bounced back from the ground to create an image From http://spaceplace.jpl.nasa.gov/en/kids/srtm_makemap3.shtml Imaging Radar From http://rst.gsfc.nasa.gov/Sect8/Sect8_7.html In this image, the color scene comes from a Landsat image from the Sahara desert in NW Sudan The diagonal strip is from an image acquired from SIR-A flown in Nov 1981 Because the sand is dry and has a low dielectric constant the radar waves penetrate up to m, imaging not just the sand, but also the bedrock below it Radar Interferometry Radar imagery taken on two different dates can be compared, and the phase difference determined to calculate the distances to point targets This was taken to the next level with the Space Shuttle Topography Mission (SRTM) in 2000 •Rather than compare two images taken on different dates, two images were collected simultaneously from slightly different locations •What is phase difference? This is an example of the use of phase difference when collecting interferometric sidescan data •Two adjacent transducer arrays •Both receive at the same time •As they are next to each other they are receiving basically the same signal •However, the signal coming to array A may have traveled slightly further than that arriving at array B •This translates into a phase difference •The phase difference can be used to determine the angle from which the signal came •Combined with travel time, this tells us the distance to that point on the seafloor SRTM A similar principle can be used from space, but the transducers (antenna) have to be further apart – in this case 60 m From http://spaceplace.jpl.nasa.gov/en/kids/srtm_make2.shtml A transmit antenna illuminates the terrain with a radar beam which is scattered by the surface Two receive antennas with a fixed separation between them (baseline) record the backscattered radar echo from slightly different positions resulting in two different radar images The two signals received at both ends of the baseline show a phase shift due to different signal paths Through the calculation of the relationship between target-receiver distances and the phase difference one obtains elevation information which can be turned into digital elevation models and maps SRTM http://www2.dlr.de/oeffentlichkeit/specials/sonderseiten/srtm/srtm_folder_02.pdf SRTM SRTM USGS G-TOP30 and digitized data Using this method, near global topography has been generated at a pixel size of arc-second (approximately 30 m) So far, data at this resolution is only available for the continental US For other regions the data is available at a pixel size of arc-seconds (90 m) The above image compares 90 m SRTM data for the Papuan Peninsula with the previously available 30 arc-second data (~1 km grid size) References Used Basic gravitational theory: • Kearey, P., M Brooks, and I Hill, An Introduction to Geophysical Exploration The geoid, satellite altimetry • http://rst.gsfc.nasa.gov/Intro/Part2_1b.html Calculating bathymetry from satellite gravity • http://topex.ucsd.edu/marine_topo/text/topo.html • Smith, W H F and D T Sandwell, Bathymetric prediction from dense satellite altimetry and sparse shipboard bathymetry, J Geophys Res., 99, 2180321824, 1994 • Sandwell, D.T., Gille, S.T., and W.H.F Smith, eds., Bathymetry from Space: Oceanography, Geophysics, and Climate, Geoscience Professional Services, Bethesda, Maryland, June 2002, 24 pp., www.igpp.ucsd.edu/bathymetry_workshop Imagaing Radar (and a small amount of interferometry) • http://rst.gsfc.nasa.gov/Sect8/Sect8_7.html Radar Interferometry (SRTM mission) • http://www2.jpl.nasa.gov/srtm/ • http://spaceplace.jpl.nasa.gov/en/kids/srtm_make2.shtml • http://www2.dlr.de/oeffentlichkeit/specials/sonderseiten/srtm/srtm_folder_02.pdf [...]... prediction from dense satellite altimetry and sparse shipboard bathymetry, J Geophys Res., 99, 2180321824, 1994 • Sandwell, D.T., Gille, S.T., and W.H.F Smith, eds., Bathymetry from Space: Oceanography, Geophysics, and Climate, Geoscience Professional Services, Bethesda, Maryland, June 2002, 24 pp., www.igpp.ucsd.edu/bathymetry_workshop 4 Imagaing Radar (and a small amount of interferometry) • http://rst.gsfc.nasa.gov/Sect8/Sect8_7.html ... where G is the Gravitational Constant (6.67x1 0-1 1 m3kg-1s-2) Gm1m2 F= r2 r m1 m2 Some Gravity Basics For the case of a mass above the earth we can re-write the previous equation using M as the... series analysis – Seismic Reflection – Heat Flow – Down-hole logging – Vertical seismic profiles (VSP) – Geophysical analysis of rock samples Geophysics at the Coarsest Scale • What techniques we... technology gets a little more complicated For example: •Imaging radar systems (for example the SIR-A, -B, and –C flown on the Space Shuttle) typically use variations in the signal bounced back from