On the previous page we mentioned that sea surface heights and seafloor variations can be determined from altimeter studies. Some of the satellites dedicated to, or including systems for, altimeter studies of the sea surface and, where pertinent, the land surface are shown in this schematic summary:
These satellites have one thing in common: They all have corner-cube reflectors emplaced uniformly in spacing over their outer surface. These, and many other satellites, are used in a technology called Satellite Laser Ranging (SLR). This is the reverse of laser altimetry where the instrument transmitting the laser beam is on the satellite itself. Check this Website for a good review of the basic principles underlying SLR. In SLR, the laser generator is fixed at a ground station and its beam is directed into space to intercept and track the satellite equipped with silicon glass plates (usually two per port, mounted at right angles [corner-cube configuration] so as to accept a beam from any direction and return it) in an array termed retrograde reflectors. Here is Lageos (sometimes written as LAGEOS; it stands for Laser Geodynamics Satellite), one of the first satellites dedicated to geodesic measurements, orbited at 6000 km in 1976.
This schematic diagram synopsizes the mode of operation of a typical SLR system:
These are two of the now more than 46 active SLR stations in a global network:
The laser beam consists of coherent light generated at the site and sent out through a telescope as bursts of very short pulses. The beam seeks out an orbiting satellite and after finding it will continue to track the orbit for a time. A given pulse is returned to the SLR site in a very brief period of time; the time of start and the time of the same pulse's return can be very precisely determined by a maser clock (interval differences in picoseconds [10-12 sec] can be separated). Since the speed of light is known very precisely, the distance between the SLR station and the satellite at the instant of intercept can be determined to an accuracy of less than a meter. Because separate ephemeral (very short time duration) data can fix the changing position of the satellite quite accurately, the distance plus angle of beam projection allow the location of the transmitting station and the orbiting satellite to be well set. (As an aside, the "trick" in the whole operation is to be able to "hit" the fast-moving satellite [whose size may be a meter or the mirror-bearing part of a larger satellite may not be much bigger], and then follow it; ancillary optical and radio [S-band; 10 cm wavelength] tracking or Doppler tracking allow the orbital path to be so exactly known that its position at any moment can predicted close enough that the chances of an intercept are quite good [and, typically, one in every 5 to 10 pulses will reach a mirror].)
Commonly, a satellite is within the line of sight of several SLR stations. These may operate simultaneously such that they in effect "triangulate" on a satellite, a situation that greatly favors more accurate determinations of position, both of the satellite and the ground stations. Thus:
There are now a fair number of satellites fitted with retroreflectors suited to SLR tracking. Some of these have other prime purposes; a few are simply targets for geodetic studies. This next chart shows the past, presently functioning, and plannned satellites equipped with reflecting mirrors:
Because the lettering on the ordinate that identifies a satellite is hard to read, here is a partial listing of SLR-compatible satellites: ADEOS; ALOS; CHAMP; Envisat; ERS1 and 2; Etalon; Geos3; GLONASS; GPS35 and 36; Lageos1 and 2; Meteor3 and 6, Starlette; Stella. The designation LLR refers to Lunar Laser Ranging; three reflectors were left on the Moon's surface by Apollo astronauts and two more were on Russian probes that successfully landed; their presence has allowed much improved determinations of the lunar orbit and distance variations. A list of satellites that participate in SLR measurements is found at this Goddard Website.
Notice that the listing includes two GPSs (Global Positioning Satellites). This is an entirely separate system for location of points on the Earth or in space (it is the basis of civilian commercial hand-held instruments that allow anyone to locate himself/herself on the ground [for instance, when hiking in the wilderness] or in a car or boat). The system in likewise invaluable as a surveying method and in conducting some of the studies mentioned in the next paragraph. More is said about GPS on page 11-6 or you can find a good review online at this University of Colorado summary site. The Russians operate a similar system called GLONASS.
The use of SLR has become sophisticated enough that its data are now collected and available through the International Laser Ranging Service. From these data the International Terrestrial Reference Frame, a compilation of data on movements and relevant conditions of the land and ocean surfaces, is derived.
SLR has found many important applications, chief among which are: Polar motions and Earth rotations; gravitational variations of land surfaces; ocean surface configurations; ocean tides in basins; ice mass and volume; post-glacial rebound measurements; crustal deformation and tectonic plate motions.
This last category involves a major NASA, NOAA, U.S.Geological Survey program, joined by a group of international organizations that is commonly referred to as the Crustal Dynamics (now run out of Codes 920.2 and 926, NASA Goddard, check their Website); related information is found at the Geodynamics Branch site. The main objective is to measure both horizontal and vertical movements of the Earth's tectonic plates and their influence on crustal deformation. These motions, particularly at plate boundaries, amount to a few millimeters to centimeters a year. By checking periodically every several years on the precise locations of geodetic control points, mainly at specific stations established as bench sites, the actual movements of these points translate into approximately the same displacements within the plates driven away from spreading ridges. Very precise measurement of these points as they shift over time has allowed reliable estimates of the movements (both direction and velocity) both within plates and at their edges. Different plates move at different rates and directions.
SLR has proved capable of high accuracy. But another method, Very Long Baseline Interferometry (VLBI), also is also able to pinpoint a station's position and changes since its previous location(s). It relies on extremely accurate analysis of radio signals from distance sources that are received simultaneously at several radio telescope stations. Some of the basics of VLBI are covered at this Goddard site. The Canadians have prepared another good review site covering Principles of Radio Interferometry; a clear overview of some general principles of interferometry is found at this Canadian site.Some of the main ideas behind radio interferometry are summarized in the next several paragraphs. Look first at this diagram:
The radio wave source is usually some radio galaxy or a quasar sending strong radio signals. These, of course, travel at light speed. Two or more widely separated (very long baseline; resolution increases with length or separation) radio telescope receivers each record the signal. Each is also tied to a very accurate (hydrogen maser) clock which synchronizes the timing. Data reduction includes removing atmospheric effects and then checking time differentials for a given signal which result because one station is further away from the distant signal source than the other. The wave forms of the signals thus do not coincide precisely, i.e., they show partial to complete constructive or destructive interference (in optical interferometry, these slight differences in phase would appear as light fringes). Using a computer-based correlator, signal analysis by cross-correlation is conducted, as displayed in this diagram:
The result is a new data set from which the positions of reference points can be calculated and compared. Presently, about 40 receiver sites, mostly in North America and Europe, are participating in this effort.
The most accurate data on displacement rates come from averaging years of measurements. For instance, this plot covers 10 years of continuing data for the increasing separation between a European and a North American station. While there is scatter (uncertainty) for each measurement, the straight line fit (17 mm/yr) is credible.
Two maps indicate the emerging patterns of relative plate motion for those plates with enough site points to indicate both magnitude and direction of movement. The first is more general:
The plate containing the European "continent" is moving northeast, as the African plate pushes against it. The Pacific plate's direction is to the northwest. Two contrasting movement patterns occur in the North American plate; most is being driven west from the Mid-Atlantic Ridge but a small segment or sliver of western California southward is moving north-northwest to northwest where complex interactions (in part related to the overriding of the Cocos plate with the eastern Pacific plate cause sliding along a series of transverse faults (e.g., San Andreas). This next diagram offers more details and shows the fastest movements now occur in California (turquoise) and in southeast Australia (purple).
This is more evident in the following map on which velocity vectors are plotted relative to the plates involved:
With accumulating years of measurements, the movement picture for the North American plate has improved in detail. Here is one of the recent plots concentrating on plate shifts as measured at the stations indicated:
One of the by-products from NASA Goddard's Geodynamic Branch work on crustal deformation is the splendid plate tectonics map made by Dr. Paul D. Lowman, Jr. (author of Section 12 of this Tutorial). It now shows plate locations, plate velocities, and volcanic activity for the last one million years. We show this important map below but because at this smaller size it is unreadable we offer the option of looking at a large, more readable version by clicking here (to return to this page, hit your Back button).
Crustal movements of another kind have now been further detailed using positional information obtained from ranging to the Global Positioning System array of satellites. Parts of the Earth's crust were depressed (pushed downwards) from the weight of several kilometers thickness of ice at various times during the Pleistocene Ice Ages. As the more rigid upper crust bends down, the more plastic lower crust/upper mantle is driven sidewards to make room. When the ice sheets melt, there is a slow, generally steady upwards rebound that is now being measured by GPS and other satellites. A group of scientists at the University of Toronto and other institutions have recently reported on results obtained for the Fennoscandinavian crust. The illustration on the left shows measured vertical rebound; that on the right indicates that these workers also detected some horizontal movement outward from a depression center.
The third major subdivision of applied geophysics, Seismology, obviously does not benefit directly from air and space observations. But it is clearly a mode of remote sensing, similar to radar in measuring travel times of seismic waves. Seismometers must of necessity be on the surface (or in drill holes), preferably mounted on bedrock, to pick up the various kinds of seismic disturbances, especially from earthquakes. The seismic waves are oscillations of several types - P or primary waves (push-pull [compressional], analogous to acoustic wave motion), S or secondary waves (sinusoidal, also called shear waves), and others, such as surface Love and Rayleigh waves. Each wave type travels from its source to a seismic monitoring instrument (at a earthquake receiving station or, in geophysical surveying, in shallow holes near the point where wave-producing explosive detonations are set off) at some velocity, dependent on its depth and the seismic properties of materials along its particular path to the station. The oscillations at the station are recorded as "wiggles" or tracings on a seismogram. Here is a seismogram recorded at the Weston Station of Boston College for a recent earthquake in Turkey. The seismic waves traveled first through the crust under Turkey, then the Upper Mantle, and finally through the crust of the North American Plate under Boston.
Much can be learned from a seismogram: the distance to the earthquakes epicenter (point at the surface about its zone of origin); depth to the origin; magnitude or intensity (and kinetic energy released); duration; direction of movement; and often the type of fault along which the earthquake movement results in strain release. Each station has developed nomographs that can be used to solve for some of these parameters. Examine this one:
The distance to the epicenter can be in any direction (360°) from the station. But when distance information from three stations is used to draw a circle from each with the radius taken as its distance to the epicenter, there will emerge a narrow zone of intersection that defines the epicenter common to the three; thus:
Seen from the viewpoint of looking down at the northern hemisphere (North Pole at center), and with zones of active volcanism in red and continents in blue, the pattern again reveals that most earthquakes and volcanic eruptions occur at plate boundaries.
Such plots help to define zones or regions of higher earthquake activity. This distribution plus information about the nature and extent of damage allows estimates of seismic risk or likelihood of degree of destruction within some time span. This risk map has appeared in the newspaper USA Today shortly after an earthquake in Seattle, Washington in late February of 2001.
Another aspect of seismology is the technique called seismic tomography. In general, the term "tomography" refers to a representation in cross-section in which neighboring 2-dimensional cross-sections are combined to provide a 3-dimensional model. Computer-assisted tomography (CAT) as used in medical diagnosis is well-known as a non-invasive method of examining internal organs for abnormal regions (see page I-26c). X-rays or ultrasonic waves are absorbed unequally be different materials, and computer-aided tomography consists of studying the attenuation of X-rays, Gamma-rays, or ultrasonic waves that pass through the body in distinctly controlled planar sections. Seismic tomography uses the same principles, with the difference that the travel-times of the signals, rather than their attenuation, are observed. Seismic tomography involves the 3-dimensional modeling of the velocity distribution of seismic waves in the Earth. Sophisticated computer programs analyze arrival time differences to construct, in effect, slices into the Earth which when stacked together yield 3-D images.
Seismic tomography is at the leading edge of research into the nature of the crust, mantle, and core of Earth. It is too specialized to be considered in this brief survey. But these several figures may add a bit to your understanding. The first shows a series of seismic reflection waves beneath a small segment of crust; materials known to be at and just below the surface are indicated
Tomographic analysis can integrate seismic velocity variations into this type of 3-D representation in which the bodies in blue and red are also indicative of lithologic materials and/or structures within the mantle:
As said above, satellites cannot directly record earthquake (seismic) events. However, some seismometers are located in isolated places where direct telephone lines do not reach. Transmitters at the field station can telemeter data to satellites overhead that then relay the recorded seismic signals to receiving stations. Magnetic or gravity measuring satellites do reveal much about the Earth's interior which is of use in interpreting internal conditions related to plate motions that are factored in models used to analyze earthquake mechanisms.
There is one other use of satellites that bears on earthquake effects. This employs radar interferometry to examine displacements of the surface (SLR and altimeter profiling also can contribute to determining changes in surface position and elevations brought on by an earthquake). Color fringes produced from interferometric data indicate the distribution of changes in the surface (data from a pre-earthquake passage of the satellite are used to work out the differences in position).
An excellent case study which used this technology is the 7.1 magnitude Hector Mine earthquake of October 16, 1999 40 km (25 miles) northwest of Barstow, California (edge of Mojave Desert). This map shows the intensity distribution (relates to how people perceived the quake) around the epicenter.
This quake occurred in what is known as the Mojave shear fault zone. Lateral displacement along this predominantly strike-slip fault ranged from 3.8 to 4.7 meters (almost 15 ft). Surface rupture extending for 41 km can be seen in this aerial photo taken near the mine; the offset road (right) reveals this displacement.
Radar interferometry from aircraft and spacecraft have provided useful information on vertical ground displacements (see page 11-10 that may be precursor hints of impending events (this also applies to volcanoes where eruptions are accompanied by ground swelling [Mt. St. Helens case study; page 3-7]). Here is an interferogram (displayed by color fringes) showing displacement strain along the surface around the Lavic Lake fault as determined from analysis of SAR data from the ERS-1 satellite, soon after the day on which the Hector Mine earthquake was triggered:
A similar plot, superimposed on an image background, shows vertical displacement after an earthquake near Landers, California in the Mojave Desert. This occurred in 1992 within the same shear zone as the Hector Mine earthquake.
This subsection on geophysics as a component of remote sensing in its broadest meaning is, from what you have seen, a worthy diversion from the main theme. We move on to that theme - the more customary types of remote sensing - in the rest of this Introduction and the Sections that follow, first by investigating the role of the photon and its place in determining the characteristics of the electromagnetic spectrum.