27 Jan 12

How Does a Seismometer Work?

iris.edu

A seismograph is a device for measuring the movement of the earth, and consists of a groundmotion detection sensor, called a seismometer, coupled with a recording system. A simple seismometer that is sensitive to up-down motions of the earth can be understood by visualizing a weight hanging on a spring. The spring and weight are suspended from a frame that moves along with the earthʼs surface. As the earth moves, the relative motion between the weight and the earth provides a measure of the vertical ground motion. If a recording system is installed, such as a rotating drum attached to the frame, and a pen attached to the mass, this relative motion between the weight and earth can be recorded to produce a history of ground motion, called a seismogram.

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Seismographs operate on the principle of inertia — stationary objects, such as the weight in the above picture, remain stationary unless a force is applied to them. The weight thus tends to remain stationary while the frame and drum are moving. Seismometers used in earthquake studies are designed to be highly sensitive to ground movements, so that movements as small as 1/10,000,000 centimeters (distances almost as small as atomic spacing) can be detected at very quiet sites. The largest earthquakes, such as the magnitude 9.1 Sumatra-Andaman Islands earthquake in 2004, create ground motions over the entire Earth that can be several centimeters high.

Modern research seismometers are electronic, and instead of using a pen and drum, the relative motion between the weight and the frame generates an electrical voltage that is recorded by a computer. By modifying the arrangement of the spring, weight and frame, seismometers can record motions in all directions. Seismometers also commonly record ground motions caused by a wide variety of natural and man-made sources, such as trees blowing in the wind, cars and trucks on the highway, and ocean waves crashing on the beach.

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13 Jan 12

Geophysical Surveying

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Geophysical traverses can be performed using a combination of electronic devices, including a proton ground magnetometer, a gamma-ray spectrometer, and a VLF-electromagnetic unit. These devices are characterized by their ease of operation and lightweight portability, facilitating operation in areas that are difficult to access.

The magnetometer measures the vertical magnetic intensity of subsurface materials. Higher values can generally be attributed to rocks of relatively high magnetism. Lower values can indicate materials of low magnetism or discontinuous zones, such as fractures, faults, channels, or contacts, where the magnetic materials have been leached out.

G-856 Proton Magnetometer. Get yours at heritagegeophysics.com.

Care must be taken in the location of the traverses, because buried pipes, power lines, and metal objects can adversely affect the readings and cause abnormal magnetic peaks. The magnetometer helps locate areas of fracturing or channeling in the underlying formations, which may form favorable geologic conditions for ground-water transmission or storage.

The gamma-ray spectrometer is used to measure the natural radiation from near-surface earth materials, primarily uranium, potassium, and thorium. This technique is useful in mineral exploration for radioactive materials as well as in geologic mapping and fault location, where contrasting, weakly radioactive rocks are buried beneath the soil cover.

The very low frequency electromagnetic (VLF/EM) survey measures secondary fields generated by conducting bodies in the subsurface, when subjected to a primary electromagnetic signal initiated from a fixed military transmitter. Readings are taken at closely spaced intervals and the vertical in-phase and vertical out-of-phase, or quadrature, components of the secondary fields are measured. Anomalous conditions, generally indicated by a “cross-over” may be related to subsurface contacts or faulting.

Electrotelluric soundings

Electrotelluric soundings measure the relative conductivity throughout a specific depth interval and allow an accurate evaluation of ground-water production potential to depths of up to 2,500 feet. This is particularly useful in desert environments, where deep wells are often necessary. In the past, traditional resistivity soundings were only able to penetrate to depths of 600 feet before the vertical accuracy became suspect. Now an analysis of potential deep aquifers can be made before attempting expensive exploratory drilling.

The electrotelluric survey measures the electrical fields generated by telluric currents flowing in the subsurface formations of differing lithology. These currents are induced by the ionosphere as pulses, with duration depending on the depth of penetration. A surface receiver transforms the electrotelluric radiation into an audible signal.

VLF/Electromagnetic soundings

Very low frequency electromagnetic (VLF/EM) resistivity soundings measure the ratio and phase angle between the horizontal magnetic field and the horizontal electric field components of the plane wave initiated from a fixed military transmitter. Apparent resistivity of one or two earth layers can be determined, with the effective depth of penetration dependent on both the electrical resistivity and the frequency of the radio wave received. The equipment is both lightweight and quick to set up, making these soundings useful in areas of limited accessibility and as a check on suspected resistivity conditions.

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5 Jan 12

The Proton Magnetometer: New Tool for Archeologists and Treasure-Hunters

By E. H. Marriner | geotech1.com

EXTRACT

Archeologists can save precious time with a new instrument: the proton magnetometer. Just recently, it has found stone-age civilization sites in England and probed Etruscan tombs in Italy. Now investigators are using it in the USA. No longer must archeologists drill holes and dig trenches just to learn whether there might be anything down there worth looking for. With the proton magnetometer, the center of a site can be plotted on a map long before anyone brings out the shovels.

An electrical method of probing has been used for some time. It works by passing a current into the ground through two outer electrodes in a line of four. The resistivity can then be calculated from a simple formula involving the measured resistance between the inner pair of electrodes and their distances apart. The success of a site survey depends on having a measurable contrast in resistivity between the structures sought and the surrounding ground. The contrast depends on the moisture content of the ground; this, being a variable, can be compensated for on the chart. Information is gathered by making a measurement spread through a right angle. Then the data are processed. A pattern 25 feet wide and 55 feet long can be plotted in hour, against days of digging by the old method.

This method of locating sites has been used in Europe for several years but has only recently been tried in this country. The University of California found a new San Dieguito Indian complex in San Diego County, California, on the first field trial. But the resistivity method is being supplanted by a new method, using the proton magnetometer.

Any instrument that measures a magnetic field is called a magnetometer. Such instruments have been used in geophysical studies for many years.

The original magnetometer was called the Swedish mining compass. It was developed about 100 years ago and used only to detect ore bodies. About the turn of the century the Hotchkiss dip needle and super-dip were developed. During World War II, aircraft detection of submarines came out of this principle, although these devices were actually a type of variometer (instrument used to measure variations in magnetic fields).

In 1956 a group of research geophysicists developed the proton magnetometer. This instrument measures the magnitude of the earth’s magnetic field. The operator of the device can detect a magnetic anomaly spatially on a chart.

Systematic variations in the physical properties of the earth are what enable the magnetometer to find the structures that cause them. These variations are called anomalies. Anomalies due to magnetic material can be detected by the proton magnetometer. Two distinct types sought by the archeologist are those produced by soil of high susceptibility in comparison with its surrounding material, and those caused by the ferromagnetic properties of heated material, which is termed thermoremanent magnetism.

The magnetometer is one of the few devices that have been adapted in the past few years to archeological surveying.

The first use of the proton magnetometer for archeological surveying was in the winter of 1957 at Huntingtonshire, England, by a group of Oxford University scientists. They detected a number of Roman and medieval walls and hose outlines made of clay.

A few years later a group from the Indiana Historical Society used the device to find a pre-Columbian village in southern Indiana and produced enough evidence to map in detail the entire fortified village.

A proton magnetometer of different design was used in the German rhineland. This instrument produced readings unaffected by diurnal variations or magnetic storms. It proved of real value in this area since there is a great variety of subsoils in the pits and ditches. The Roman city of Xanten was explored with the proton magnetometer, and the survey data produced detailed maps of the city’s foundations and defenses.

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29 Dec 11

Resistivity: Principles and Applications

tricon-online.de

1. Principles

An electric current is injected to the earth’s ground by two electrodes (A and B). A potential field is produced in the underground inclined from the dispersion of the specific electrical resistance. Is the resistance spread in a homogenous way the electric current- and potenial-field lines are produced (fig 1).

Fig 1: Current dipole (red) and potential field (blue) showed in the situation of homogenous dispersion of the specific underground’s resistance.

The potential differences can be recorded with two other electrodes (M and N) on the earth`s surface. You can calculate the apparent resistivity with the two values of tension and electric current. (Inclined to the distance of the electrodes A and B and also inclined to the dispersion of the conductivity in the underground the produced potential field is spread in a specific way. You measure the tension with the two potenial electrodes A and B. Then you can calculate the apparent resistivity: It is called like this because it is the integral of the volumetric dispersion of the specific resistances of this volume.)

Is there a variation of the conductivity in the underground caused by changing bedding, changes of the fine fraction in a bedding or different rates of cleaves in a rock the current dipole is teared in a characteristical way (fig 2).

Fig. 2: Current-lines running through a bedding with bad conductivity above a bedding with good conductivity

Usually you differ between geoelectrical soundings and mappings. The sounding delivers an image of the resistance’s dispersion inclined to the depth. You use mapping to get information about changes of the resistance in lateral dimensions in a certain area.

1.1 Geoelectrical Sounding

The Schlumberger-Sounding is an example for one of many possible electrode-patterns. The distance between the potential electrodes (M and N) is kept constant. The distance between the electrodes A and B is made bigger after every measurement: So a growing part of the underground is catched by the system of the electrodes.

When you show the measured values inclined to half of the distance between electrodes A and B on/in a double-logarithmic co-ordinate system you can see: a sounding-graph which you can use to create a model about the dispersion of the specific resistance in the underground, This model can be used in addition to other geological information about the area and calibriation at/on drillings to build the geological structures of the underground.

Fig 3: a) Measured Schlumberger-sounding-graph; b) Layer model (best fit model) of the resistivity; c) Geological situation on site.

How deep you can explore with the pattern of the Schlumberger-sounding depends mainly on the maximum separation (distance between electrodes A and B) and the resistivity-differences of the bedding.

1.2. Geoelectric mapping

Here the distance between sondes and electrodes are kept constant. Those constructions lead along the profiles to get changes of the resistance in the lateral dimension (e.g. fault zones in the rock, boundaries of landfills). That means that there is a defined maximum depth of sounding. The accuracy of the method depends on the distance between profile and measuring point and the chosen configuration of the electrodes.

2. Applications

Those geoelectric methods are used for the following problems in the geotechniques and geosciences in general:

  • Prospection of deposits (e.g. finding out of gravel-, sand-, clay- or calcareous deposits)
  • Landfills
  • Measurement of boundaries
  • Bedding
  • Scouting of the hydrogeological situation near landfills
  • Exploration of planned landfill
  • Are there any aquicludes?
  • Structure of geological fault zones and cleaved zones
  • Capture of ground-water
  • Determination of the aquifer and impermeable layers
  • Mapping of fault zones in the rock
  • Localization of drilling points to build new wells and levels
  • Distinguish between salt water and fresh water occurrence
  • Archaeological explorations e.g. detection of historic settlements
  • Exploration of buildings e.g. determination of leakage on dams

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22 Dec 11

Geomagnetics: Principles and Applications

tricon-online.de

1. Principles

Magnetic explorations are based on the measurement of the magnetic field on the observation surface. The regular magnetic field of the earth (standard field) is corresponding to a dipol field. Differences to the standard field (anomalies of the magnetic field) are caused by magnetic void objects in the underground (e.g. accumulations of ferromagnetic materials like barrels, steel or iron scrap, or ore deposits etc.). Those anomalies can be recorded with sensors. In this way the positions of the objects causing the anomalies arc detected (fig. 1).

fig. 1: a) Schematic illustration of the measured anomaly of the earth’s field above a magnetic void object in the underground b) Shape of the anomalous source field to produce the observed total magnetic intensity plotted at figure a.

The Gradiometeranordnung is used for magnetic field measurement which should be indenpendent from temporal variations of the magnetic field of the earth. Two magnetic sensors with a constant vertical distance arc used to get the measurement results. In regard to magnetic void objects in the underground in the analysis the difference between the measurement results of the two probes are pulled up.

Usually certain colors are used on a map to show the results. With the knowledge of the positions of the objects (e.g. ore, barrels, vehicles, unexploded bombs) additional explorations to estimate the risk-potential of the investigated area can be done. Geomagnetics is one of the fastest and cheapest geophysical exploration methods. It is often used in the beginning of explorations and it is combined with other geophysical methods like GPR, resistivity or seismic.

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2. Applications

The Geomagnetic method is one of the most accurate methods of the geophysics, because the magnetic exploration is more specific in regard to materials than other geophysical methods. The measurements can be done in large areas on the earth or from the sky with less expenditure compared to other methods. Those results are often added to the results of other geophysical methods. Geomagnetic is often used, but not limited for:

  • Survey on landfill: Mapping the borders of the deposit (usually garbage or rubbish have a high percentage of ferromagnetic materials), to localize ferromagnetic objects (e.g. barrels) on site.
  • Detection of metal pipes and cables
  • Localisation of unexploded bombs, UXO, EOD and mines, exploration of armored foundations and buildings on former industrial areas.
  • Archaeological explorations: Detection of settlements (eg. castles and ramparts) and archaic fireplaces in the underground
  • Prospection of iron ore deposits and sediments of iron containing/ferromagnetic minerals.

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15 Dec 11

The Resistivity Meter; Applications in the Electrical and Mining Industries

by prettyone | thecgisite.com

Resistivity is the term used when describing the specific electrical resistance of a material. Put into lay mans terms it is a measurement of the resistance of any given material to electric current; it is normally measured using a resistivity meter. A material that has a high resistivity will impede the flow of electricity greatly while those with low resistivity will freely allow the flow of electrons.

Although materials all have a unique resistivity measurement, there are certain generalisations that can be made without having to use a meter. Metals and substances such as salt water tend to be great conductors of electricity where as materials like rubber, plastic and glass are extremely resistant. Because of the low resistance of metals, and especially copper, these materials are used for electrical wiring while the materials that cover them to prevent short circuits and shocks are made of plastic or rubber. This is where the resistivity meter is used most extensively, for finding the resistance of lengths of wire in electrical devices.

The formula for resistivity of a wire that a meter automatically computes is a relatively simple equation based upon three factors. The first of these is the resistance, measured in Ohms and can be found using a table of resistances for a number of different materials. The second component of the equation is the area of the circumference of the wire. The final piece of information needed to find the resistivity of a wire is the length of the piece of wire. The equation simply multiplies the area and resistance and divides the resulting figure by the length; in terms of units of measurement the Greek symbol rho is used and will usually be present on the readout of the meter.

It is not just wires however that are measured using a resistivity meter, these pieces of equipment are extensively used in measuring rock resistivities. Naturally rocks are not going to be used for electrical wiring but as the metallic minerals in rocks conduct electricity better than other non-metallic minerals, a meter is a useful tool in finding the metallic mineral content of rocks. The uses for this are vast but predominantly resistivity meters are used for geophysical exploration and more precisely the metal content of rock formations. Methods of measurement vary from surface testing to drilling holes in the ground.

SYSCAL Kid, entry level model for depths to 50m, available at heritagegeophysics.com.

These methods of resistivity measurement do not always use a handheld meter but a far larger and more industrial technique. The two predominant forms of measurement are termed as active and passive. Active measurement involves introducing electricity to the ground and measuring the strength of the electrical field created between electrodes giving an indication of resistance. Passive methods on the other hand measure the natural flow of electricity caused by electrochemical reactions between minerals and fluids.

Due to the benefits of this type of measurement the resistivity meter is extensively used in a variety of industries. The mining industries, especially those involved with the search of precious and semi-precious metals in rocks heavily use this equipment when prospecting. It is also extensively used in the oil industry. In terms of the resistivity of wires, electrical manufacturers use this type of meter to find the qualities of their components. Today it is hard to imagine a world without this type of meter, not only would there be less metals mined in the world but the electrical equipment we have in are homes would almost certainly be less well developed. About the author of this article: industry expert thomas pretty looks into the variety of uses for the resistivity meter.

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9 Dec 11

Geohydrology

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Hydrogeology (hydro- meaning water, and -geology meaning the study of the Earth) is the area of geology that deals with the distribution and movement of groundwater in the soil and rocks of the Earth’s crust, (commonly in aquifers). The term geohydrology is often used interchangeably. Some make the minor distinction between a hydrologist or engineer applying themselves to geology (geohydrology), and a geologist applying themselves to hydrology (hydrogeology).

Introduction

Hydrogeology is an interdisciplinary subject; it can be difficult to account fully for the chemical, physical, biological and even legal interactions between soil, water, nature and society. The study of the interaction between groundwater movement and geology can be quite complex. Groundwater does not always flow in the subsurface down-hill following the surface topography; groundwater follows pressure gradients (flow from high pressure to low) often following fractures and conduits in circuitous paths. Taking into account the interplay of the different facets of a multi-component system often requires knowledge in several diverse fields at both the experimental and theoretical levels. The following is a more traditional introduction to the methods and nomenclature of saturated subsurface hydrology, or simply hydrogeology.

SYSCAL Pro Switch 48 Imaging Resistivity System, available at heritagegeophysics.com.

Hydrogeology in relation to other fields

Hydrogeology, as stated above, is a branch of the earth sciences dealing with the flow of water through aquifers and other shallow porous media (typically less than 450 m or 1,500 ft below the land surface.) The very shallow flow of water in the subsurface (the upper 3 m or 10 ft) is pertinent to the fields of soil science, agriculture and civil engineering, as well as to hydrogeology. The general flow of fluids (water, hydrocarbons, geothermal fluids, etc.) in deeper formations is also a concern of geologists, geophysicists and petroleum geologists. Groundwater is a slow-moving, viscous fluid (with a Reynolds number less than unity); many of the empirically derived laws of groundwater flow can be alternately derived in fluid mechanics from the special case of Stokes flow (viscosity and pressure terms, but no inertial term).

The mathematical relationships used to describe the flow of water through porous media are the diffusion and Laplace equations, which have applications in many diverse fields. Steady groundwater flow (Laplace equation) has been simulated using electrical, elastic and heat conduction analogies. Transient groundwater flow is analogous to the diffusion of heat in a solid, therefore some solutions to hydrological problems have been adapted from heat transfer literature.

Traditionally, the movement of groundwater has been studied separately from surface water, climatology, and even the chemical and microbiological aspects of hydrogeology (the processes are uncoupled). As the field of hydrogeology matures, the strong interactions between groundwater, surface water, water chemistry, soil moisture and even climate are becoming more clear.

For example: Aquifer drawdown or overdrafting and the pumping of fossil water increases the total amount of water within the hydrosphere subject to transpiration and evaporation processes, thereby causing accretion in water vapour and cloud cover, the primary absorbers of infrared radiation in the earth’s atmosphere. Adding water to the system has a forcing effect on the whole earth system. An accurate estimate of the climatic forcing effect due to this hydrogeological fact is yet to be quantified.

Extract from the article “Geohydrology -  Wikipedia”.

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1 Dec 11

Magnetometer

en.citizendium.org

A magnetometer is a scientific instrument used to measure the strength of magnetic fields.

Earth’s magnetism varies from place to place and differences in the Earth’s magnetic field can be caused by a couple of things:

  • The differing nature of rocks
  • The interaction between charged particles from the Sun and the magnetosphere and the ionosphere.

Uses

Magnetometers are used in geophysical surveys to find changes in concentration of magnetic minerals in geologic formations (and also deposits of iron because they can measure the magnetic pull of iron). Magnetometers are also used to detect archeological sites, shipwrecks and other buried or submerged objects.

A magnetometer can also be used by satellites like NASA’s Magsat or German CHAMP satellites to measure both the magnitude and direction of the Earth’s magnetic field.

Magnetometers are very sensitive, and can give an indication of possible auroral activity before one can even see the light from the aurora. There is a grid of magnetometers around the world constantly measuring the effect of the solar wind on the Earth’s magnetic field.

There are a variety of military applications for magnetometers. These include magnetic anomaly detectors for sensing submerged submarines from aircraft, unmanned remote detectors of the metal of vehicles and personnel, detonators that trigger when sensing a large metal mass (e.g., a ship), etc.

Types

Magnetometers can be divided into two basic types:

  • Scalar magnetometers

Measure the total strength of the magnetic field to which they are subjected, and

  • Vector magnetometers

Have the capability to measure the component of the magnetic field in a particular direction. The use of three orthogonal vector magnetometers allows the magnetic field strength, inclination and declination to be uniquely defined.

  • Magnetograph

A special magnetometer that continuously records data.

Examples of vector magnetometers are fluxgates and superconducting quantum interference devices, or SQUIDs. Some scalar magnetometers are discussed below.

Proton precession magnetometer

One type of magnetometer is the proton precession magnetometer, which operates on the principle that protons are spinning on an axis aligned with the magnetic field.

An inductor creates a strong magnetic field around a hydrogen-rich fluid, causing the protons to align themselves with the newly created field. The field is then interrupted, and as protons are realigned with Earth’s magnetic field, spinning protons precess at a specific frequency. This produces a weak magnetic field that is picked up by the same inductor. The relationship between the frequency of the induced current and the strength of Earth’s magnetic field is called the proton gyromagnetic ratio, and is equal to 0.042576 Hertz per nanotesla (Hz/nT).

Because the precession frequency depends only on atomic constants and the strength of the external magnetic field, the accuracy of this type of magnetometer is very good. Magnetic impurities in the sensor and errors in the measurement of the frequency are the two causes of errors in these magnetometers.

If several tens of watts are available to power the aligning process, these magnetometers can be moderately sensitive. Measuring once per second, standard deviations in the readings in the 0.01 nT to 0.1 nT range can be obtained.

Overhauser magnetometer

The Overhauser effect takes advantage of a quantum physics effect that applies to the hydrogen atom. This effect occurs when a special liquid (containing free, unpaired electrons) is combined with hydrogen atoms and then exposed to secondary polarization from a radio frequency (RF) magnetic field (i.e. generated from a RF source).

RF magnetic fields are ideal for use in magnetic devices because they are transparent to the Earth’s DC magnetic field and the RF frequency is well out of the bandwidth of the precession signal (i.e. they do not contribute noise to the measuring system).

The unbound electrons in the special liquid transfer their excited state (i.e. energy) to the hydrogen nuclei (i.e. protons). This transfer of energy alters the spin state populations of the protons and polarizes the liquid – just like a proton precession magnetometer – but with much less power and to much greater extent.

The proportionality of the precession frequency and magnetic flux density is perfectly linear, independent of temperature and only slightly affected by shielding effects of hydrogen orbital electrons. The constant of proportionality is known to a high degree of accuracy and is identical to the proton precession gyromagnetic constant.

Overhauser magnetometers achieve some 0.01 nT/√Hz noise levels, depending on particulars of design, and they can operate in either pulsed or continuous mode.

Cesium vapor magnetometer

A basic example of the workings of a magnetometer may be given by discussing the common “optically pumped cesium vapour magnetometer” which is a highly sensitive (0.004 nT/√Hz) and accurate device used in a wide range of applications. Although it relies on some interesting quantum mechanics to operate, its basic principles are easily explained.

Portable Cesium Vapor Magnetometer Model G-859 Mineral Mag, available at heritagegeophysics.com.

The device broadly consists of a photon emitter containing a cesium light emitter or lamp, an absorption chamber containing cesium vapour and a “buffer gas” through which the emitted photons pass, and a photon detector, arranged in that order.

SQUID magnetometer

SQUIDs, or Superconducting Quantum Interference Devices, are used to measure extremely small magnetic fields; they are currently the most sensitive vector magnetometers known, with noise levels as low as 3 fT·Hz−0.5.

These magnetometers require cooling with liquid helium (4.2 K) or liquid nitrogen (77 K) to operate, hence the packaging requirements to use them are rather stringent both from a thermal-mechanical as well as magnetic standpoint.

  • Calibration

The basic principle that allows the device to operate is the fact that a cesium atom can exist in any of nine energy levels, which is the placement of electron atomic orbitals around the atomic nucleus. When a cesium atom within the chamber encounters a photon from the lamp, it jumps to a higher energy state and then re-emits a photon and falls to an indeterminate lower energy state. The cesium atom is ‘sensitive’ to the photons from the lamp in three of its nine energy states, and therefore eventually, assuming a closed system, all the atoms will fall into a state in which the all the photons from the lamp will pass through unhindered and be measured by the photon detector. At this stage the device can be said to be perfectly calibrated.

  • Detection

Given that this theoretically perfect magnetometer is now calibrated it can be exposed to the environment. It is easy to imagine that the environment is constantly emitting quanta of energy and that some of these will pass through the chamber. When they do, they may hit one of our cesium atoms and cause it to jump into a new energy state, which may in turn be one in which it can absorb a photon from our cesium emitter. If this is the case it will cause a decrease in the number of photons reaching our detector and this can be easily recorded. Scaling from this simple example to account for the vast number of energy transactions occurring within the cesium vapour, it is easy to see how the system works.

  • Applications

When removed from an isolated environment, the cesium vapour can never be ‘perfectly’ calibrated and the system is subject to environmental interference as are all scalar magnetometers. However, by the application of feedback systems and an averaging of the detection rates seen in a benign environment, the instrument can be calibrated sufficiently well in a real-world environment to make it accurate and useful for detection.

Early magnetometers

In 1833 Carl Friedrich Gauss, head of the Geomagnetic Observatory in Göttingen, published a paper entitled “On the intensity of the Earth’s magnetic field expressed in absolute measure”. It described a new instrument that Gauss called a “magnometer” (a term which is still occasionally used instead of magnetometer). It consisted of a permanent bar magnet suspended horizontally from a gold fever. A magnetometer is also called a gaussmeter.

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24 Nov 11

Seismographs – Keeping Track of Earthquakes

earthquake.usgs.gov

Throw a rock into a pond or lake and watch the waves rippling out in all directions from the point of impact. Just as this impact sets waves in motion on a quiet pond, so an earthquake generates seismic waves that radiate out through the Earth.

Seismic waves lose much of their energy in traveling over great distances. But sensitive detectors (seismometers) can record theses waves emitted by even the smallest earthquakes. When these detectors are connected to a system that produces a permanent recording, they are called seismographs.

There are many different types seismometers, but they all are based on the fundamental principle – that the differential motion between a free mass (which tends to remain at rest) and a supporting structure anchored in the ground (which moves with the vibrating Earth) can be used to record seismic waves.

Figure 1. Seismographs are designed so that slight earth vibrations move the instruments; the suspended mass (M), however, tends to remain at rest, and its recording stylus records this difference in motion. The horizontal seismograph shown here moves only in the horizontal plane. Vertical seismographs, like the simple one shown here, use a “soft” link between the earth-anchored instrument and the suspended mass. In this design, the mass hangs from a spring, which absorbs some of the motion and causes the mass to lag behind actual motion.

This principle is illustrated in figure. Vertical support AB holds mass M in position by wire AM and by strut BM at point B; the system becomes a seismometer when the vertical support is embedded in a concrete pier attached to the Earth. If there is no friction at the point B and mass M is reasonably large, the movement of the pier and the attached upright support in response to an earthquake wave will set up a differential motion between the mass and the pier (the inertia of the mass will make it remain at rest). This motion – the signal of an earthquake wave – can then be recorded on a revolving drum. When the pier is steady, the pen attached to the mass writes a straight line. But when the pier shakes, the mass and strut wiggle, recording waves from the earthquake that started the boom in motion.

Usually, the drum rotates on a screw-threaded axle so that the recording pen moves on a continuously advancing record and does not simply repeat the same circle over and over. Because time – both the time of day and the synchronization of events – is an important element in seismology, clocks are always part of a seismograph system.

A single seismograph pendulum works in only one direction, and cannot give a complete picture of wave motions from other directions. To overcome this problem, modern seismograph stations have three separate instruments to record horizontal waves – (1) one to record the north-south waves, (2) another to record east-west waves, and (3) a vertical one in which a weight resting on a spring tends to stand still and record vertical ground motions. The spring-suspended mass lags behind the motion caused by the earthquake, making the pen record the waves on the drum. This combination of instruments tells a seismologist the general direction of the seismic wave source, the magnitude at its source, and the character of the wave motion. Instruments at other stations must be used to get a precise fix on the earthquake’s epicenter.

An earthquake generates a series of waves that penetrate the entire Earth and travel at and through its surface. Each wave has a characteristic time: each has its own move of travel. They are quite complex, but a few basic facts will explain how they travel through the Earth and how an earthquake’s epicenter can be determined from seismograph records.

There are four basic types of seismic waves; two preliminary body waves that travel through the Earth and two that travel only at the surface (L waves). Combinations, reflections, and diffractions produce an infinity of other types, but body waves are the main interest in this discussion.

Body waves are composed of two principal types; the P (primary) wave, comparable to sound waves, which compresses and dilates the rock as it travels forward through the Earth; and the S (secondary) wave, which shakes the rock sideways as it advances at barely more than half the P-wave speed.

Figure 2. Travel-time curves with idealized seismograms (earthquake records superimposed). The P wave is designated the primary preliminary wave because it is the first to arrive at a seismic station after an earthquake. It travels at a speed usually less than 6 kilometers per second in the Earth’s crust and jumps to 13 kilometers per second through the core.

The S wave is the secondary preliminary wave to be recorded. It follows paths through the Earth quite similar to those of the P-wave paths, except that no consistent evidence has yet been found that the S wave penetrates the Earth’s core.

The lines labeled P, S, and L in the curves shown on figure 2 represent the travel time required for each phase at distances of 0 to 1300 kilometers from the earthquake’s epicenter. They mark the points on the record at which these waves first arrive at the station.

The simplest method of locating an earthquake on a globe is to find the time interval between the P- and S-wave arrivals at several seismograph stations. The distance to the earthquake from each station is then determined from standard travel-time tables and travel-time curves. Great-circle arcs are drawn on the globe using the distance of the earthquake to the station as a radius. All the arcs should intersect at a common point – the epicenter.

Another method of locating an earthquake is to use the P-wave arrival-time minus origin-time (P – O) interval instead of distance. This method is more common because the time can be taken directly from surface focus travel-time tables assuming an origin of 00 hours. This method, however, requires that travel-time tables be available for various depths of focus. For locating a deep shock, one 700 kilometers deep, for example, travel-time tables and travel-time curves for that depth have to be used to calculate the origin time and distances.

Other wave types can be generated inside the Earth by P and S waves, as shown in figure 3. As many as five different wave groups or phases can emerge when a P or S wave encounters a discontinuity or interface within the Earth.

Figure 3. Propagation paths of combinations of P, S, and L waves from an earthquake focus.

Abridged from Earthquake Information Bulletin. Vol. 2, No. 5, September – October, 1970.

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18 Nov 11

Seismograph

neohumanism.org

Seismographs are used by seismologistss to record seismic waves. By studying seismic waves, geologists can map out the interior of the Earth.

When an earthquake occurs, seismographs near the epicenter are able to record both S and P waves, but those on the other side of the Earth can only record P waves.

ES 3000 Low-Cost with 8 or 12 channels, available at heritagegeophysics.com.

Design

Seismographs consist of a foundation, a suspension, a damper, a suspended mass, a motion sensor attached to the mass, an amplifying device, and a recording device.

Prefessional seismographic observatories usually have instruments for three axes, north-south, east-west, and up-down. One set of three instruments is tuned to oscillate at three seconds, and the other at thirty seconds. Seismologists generally prefer a vertical seismograph if only one instrument is possible.

A professional observatory is best when mounted on bedrock with an uncracked connection to a continental plate. Amateur and field instruments are mounted on small 30cm (1foot) buried concrete piers.

A popular modern design uses strain-beam accelerometers constructed as integrated circuits. This makes it easier to construct a three-axis observatory in a small field or amateur unit.

The classic recorder was once a drum, recording one week per sheet on special graph paper. Now, the most common recorder is a computer with an analog-to-digital converter, a disk drive and an internet connection. Many observatories now use personal computers. For amateurs, a PC with a recording sound card provides a computer with an inexpensive analog-to-digital converter.

An algorithm often used to eliminate insignificant observations uses a short-term average and a long term average. When the short term average is statistically significant compared to the long term average, the event is worth recording.

One classic seismograph is a “gate” design. A heavy weight is mounted on the point of a long triangle, hinged at a vertical edge. The hinge has very low friction. As the ground moves, the weight stays unmoving, swinging the gate on the hinge. The gate is slightly tilted, so the weight tends to slowly return to a central position. The gate is adjusted to oscillate once per three seconds, or thirty seconds. Amateur devices usually oscillate once per ten seconds.

A pan of oil is placed on the ground, and a small sheet of metal mounted on the underside of the gate drags in the oil to damp oscillations in the gate.

An arrangement of levers and mirrors reflects a light beam to a direct-recording roll of film, or in modern systems, a pair of differential electronic photosensors. The recording device was traditionally paper on a drum.

Vertical accelerometers use a variation on this design, but use a spring to suspend the arm.

Another gate design has a vertical torsion wire several feet long, and attaches a copper flag to it. A mirror on the flag reflects a light beam. A magnet induces eddy currents in the flag to damp motion.

One of the best amateur designs was commissioned by Scientific American. Basically, the design is a gate. The weight is a large sense coil, moving in the magnetic field of a magnetron magnet (cheaply available from microwave oven repair shops). The damper is a one-megaohm variable resistance across the sense coil. The hinges are sheets of brass, held in clamps. The frame is square aluminum tubing. The device senses velocity rather than position, but requires very little care, and is easy to construct and tune.

Another design actually measures the distance between two piers, which will change when a wave passes the instrument.

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