Sense

Senses are the physiological methods of perception. The senses and their operation, classification, and theory are overlapping topics studied by a variety of fields, most notably neuroscience, cognitive psychology or cognitive science, and philosophy of perception. The nervous system has a specific sensory system, or organ, dedicated to each sense.Definition of senseThere is no firm agreement among neurologists as to the number of senses because of differing definitions of what constitutes a sense. One definition states that an exteroceptive sense is a faculty by which outside stimuli are perceived. The traditional five senses are sight, hearing, touch, smell, taste a classification attributed to Aristotle. Humans also have at least six additional senses a total of eleven including interoceptive senses that include nociception pain, quilibrioception balance, proprioception & kinesthesia joint motion and acceleration, sense of time, thermoception temperature differences, and in some a weak magnetoception direction.One commonly recognized catagorisation for human senses is as follows chemoreception photoreception mechanoreception and thermoception. Indeed, all human senses fit into one of these four categories.Different senses also exist in other organisms, for example electroreception.A broadly acceptable definition of a sense would be a system that consists of a group sensory cell types that responds to a specific physical phenomenon, and that corresponds to a particular group of regions within the brain where the signals are received and interpreted. Disputes about the number of senses arise typically regarding the classification of the various cell types and their mapping to regions of the brain.dit Sensesedit SightSight or vision is the ability of the brain and eye to detect electromagnetic waves within the visible range light interpreting the image as sight. There is disagreement as to whether this constitutes one, two or three senses. Neuroanatomists generally regard it as two senses, given that different receptors are responsible for the perception of colour the frequency of photons of light and brightness amplitudeintensity  number of photons of light. Some arguecitation needed that stereopsis, the perception of depth, also constitutes a sense, but it is generally regarded as a cognitive that is, postsensory function of brain to interpret sensory input and to derive new information. The inability to see is called blindness.edit HearingHearing or audition is the sense of sound perception. Since sound is vibrations propagating through a medium such as air, the detection of these vibrations, that is the sense of the hearing, is a mechanical sense akin to a sense of touch, albeit a very specialized one. In humans, this perception is executed by tiny hair fibres in the inner ear which detect the motion of a membrane which vibrates in response to changes in the pressure exerted by atmospheric particles within a range of  to  Hz, with substantial variation between individuals. Sound can also be detected as vibrations conducted through the body by tactition. Lower and higher frequencies than that can be heard are detected this way only. The inability to hear is called deafness.

edit Taste    This article or section is missing citations or needs footnotes.Using inline citations helps guard against copyright violations and factual inaccuracies. March Taste or gustation is one of the two main chemical senses. There are at least four types of tastes that buds receptors on the tongue detect, and hence there are anatomists who arguecitation needed that these constitute five or more different senses, given that each receptor conveys information to a slightly different region of the braincitation needed. The inability to taste is called ageusia.The four wellknown receptors detect sweet, salt, sour, and bitter, although the receptors for sweet and bitter have not been conclusively identified. A fifth receptor, for a sensation called umami, was first theorised in  and its existence confirmed in . The umami receptor detects the amino acid glutamate, a flavor commonly found in meat and in artificial flavourings such as monosodium glutamate.Note that taste is not the same as flavor flavor includes the smell of a food as well as its taste.edit SmellSmell or olfaction is the other chemical sense. Unlike taste, there are hundreds of olfactory receptors, each binding to a particular molecular feature. Odor molecules possess a variety of features and thus excite specific receptors more or less strongly. This combination of excitatory signals from different receptors makes up what we perceive as the molecules smell. In the brain, olfaction is processed by the olfactory system. Olfactory receptor neurons in the nose differ from most other neurons in that they die and regenerate on a regular basis. The inability to smell is called anosmia.edit TouchTouch, also called tactition, mechanoreception or somatic sensation, is the sense of pressure perception, generally in the skin. There are a variety of nerve endings that respond to variations in pressure e.g., firm, brushing, and sustained. The inability to feel anything or almost anything is called anesthesia. Paresthesia is a sensation of tingling, pricking, or numbness of a persons skin with no apparent long term physical effect.edit Balance and ccelerationBalance, Equilibrioception, or vestibular sense, is the sense which allows an organism to sense body movement, direction, speed, and acceleration, and to attain and maintain postural equilibrium and balance. The organ of equilibrioception is the vestibular labyrinthine system found in both of the inner ears. Technically this organ is responsible for two senses, angular momentum and linear acceleratin which also senses gravity, but they are known together as equilibrioceptionThe vestibular nerve conducts information from the three semicircular canals, corresponding to the three spatial planes, the utricle, and the saccule. The ampulla, or base, portion of the three semicircular canals each contain a structure called a crista. These bend in response to angular momentum or spinning. The saccule and utricle, also called the otolith organs, sense linear acceleration and thus gravity. Otoliths are small crystals of calcium carbonate that provide the inertia needed to detect changes in acceleration or gravity.

Thermoception is the sense of heat and the absence of heat cold by the skin and including internal skin passages. The thermoceptors in the skin are quite different from the homeostatic thermoceptors in the brain hypothalamus which provide feedback on internal body temperature.edit Kinesthetic senseProprioception, the kinesthetic sense, provides the parietal cortex of the brain with information on the relative positions of the parts of the body. Neurologists test this sense by telling patients to close their eyes and touch the tip of a finger to their nose. Assuming proper proprioceptive function, at no time will the person lose awareness of where the hand actually is, even though it is not being detected by any of the other senses. Proprioception and touch are related in subtle ways, and their impairment results in surprising and deep deficits in perception and action.edit PainNociception physiological pain signals neardamage or damage to tissue. The three types of pain receptors are cutaneous skin, somatic joints and bones and visceral body organs. It was believed that pain was simply the overloading of pressure receptors, but research in the first half of the th century indicated that pain is a distinct phenomenon that intertwines with all of the other senses, including touch. Pain was once considered an entirely subjective experience, but recent studies show that pain is registered in the anterior cingulate gyrus of the brain.edit Other internal sensesAn internal sense or interoception is any sense that is normally stimulated from within the body. These involve numerous sensory receptors in internal organs, such as stretch receptors that are neurologically linked to the brain.   Pulmonary stretch receptors are found in the lungs and control the respiratory rate.Cutaneous receptors in the skin not only respond to touch, pressure, and temperature, but also respond to vasodilation in the skin such as blushing.  Stretch receptors in the gastrointestinal tract sense gas distension that may result in colic pain.Stimulation of sensory receptors in the esophagus result in sensations felt in the throat when swallowing, vomiting, or during acid reflux. Sensory receptors in pharynx mucosa, similar to touch receptors in the skin, sense foreign objects such as food that may result in a gagging reflex and corresponding gagging sensation.timulation of sensory receptors in the urinary bladder and rectum may result in sensations of fullness.Stimulation of stretch sensors that sense dilation of various blood vessels may result in pain, for example headache caused by vasodilation of brain arteries.edit Nonhuman senses

edit Analogous to human sensesOther living organisms have receptors to sense the world around them, including many of the senses listed above for humans. However, the mechanisms and capabilities vary widely.edit SmellAmong nonhuman species, dogs have a much keener sense of smell than humans, although the mechanism is similar. Insects have olfactory receptors on their antennae.edit VisionCats have the ability to see in the dark due to muscles surrounding their irises to contract and expand pupils as well as the tapetum lucidum, a reflective membrane that optimizes the image. Pit vipers and some boas have organs that allow them to detect infrared light, such that these snakes are able to sense the body heat of their prey. The common vampire bat may also have an infrared sensor on its nose. Infrared senses are, however, just sight in a different light frequency range. It has been found that birds and some other animals are tetrachromats and have the ability to see in the ultraviolet down to  nanometers. Bees are also able to see in the ultraviolet.edit BalanceCtenophores have a balance receptor a statocyst that works very differently from the mammalians semicircular canals.edit Not analogous to human sensesIn addition, some animals have senses that humans do not, including the followingElectroception or electroreception, the most significant of the nonhuman senses, is the ability to detect electric fields. Several species of fish, sharks and rays have the capacity to sense changes in electric fields in their immediate vicinity. Some fish passively sense changing nearby electric fields some generate their own weak electric fields, and sense the pattern of field potentials over their body surface and some use these electric field generating and sensing capacities for social communication. The mechanisms by which electroceptive fish construct a spatial representation from very small differences in field potentials involve comparisons of spike latencies from different parts of the fishs body.The only order of mammals that is known to demonstrate electroception is the monotreme order. Among these mammals, the platypus has the most acute sense of electroception.Body modification enthusiasts have experimented with magnetic implants to attempt to replicate this sense, however in general humans and probably other mammals can detect electric fields only indirectly by detecting the effect they have on hairs. An electrically charged balloon, for instance, will exert a force on human arm hairs, which can be felt through tactition and identified as coming from a static charge and not from wind or the like. This is however not electroception as it is a postsensory cognitive action.

 

Echolocation is the ability to determine orientation to other objects through interpretation of reflected sound like sonar. Bats and cetaceans are noted for this ability, though some other animals use it, as well. It is most often used to navigate through poor lighting conditions or to identify and track prey. There is currently an uncertainty whether this is simply an extremely developed postsensory interpretation of auditory perceptions or it actually constitutes a separate sense. Resolution of the issue will require brain scans of animals while they actually perform echolocation, a task that has proven difficult in practice. Blind people report they are able to navigate by interpreting reflected sounds esp. their own footsteps, a phenomenon which is known as Human echolocation. Magnetoception or magnetoreception is the ability to detect fluctuations in magnetic fields and is most commonly observed in birds, though it has also been observed in insects such as bees. Although there is no dispute that this sense exists in many avians it is essential to the navigational abilities of migratory birds, it is not a wellunderstood phenomenon. There is experimental and physical evidence to suggest this sense exists in a weak form in humanscitation needed.agnetotactic bacteria build miniature magnets inside themselves and use them to determine their orientation relative to the Earths magnetic field. Pressure detection uses the lateral line, which is a pressuresensing system of hairs found in fish and some aquatic amphibians. It is used primarily for navigation, hunting, and schooling. Humans have a basic relativepressure detection ability when eustachian tubes are blocked, as demonstrated in the ears response to changes in altitude. Polarized light direction  detection is used by bees to orient themselves, especially on cloudy days. Cuttlefish can also perceive the polarization of light.Sensorium means sensation The term sensorium plural sensoria refers to the sum of an organisms perception, the seat of sensation where it experiences and interprets the environments within which it lives. The term originally enters English from the Late Latin in the midth century, from the stem sens see sense. In earlier use it referred, in a broader sense, to the brain as the minds organ Oxford English Dictionary . In medical, psychological, and physiological discourse it has come to refer to the total character of the unique and changing sensory environments perceived by individuals. These include the sensation, perception, and interpretation of information about the world by senses, perceptual systems and minds MedTerms .Ratios of sensationIn the thcentury the concept behind the sensorium became a key part of the cultural theories of Marshall McLuhan, Edmund Carpenter and Walter J. Ong Carpenter and McLuhan  Ong

McLuhan, like his mentor Harold Innis, believed that media were biased according to time and space. He paid particular attention to what he called the sensorium, or the effects of media on our senses, positing that media affect us by manipulating the ratio of our senses. For example, the alphabet stresses the sense of sight, which in turn causes us to think in linear, objective terms the medium of the alphabet thus has the effect of reshaping the way in which we, collectively and individually, perceive and understand our environment. Focusing on variations in the sensorium across social contexts, these theorists collectively suggest that the world is explained and experienced differently depending on the specific ratios of sense that members of a culture share in the sensoria they learn to inhabit Howes , p. . More recent work has demonstrated that individuals may include in their unique sensoria perceptual proclivities that exceed their cultural norms even when, as in the history of smell in the West, the sense in question is suppressed or mostly ignored Classen, Howes and Synnott .This interplay of various ways of conceiving the world could be compared to the experience of synesthesia, where stimulus of one sense causes a perception by another, seemingly unrelated sense, as in musicians who can taste the intervals between notes they hear Beeli et al., , or artists who can smell colours. Many individuals who have one or more senses restricted or lost develop a sensorium with a ratio of sense which favours those they possess more fully. Frequently the blind or deaf speak of a compensating effect, whereby their touch or smell become more acute, changing the ways they perceive and reason about the world especially telling examples are found in the cases of wild children, whose early childhoods were spent in abusive, neglected or nonhuman environments, both intensifying and minimizing perceptual abilities Classen .edit Development of unique sensoria in cultures and individualsAlthough some consider these modalities abnormal, it is more likely that these examples demonstrate the contextual and socially learned nature of sensation. A normal sensorium and a synesthetic one differ based on the division, connection, and interplay of the bodys manifold sensory apparatus. A synesthete has simply developed a different set of relationships, including cognitive or interpretive skills which deliver unique abilities and understanding of the world Beeli et al., . The sensorium is a creation of the physical, biological, social and cultural environments of the individual organism and its relationships while being in the world.

What is considered a strange blurring of sensation from one perspective, is a normal and natural way of perception of the world in another, and indeed many individuals and their cultures develop sensoria fundamentally different from the visioncentric modality of most Western science and culture. One revealing contrast is the thought of a former Russian on the matter The dictionary of the Russian language...defines the sense of touch as follows In reality all five senses can be reduced to onethe sense of touch. The tongue and palate sense the food the ear, sound waves the nose, emanations the eyes, rays of light. That is why in all textbooks the sense of touch is always mentioned first. It means to ascertain, to perceive, by body, hand or fingers Anonymous .As David Howes explainse reference to Russian textbooks treating touch first, in contrast to American psychology textbooks which always begin with sight, is confirmed by other observers Simon  and serves to highlight how the hierarchization of the senses can vary significantly even between cultures belonging to the same general tradition here, that of the West , pp. .edit Sensory ecology and anthropologyThese sorts of insights were the impetus for the development of the burgeoning field of sensory anthropology, which seeks to understand other cultures from within their own unique sensoria. Anthropologists such as Paul Stoller  and Michael Jackson ,  have focused on a critique of the hegemony of vision and textuality in the social sciences. They argue persuasively for an understanding and analysis that is embodied, one sensitive to the unique context of sensation of those one wishes to understand. They believe that a thorough awareness and adoption of other sensoria is a key requirement if ethnography is to approach true understanding.A related area of study is sensory or perceptual ecology. This field aims at understanding the unique sensory and interpretive systems all organisms develop, based on the specific ecological environments they live in, experience and adapt to. A key researcher in this field has been psychologist James J. Gibson, who has written numerous seminal volumes considering the senses in terms of holistic, selfcontained perceptual systems. These exhibit their own mindful, interpretive behaviour, rather than acting simply as conduits delivering information for cognitive processing, as in more representational philosophies of perception or theories of psychology , . Perceptual systems detect affordances in objects in the world, directing attention towards information about an object in terms of the possible uses it affords an organism.The individual sensory systems of the body are only parts of these broader perceptual ecologies, which include the physical apparatus of sensation, the environment being sensed, as well as both learned and innate systems for directing attention and interpreting the results. These systems represent and enact the information as an influence which leads to a transformation required to perceive, identify or reason about the world, and are distributed across the very design and structures of the body, in relation to the physical environment, as well as in the concepts and interpretations of the mind.

This information varies according to species, physical environment, and the context of information in the social and cultural systems of perception, which also change over time and space, and as an individual learns through living. Any single perceptual modality may include or overlap multiple sensory structures, as well as other modes of perception, and the sum of their relations and the ratio of mixture and importance comprise a sensorium. The perception, understanding, and reasoning of an organism is dependent on the particular experience of the world delivered by changing ratios of sense.edit Clouded SensoriumA clouded sensorium is a medical term used to describe an inability to think clearly or concentrate. It is associated with a huge variety of underlying causes from drug induced states to pathogenic states induced by disease or mineral deficiency.Although the sense of time is not associated with a specific sensory system, the work of psychologists and neuroscientists indicates that our brains do have a system governing the perception of time. This is a highly distributed system including the cerebral cortex, cerebellum and basal ganglia as its components. One particular component, the suprachiasmatic nucleus, is responsible for the circadian or daily rhythm, while other cell clusters appear to be capable of shorterrange timekeeping. The sense of time is impaired in some patients with neurological diseases such as Parkinsons Disease and Attention Deficit Disorder.Psychoactive drugs can also impair a persons perception of time, as those on stimulants tend to overestimate time intervals, whereas those on depressants tend to underestimate them. Those lack of proper estimations are generally attributed to the idea that the number of neurotransmitters active in our brain determines the ratio to which our conscious and subconscious selves can perceive perception in relation to time. Neurotransmitters such as dopamine and adrenaline are also thought to be partly responsible for ones perception of time.Imagine you are traveling in a car, without a speedometer, at a speed of kmh. You then pass a road sign telling you the speed limit is now kmh. You will find that without the aid of the speedometer you will slow to a speed of around kmh. If you were in the same car with no speedometer, and told to accelerate to kmh from a standstill, you would reach kmh accurately. This phenomenon is due to the minds ability to alter conscious perception, much in the way that when you watch the ending credits of a movie and then redirect your vision elsewhere, your vision appears to roll in the reverse direction of that in which the credits were moving. This is a function of the brain believed to be used for the maintenance of a perceived balance adapted to our environment. Without this mechanism, we would be unable to attune to our current environment with its daylong and yearlong cycles.

Steven Hawking touches briefly on the subject in A Brief History of Time. Hawking suggests that the perception of time is a ratio Unit of Time  Time Lived. For example, one hour to a sixmonthold person would be approximately , while one hour to a yearold person would be , Therefore an hour appears much longer to a young child than to an aged adult, even though the measure of time is equal.A sensory system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory receptors, neural pathways, and parts of the brain involved in sensory perception. Commonly recognized sensory systems are those for vision, hearing, somatic sensation touch, taste and olfaction smell.The receptive field is the specific part of the world to which a receptor organ and receptor cells respond. For instance, the part of the world an eye can see, is its receptive field the light that each rod or cone can see, is its receptive field. Receptive fields have been identified for the visual system, auditory system and somatosensory system, so far.StimulusSensory systems code for four aspects of a stimulus type modality, intensity, location, and duration. Certain receptors are sensitive to certain types of stimuli for example, different mechanoreceptors respond best to different kinds of touch stimuli, like sharp or blunt objects. Receptors send impulses in certain patterns to send information about the intensity of a stimulus for example, how loud a sound is. The location of the receptor that is stimulated gives the brain information about the location of the stimulus for example, stimulating a mechanoreceptor in a finger will send information to the brain about that finger. The duration of the stimulus how long it lasts is conveyed by firing patterns of receptors.edit ModalityA stimulus modality sensory modality is a type of physical phenomenon that can be sensed. Examples are temperature, taste, sound, and pressure. The type of sensory receptor activated by a stimulus plays the primary role in coding the stimulus modality.In the memoryprediction framework, Jeff Hawkins mentions a correspondence between the six layers of the cerebral cortex and the six layers of the optic tract of the visual system. The primary visual cortex has areas labelled V, V, V, V, V, MT, IT, etc. Thus Area V mentioned below, is meant to signify only one class of cells in the brain, for which there can be many other cells which are also engaged in vision.Hawkins lays out a scheme for the analogous modalities of the sensory system. Note that there can be many types of senses, some not mentioned here. In particular, for humans, there will be cells which can be labelled as belonging to V, V A, A, etc.

edit V visionVisual Area , or V, is used for vision, via the visual system to the primary visual cortex. See the illustration above.arearedit A auditory  hearingAuditory Area , or A, is for hearing, via the auditory system, the primary auditory cortex.edit S somatosensory  touchSomatosensory Area , or S, is for touch and proprioception in the somatosensory system. The somatosensory system feeds the Brodmann Areas ,  and  of the primary somatosensory cortex. But there are also pathways for proprioception via the cerebellum, and motor control via Brodmann area .tonguetongueedit G gustatory  tasteGustatory Area , or G, is used for taste.edit O olfactory  smellOlfactory Area , or O, is used for smell. In contrast to vision and hearing, the olfactory bulbs are not crosshemispheric the right bulb connects to the right hemisphere and the left bulb connects to the left hemisphere.Seismometer is of Greek origin and comes from Seism  the shakes and Meteo  I measure are instruments that measure and record motions of the ground, including those of seismic waves generated by earthquakes, nuclear explosions, and other seismic sources. Records of seismic waves allow seismologists to map the interior of the Earth, and locate and measure the size of these different sources.Seismograph is another Greek term from Seism  the shakes and Grapho  I draw. It is often used for seismometer, though it is more applicable to the older instruments in which the measuring and recording of ground motion were combined than to modern systems, in which these functions are separated. Both types provide a continuous record of ground motion this distinguishes them from seismoscopes, which merely indicate that motion has occurred, perhaps with some simple measure of how large it was.Basic principlesInertial seismometers have A mass, usually called the inertial mass, that can move relative to the instrument frame, but is attached to it by a system such as a spring that will hold it fixed relative to the frame if there is no motion, and also damp out any motions once the motion of the frame stops. A means of recording the motion of the mass relative to the frame, or the force needed to keep it from moving.Any motion of the ground moves the frame. The mass tends not to move because of its inertia, and by measuring the motion between the frame and the mass the motion of the ground can be determined, even though the mass does move.

Early seismometers used optical levers or mechanical linkages to amplify the small motions involved, recording on sootcovered paper or photographic paper.Modern instruments use electronics. In some systems, the mass is held nearly motionless relative to the frame by an electronic negative feedback loop. The motion of the mass relative to the frame is measured, and the feedback loop applies a magnetic or electrostatic force to keep the mass nearly motionless. The voltage needed to produce this force is the output of the seismometer, which is recorded digitally. In other systems the mass is allowed to move, and its motion produces a voltage in a coil attached to the mass and moving through the magnetic field of a magnet attached to the frame. This design is often used in the geophones used in seismic surveys for oil and gas.Professional seismic observatories usually have instruments measuring three axes northsouth, eastwest, and updown. If only one axis can be measured, this is usually the vertical because it is less noisy and gives better records of some seismic waves.The foundation of a seismic station is critical. A professional station is sometimes mounted on bedrock. The best mountings may be in deep boreholes, which avoid thermal effects, ground noise and tilting from weather and tides. Amateur, or less exotic instruments are often mounted in insulated enclosures on small buried piers of unreinforced concrete. Reinforcing rods and aggregates would distort the pier as the temperature changes. A site should always be surveyed for ground noise with a temporary installation before pouring the pier and laying conduit.edit Zhang Hengs SeismoscopeReplica of Zhang Hengs seismoscope Houfeng Didong YiReplica of Zhang Hengs seismoscope Houfeng Didong YiIn , Zhang Heng of Chinas Han dynasty invented the first seismoscope by the definition above, which was called Houfeng Didong Yi lit. instrument for measuring the seasonal winds and the movements of the Earth. The description we have, from the History of the Later Han Dynasty, says that it was a large bronze vessel, about  meters in diameter at eight points around the top were dragons heads holding bronze balls. When there was an earthquake, one of the mouths would open and drop its ball into a bronze toad at the base, making a sound and supposedly showing the direction of the earthquake. On at least one occasion, probably at the time of a large earthquake in Gansu in , the seismoscope indicated an earthquake even though one was not felt. The available text says that inside the vessel was a central column that could move along eight tracks this is thought to refer to a pendulum, though it is not known exactly how this was linked to a mechanism that would open only one dragons mouth.

edit An early exampleThe principle can be shown by an early special purpose seismometer. This consisted of a large stationary pendulum, with a stylus on the bottom. As the earth starts to move, the heavy mass of the pendulum has the inertia to stay still in the nonearth frame of reference. The result is that the stylus scratches a pattern corresponding with the earths movement. This type of strong motion seismometer recorded upon a smoked glass glass with carbon soot. While not sensitive enough to detect distant earthquakes, this instrument could indicate the direction of the initial pressure waves and thus help find the epicenter of a local earthquake — such instruments were useful in the analysis of the  San Francisco earthquake. Further reanalysis was performed in the s using these early recordings, enabling a more precise determination of the initial fault break location in Marin county and its subsequent progression, mostly to the south.edit Early designsAfter , most seismometers were descended from those developed by the team of John Milne, James Alfred Ewing and Thomas Gray, who worked in Japan from . These seismometers used damped horizontal pendulums. Later, after World War II, these were adapted into the widely used PressEwing seismometer.Later, professional suites of instruments for the worldwide standard seismographic network had one set of instruments tuned to oscillate at fifteen seconds, and the other at ninety seconds, each set measuring in three directions. Amateurs or observatories with limited means tuned their smaller, less sensitive instruments to ten seconds. The basic damped horizontal pendulum seismometer swings like the gate of a fence. A heavy weight is mounted on the point of a long from  cm to several meters triangle, hinged at its vertical edge. As the ground moves, the weight stays unmoving, swinging the gate on the hinge.The advantage of a horizontal pendulum is that it achieves very low frequencies of oscillation in a compact instrument. The gate is slightly tilted, so the weight tends to slowly return to a central position. The pendulum is adjusted before the damping is installed to oscillate once per three seconds, or once per thirty seconds. The generalpurpose instruments of small stations or amateurs usually oscillate once per ten seconds. A pan of oil is placed under the arm, and a small sheet of metal mounted on the underside of the arm drags in the oil to damp oscillations. The level of oil, position on the arm, and angle and size of sheet is adjusted until the damping is critical, that is, almost having oscillation. The hinge is very low friction, often torsion wires, so the only friction is the internal friction of the wire. Small seismographs with low proof masses are placed in a vacuum to reduce disturbances from air currents.Zollner described torsionallysuspended horizontal pendulums as early as , but developed them for gravimetry rather than seismometry.

Early seismometers had an arrangement of levers on jeweled bearings, to scratch smoked glass or paper. Later, mirrors reflected a light beam to a directrecording plate or roll of photographic paper. Briefly, some designs returned to mechanical movements to save money. In midtwentiethcentury systems, the light was reflected to a pair of differential electronic photosensors called a photomultiplier. The voltage generated in the photomultiplier was used to drive galvanometers which had a small mirror mounted on the axis. The moving reflected light beam would strike the surface of the turning drum, which was covered with photosensitive paper. The expense of developing photo sensitive paper caused many seismic observatories to switch to ink or thermalsensitive paper.edit Modern instrumentsThis article or section needs copy editing for grammar, style, cohesion, tone or spelling.ou can assist by editing it now. A howto guide is available. September Modern instruments use electronic sensors, amplifiers, and recording instruments. Most are broadband covering a wide range of frequencies. Some seismometers can measure motions with frequencies from  Hz . seconds per cycle to / Hz  seconds per cycle. The mechanical suspension for horizontal instruments remains the gardengate described above. Vertical instruments use some kind of constantforce suspension such as the LaCoste suspension. The LaCoste suspension uses a zerolength spring to provide a long period high sensitivity.   Some modern instruments use a triaxial design in which three identical sensors that measure motion at the same angle to the vertical are  degrees apart on the horizontal. Vertical and horizontal motions can be computed from these measurements.Seismometers unavoidably introduce some distortion into the signals they measure, but professionallydesigned systems have carefullycharacterized frequency transforms.Modern sensitivities come in three broad ranges geophones,  to  V/m local geologic seismographs, about , V/m and teleseismographs, used for world survey, about , V/m. Instruments come in three main varieties short period, long period and broadband. The short and long period measure velocity and are very sensitive, however they clip or go offscale for ground motion that is strong enough to be felt by people. A bit analogtodigital conversion channel is commonplace. Practical devices are linear to roughly a part per million.
Delivered seismometers come with two styles of output analog and digital. Analog seismographs require analog recording equipment, possibly including an analogtodigital converter. Digital seismographs simply plug in to computers. They present the data in standard digital forms often SE over ethernet.

edit TeleseismometersThe low frequency direction seismometer cover removed. Two masses for x and ydirection can be seen, the third one for zdirection is below.The low frequency direction seismometer cover removed. Two masses for x and ydirection can be seen, the third one for zdirection is below.The modern broadband seismograph can record a very broad range of frequencies. It consists of a small proof mass, confined by electrical forces, driven by sophisticated electronics. As the earth moves, the electronics attempt to hold the mass steady through a feedback circuit. The amount of force necessary to achieve this is then recorded.In most designs the electronics holds a mass motionless relative to the frame. This device is called a Force Balance Accelerometer. It measures acceleration instead of velocity of ground movement. Basically, the distance between the mass and some part of the frame is measured very precisely, by a linear variable differential transformer. Some instruments use a linear variable differential capacitor.That measurement is then amplified by electronic amplifiers attached to parts of an electronic negative feedback loop. One of the amplified currents from the negative feedback loop drives a coil very like a loudspeaker, except that the coil is attached to the mass, and the magnet is mounted on the frame.The result is that the mass stays nearly motionless.Most instruments directly measure the ground motion using the distance sensor.The voltage generated in a sense coil on the mass by the magnet directly measures the instantaneous velocity of the ground.The current to the drive coil provides a sensitive, accurate measurement of the force between the mass and frame, thus directly measuring the grounds acceleration using F=MA of basic physics.One of the continuing problems with sensitive vertical seismographs is the buoyancy of their masses. The uneven changes in pressure caused by wind blowing on an open window can easily change the density of air in a room enough to cause a vertical seismograph to show spurious signals. Therefore, most professional seismographs are sealed in rigid gastight enclosures. For example, this is why a common Streckheisen model has a thick glass base that must be glued to its pier without bubbles in the glue.It might seem logical to make the heavy magnet serve as a mass, but that subjects the seismograph to errors when the Earths magnetic field moves. This is also why seismographs moving parts are constructed from a material that minimally interacts with magnetic fields.A seismograph is also sensitive to changes in temperature, and many instruments are constructed from low expansion materials such as nonmagnetic invar.The hinges on a seismograph are usually patented, and by the time the patent has expired, the art has improved. The most successful public domain designs use thin foil hinges in a clamp.Another issue is that the transfer function of a seismograph must be accurately characterized, so that its frequency response is known. This is often the crucial difference between professional and amateur instruments. Most instruments are characterized on a variable frequency shaking table.

Another type of seismometer is a digital strongmotion seismometer, or accelerograph. This data is essential to understand how an earthquake affects human structures.A strongmotion seismometer measures acceleration. This can be mathematically integrated later to give velocity and position. Strongmotion seismometers are not as sensitive to ground motions as teleseismic instruments but they stay on scale during the strongest seismic shaking.edit Other formsA Kinemetrics seismograph, formerly used by the United States Department of the Interior.A Kinemetrics seismograph, formerly used by the United States Department of the Interior.Accelerographs and geophones are often heavy cylindrical magnets with a springmounted coil inside. As case moves, the coil tends to stay stationary, so the magnetic field cuts the wires, inducing current in the output wires. They receive frequencies from several hundred hertz down to . Hz cheap to as low as  Hz pretty expensive. Some have electronic damping, a lowbudget way to get some of the performance of the closedloop wideband geologic seismographs.Strainbeam accelerometers constructed as integrated circuits are too insensitive for geologic seismographs , but are widely used in geophones.Some other sensitive designs measure the current generated by the flow of a noncorrosive ionic fluid through an electret sponge or a conductive fluid through a magnetic field.edit Modern recordingToday, the most common recorder is a computer with an analogtodigital converter, a disk drive and an internet connection for amateurs, a PC with a sound card and associated software is adequate. Most systems record continuously, but some record only when a signal is detected, as shown by a shortterm increase in the variation of the signal, compared to its longterm average which can vary slowly berconnected ismometersSeismometers spaced in an array can also be used to precisely locate, in three dimensions, the source of an earthquake, using the time it takes for seismic waves to propagate away from the hypocenter, the initiating point of fault rupture See also Earthquake location. Interconnected seismometers are also used to detect underground nuclear test explosions.In reflection seismology, an array of seismometers images subsurface features. The data are reduced to images using algorithms similar to tomography. The data reduction methods resemble those of computeraided tomographic medical imaging Xray machines CATscans, or imaging sonars.A worldwide array of seismometers can actually image the interior of the Earth in wavespeed and transmissivity. This type of system uses events such as earthquakes, impact events or nuclear explosions as wave sources. The first efforts at this method used manual data reduction from paper seismograph charts. Modern digital seismograph records are better adapted to direct computer use. With inexpensive seismometer designs and internet access, amateurs and small institutions have even formed a public seismograph network.Seismographic systems used for petroleum or other mineral exploration historically used an explosive and a wireline of geophones unrolled behind a truck. Now most shortrange systems use thumpers that hit the ground, and some small commercial systems have such good digital signal processing that a few sledgehammer strikes provide enough signal for shortdistance refractive surveys. Exotic cross or twodimensional arrays of geophones are sometimes used to perform threedimensional reflective imaging of subsurface features.

Basic linear refractive geomapping software once a black art is available offtheshelf, running on laptop computers, using strings as small as three geophones. Some systems now come in an  . m plastic field case with a computer, display and printer in the cover!Small seismic imaging systems are now sufficiently inexpensive to be used by civil engineers to survey foundation sites, locate bedrock, and find subsurface water.Earthquakes, and other sources, produce different types of seismic waves which travel through rock, and provide an effective way to image both sources and structures deep within the Earth. There are three basic types of seismic waves in solids Pwaves, Swaves both body waves and surface waves. The two basic kinds of surface waves Rayleigh and Love, can be fundamentally explained in terms of interacting P and/or Swaves.Propagation of seismic wave in the ground and the effect of presence of land mine.Propagation of seismic wave in the ground and the effect of presence of land mine.Pressure waves Pwaves, are longitudinal waves that travel at maximum velocity within solids and are therefore the first waves to appear on a seismogram.Swaves, also called Shear waves or secondary waves, are transverse waves that travel more slowly than Pwaves and thus appear later than Pwaves on a seismogram. Particle motion is perpendicular to the direction of wave propagation. Shear waves do not exist in fluids such as air or water.Surface waves travel more slowly than Pwaves and Swaves, but because they are guided by the surface of the Earth and their energy is thus trapped near the Earths surface they can be much larger in amplitude than body waves, and can be the largest signals seen in earthquake seismograms. They are particularly strongly excited when the seismic source is close to the surface of the Earth, such as the case of a shallow earthquake.For large enough earthquakes, one can observe the normal modes of the Earth. These modes are excited as discrete frequencies and can be observed for days after the generating event. The first observations were made in the s as the advent of higher fidelity instruments coincided with two of the largest earthquakes of the th century  the  Great Chilean earthquake and the  Great Alaskan earthquake. Since then, the normal modes of the Earth have given us some of the strongest constraints on the deep structure of the Earth.One of the earliest important discoveries suggested by Richard Dixon Oldham in  and definitively shown by Harold Jeffreys in  was that the outer core of the Earth is liquid. Pressure waves Pwaves pass through the core. Transverse or shear waves Swaves that shake sidetoside require rigid material so they do not pass through the outer core. Thus, the liquid core causes a shadow on the side of the planet opposite of the earthquake where no direct Swaves are observed. The reduction in Pwave velocity of the outer core also causes a substantial delay for P waves penetrating the core from the seismically faster velocity mantle.Seismic waves produced by explosions or vibrating controlled sources are the primary method of underground exploration. Controlled source seismology has been used to map salt domes, faults, anticlines and other geologic traps in petroleumbearing rocks, geological faults, rock types, and longburied giant meteor craters. For example, the Chicxulub impactor, which is believed to have killed the dinosaurs, was localized to Central America by analyzing ejecta in the cretaceous boundary, and then physically proven to exist using seismic maps from oil exploration.Using seismic tomography with earthquake waves, the interior of the Earth has been completely mapped to a resolution of several hundred kilometers. This process has enabled scientists to identify convection cells, mantle plumes and other largescale features of the inner Earth.