Digital Recording

Finally, digital recording and archiving images lets the inspector compare current radiographs with those made previously or to compare the radiographs of two different specimens.

From: Comprehensive Composite Materials II , 2018

Magnetic Recording

Kelly Knudson Fitzpatrick , in Encyclopedia of Physical Science and Technology (Third Edition), 2003

I.A Digital Recording

Digital recording is the process of storing binary data in the form of a two-level magnetization pattern. Digital recording is also referred to as saturation recording since channel bits are recorded by saturating the media either in the positive or negative direction. The resulting magnetic pattern alternates between the two remanent magnetization levels, + M r and −M r .

During writing, binary channel bits with values 0 and 1 are converted to square pulses with duration T and amplitudes −1 and 1. These square pulses make up a two-level write current signal that is sent to the write head. The resulting magnetization pattern is a track of contiguous bar magnets with magnetization M r or −M r, depending on the channel bit values. The physical length of a channel bit is L  = vT, where v is the relative velocity between the write head and the media.

When the input to the digital recording channel is a unit pulse, with amplitude 1 and duration T, the output is called the pulse response. A pulse response has a negative peak and a positive peak and looks something like Fig. 2.

FIGURE 2. An example pulse response for a magnetic recording channel.

During reading, the readback signal is a linear combination of pulse responses staggered in time and scaled by +1 or −1. Figure 3 shows an example of the two-level write current signal, the magnetization pattern, and the readback signal for a typical channel bit sequence. The dashed lines in the figure show the individual pulse responses that are summed together to form the readback signal.

FIGURE 3. From top to bottom: binary channel bits, two-level write current signal, magnetization pattern, and readback signal.

One of the major benefits of using digital recording is the ability to use error-correction coding to improve the integrity of the recorded data. Error-correction coding encases the data in a mathematical superstructure, so that if some of the data are detected incorrectly, the overriding structure is still evident and the data can be corrected. Signal processing and coding for digital magnetic recording are explored further in Section III.

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Anechoic (NVH) and electromagnetic compatibility (EMC) testing and test cells

Anthony J. Martyr , David R. Rogers , in Engine Testing (Fifth Edition), 2021

Control and instrumentation room

The advances in digital recording and processing has tended to mean that NVH test cells have short periods of shaft turning and spend most of their time having the UUT prepared for short multistage test sequences, the recorded sounds from which can be recorded, then analyzed, and digitally altered without further running. In anechoic control rooms, UUT performance displays tend to be replaced by banks of multichannel amplifiers and sound analysis equipment. The air conditioning load and heating in these rooms will vary considerably and must be designed accordingly.

While most multidisciplinary test facilities will use the same cell control system, the demands on the engine/dynamometer control hardware and software system is usually much lower in anechoic cells. It is, therefore, possible to use simpler and cheaper control systems as the data acquisition systems used are of a different type to the nonspecialist performance cells. Because of the need to suppress ambient noise and because the typical test sequences tend to consist of warm-up, short stages at different power outputs or inputs, precise control of the cell services is not entirely straightforward. Ventilation in the cell may be turned down to minimum during critical measurements and cooling water kept at a set position to prevent varying valve throughput noise; therefore monitoring UUT temperatures is critical.

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Compact Disc System and Beyond

Ian Sinclair , in Electronics Simplified (Third Edition), 2011

Error Correction

Definition

The errors in a digital recording and replay system can be of two main types, random errors, and burst errors. Random errors, as the name suggests, are errors in a few bits scattered around the disc at random and, because they are random, they can be dealt with by relatively simple methods. A burst error is quite a different beast, and is an error that involves a large number of consecutive bits.

Randomness also implies that in a frame of 588 bits, a bit that was in error might not be a data bit, and the error could in any case be corrected reasonably easily. Even if it were not, the use of EFM means that an error in one bit does not have a serious effect on the data. The EFM system is by itself a considerable safeguard against error, but the CD system needs more than this to allow, as we have seen earlier, for scratches on the disc surface that cause long error sequences. The main error correction is therefore done by the CIRC coding and decoding, and one strand of this system is the principle of interleaving. Before we try to unravel what goes on in the CIRC system, then, we need to look at interleaving and why it is carried out.

A burst or block error could be caused by a bad scratch in a disc or a major dropout on tape, and its correction is very much more difficult than the correction of a random error. Now if the bits that make up a set were not actually placed in sequence on a disc, then block errors would have much less effect. If, for example, a set of 24 data units (bytes) of 8   bits each that belonged together were recorded so as to be on eight different frames, then all but very large block errors would have much the same effect as a random error, affecting only one or two of the byte units. This type of shifting is called interleaving, and it is the most effective way of dealing with large block errors. The error-detection and correction stages are placed between the channel alternation stage and the step at which control and display signals are added prior to EFM encoding.

The CIRC method uses the Reed–Solomon system of parity coding along with interleaving to make the recorded code of a rather different form and different sequence from the original code. Two Reed–Solomon coders are used, each of which adds four parity 8-bit units to the code for a number of 8-bit units. The parity system that is used is a very complicated one, unlike simple single-bit parity, and it allows an error to be located and signaled. The CD system uses two different Reed–Solomon stages, one dealing with 24 bytes (8-bit units) and the other dealing with 28 bytes (the data bytes plus parity bytes from the first one), so that one frame is processed at a time.

In addition, by placing time delays in the form of serial registers between the coders, the interleaving of bytes from one frame to another can be achieved. The Reed–Solomon coding leaves the signal consisting of blocks that consist of correction code(1), data(1), data(2) and correction code(2), and these four parts are interleaved. For example, a recorded 32-bit signal may consist of the first correction code from one block, the first data byte of the adjacent block, the second data byte of the fourth block, and the second correction code from the eighth data block. These are assembled together, and a cyclic redundancy check number can be added.

At the decoder, the whole sequence is performed in reverse. This time, however, there may be errors present. We can detect early on whether the recorded 'scrambled' blocks contain errors and these can be corrected as far as possible. When the correct parts of a block (error code, data, data, error code) have been put together, then each word can be checked for errors, and these corrected as far as possible. Finally, the data is stripped of all added codes, and will either be error free or have caused the activation of some method (such as interpolation) of correcting or concealing gross errors.

Summary

Further error protection is achieved by interlacing frames, so that an error is less likely to affect a set of consecutive frames. The Reed–Solomon system provides a very considerable amount of detection and correction ability, and if all else fails, the signal will be interpolated, meaning that the missing digits will be replaced by values between the surrounding values. For example, in the sequence 1, 2, 3, 6, 7, 8, interpolation would fill in values of 4 and 5.

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Subsea holography and submersible 'holocameras'

J. Watson , N.M. Burns , in Subsea Optics and Imaging, 2013

12.3.1 Reconstruction of digital holograms

Generally the computer algorithms developed for digital recording and numerical replay of a hologram are based upon the Fresnel–Kirchhoff formulation of a complex monochromatic wavefield propagating from a scene through the aperture of an optical sensor (see Champeney, 1973 or Born and Wolf, 1980). The algorithms achieve computational efficiency by applying appropriate approximations and simplifications, making the Fresnel–Kirchhoff equation more suitable for digitisation and computer implementation. In most cases, the integral is implemented as a Fourier transform, thereby allowing the use of existing Fast Fourier Transform (FFT) libraries, which can be easily incorporated into software. Two common approaches are the Fresnel approximation and the convolution (sometimes known as 'angular spectrum') method. Using either approach, it is possible to recreate the intensity and phase distribution of the wavefield at any plane in the reconstructed volume of the hologram; thereby simulating the effect of traversing an image sensor through an optically replayed hologram (but with the added advantage that phase information can also be extracted). The main differences between the two methods are in the speed of processing, the different scaling properties of the reconstructed images, and their applicability for IL or off-axis (OA) recording.

Many authors (e.g. Demetrakopolous and Mittra, 1974; Yaroslavskii and Merzlyakov, 1980; Schnars and Jueptner, 1994, 2002, 2005; Cuche et al., 1999; Sun et al., 2002, 2007, 2008a; Pan and Meng, 2003; Coppola et al., 2004; Dong et al., 2004; Kreis, 2004) discuss these procedures and the reader is referred to these for more detail. Here we will give only an overview of their properties and implementation, and of their use in underwater digital holography.

Figure 12.8 shows the generalised recording and replay geometry for a hologram. The object is located in the (ξ, η), plane, and located a distance zr from the hologram (sensor) plane (x,y) along the optic axis z. On replay an image is formed in the plane (ξ′, η′) at a distance zp from the sensor. The distance r represents the radius of a spherical wavefront at the hologram plane emanating from a point in the object plane; correspondingly, r' is the radius of the wavefront at the image plane emanating from a point in the hologram plane. The holographic image is reconstructed a distance zp along the optic axis. The hologram plane is located at z  =   0.

12.8. Generalised diagram of hologram recording and replay.

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Video Enhancement and Restoration

Reginald L. Lagendijk , ... Marcel J.T. Reinders , in The Essential Guide to Video Processing, 2009

4.1 INTRODUCTION

Even with the advancing camera and digital recording technology, there are many situations in which recorded image sequences—or video for short—may suffer from severe degradations. The poor quality of recorded image sequences may be due to, for instance, the imperfect or uncontrollable recording conditions, such as one encounters in astronomy, forensic sciences, and medical imaging. Video enhancement and restoration has always been important in these application areas not only to improve the visual quality but also to increase the performance of subsequent tasks such as analysis and interpretation.

The available bandwidth for transmission or storage can also affect the quality of digital video data. In general, lossy compression is applied using industry compression standards, such as JPEG, H.26x, and MPEG-x, which may result in visible coding artifacts, such as blocking, ringing, and mosquito noise. Then, the challenge is to design noise or artifact reduction filters—so-called video enhancement filters—that provide a graceful trade-off between the amount of noise reduction and the resulting loss of perceptual picture quality.

Another important application of video enhancement and restoration is preserving motion pictures and video tapes recorded over the last century. These unique records of historic, artistic, and cultural developments are deteriorating rapidly due to aging effects of the physical reels of film and magnetic tapes that carry the information. The preservation of these fragile archives is of interest not only to professional archivists but also to broadcasters as a cheap alternative to fill the many television channels that have come available with digital broadcasting. However, reusing old film and video material is only feasible if the visual quality meets the today's standards. First, the archived film and video is transferred from the original film reels or magnetic tape to digital media. Then, all kinds of degradations are removed from the digitized image sequences, in this way increasing the visual quality and commercial value. Because the objective of restoration is to remove irrelevant information such as noise and blotches, it restores the original spatial and temporal correlation structure of digital image sequences. Consequently, restoration may also improve the efficiency of the subsequent MPEG compression of image sequences.

An important difference between the enhancement and restoration of 2D images and video is the amount of data to be processed. While for the quality improvement of important images elaborate processing is still feasible, this is no longer true for the absolutely huge amounts of pictorial information encountered in medical sequences and film/video archives. Consequently, enhancement and restoration methods for image sequences should have a manageable complexity and should be semiautomatic. The term semiautomatic indicates that in the end professional operators control the visual quality of the restored image sequences by selecting values for some of the critical restoration parameters.

The most common artifact encountered in the above-mentioned applications is noise. Over the last two decades, an enormous amount of research has focused on the problem of enhancing and restoring 2D images. Clearly, the resulting spatial methods are also applicable to image sequences, but such an approach implicitly assumes that the individual pictures of the image sequence, or frames, are temporally independent. By ignoring the temporal correlation that exists, suboptimal results maybe obtained, and the spatial intraframe filters tend to introduce temporal artifacts in the restored image sequences. In this chapter, we focus our attention specifically on exploiting temporal dependencies, yielding interframe methods. In this respect, the material offered in this chapter is complementary to that on image enhancement in the Chapters 10–13 Chapter 11 Chapter 12 Chapter 13 of The Essential Guide to Image Processing [1]. The resulting enhancement and restoration techniques operate in the temporal dimension by definition, but often have a spatial filtering component as well. For this reason, video enhancement and restoration techniques are sometimes referred to as spatiotemporal filters or 3D filters. Section 4.2 of this chapter presents three important classes of noise filters for video frames, namely linear temporal filters, order-statistic (OS) filters, and multiresolution filters. Section 4.3 focuses on coding artifact reduction, in particular on blockiness reduction due to the lossy Discrete Cosine Transform (DCT).

In forensic sciences and film and video archives, a large variety of artifacts are encountered. Besides noise, we discuss the removal of other important impairments that rely on spatial or temporal processing algorithms, namely blotches (Section 4.4), vinegar syndrome (Section 4.5), intensity flicker (Section 4.6), kinescope moiré (Section 4.7), and scratches (Section 4.8). Blotches are dark and bright spots that are often visible in damaged film image sequences. The removal of blotches is essentially a temporal detection and interpolation problem. In certain circumstances, their removal needs to be done by means of spatial algorithms, as will be shown later in this chapter. Vinegar syndrome represents a special type of impairment related to film, and it may have various appearances (e.g., partial loss of color, blur). In some cases, blotch removal algorithms can be applied for blotch removal. In general, however, the particular properties of their appearance need to be taken into account. Intensity flicker refers to variations in intensity in time, caused by aging of film, by copying and format conversion (e.g., from film to video), and—in case of earlier film—by variations in shutter time. While blotches are spatially highly localized artifacts in video frames, intensity flicker is usually a spatially global, but not stationary, artifact. The kinescope moiré phenomenon appears during film-to-video transfer using telecine devices. It is caused by the superposition of (semi-)periodical signals from the film contents and the scan pattern used by the telecine device. Because of its (semi-)periodical nature, spectrum analysis is generally used for its removal. Film scratches are either bright or dark vertical lines spanning the entire frame. They appear approximately at the same place in consecutive frames.

It becomes apparent that image sequences may be degraded by multiple artifacts. For practical reasons, restoration systems follow a sequential procedure, where artifacts are removed one by one. As an example, Fig. 4.1 illustrates the order in which the removal of flicker, moiré, noise, scratches, blotches, and vinegar syndrome takes place. The reasons for this modular approach are the necessity to judge the success of the individual steps (e.g., by an operator) and the algorithmic and implementation complexity. The coding artifacts removal has not been displayed because it usually takes place in a preprocessing step.

FIGURE 4.1. Some processing steps in the removal of various video artifacts.

As already suggested in Fig. 4.1, most temporal filtering techniques require an estimate of the motion in the image sequence. Motion estimation has been discussed in detail in Chapters 3, 8, and 11 of this book. However, the estimation of motion from degraded image sequences is problematic. We are faced with the problem that the impairments of the video disturb the motion estimator, but at the same time correct motion estimates are assumed in developing enhancement and restoration algorithms. In this chapter, we do not discuss the design of new motion estimators [2–4] that are robust to the various artifacts, but we assume that existing motion estimators can be modified appropriately such that sufficiently correct and smooth motion fields are obtained. The reason for this approach is that even under ideal conditions motion estimates are never perfect. Usually, incorrect or unreliable motion vectors are dealt with in a few special ways. First, clearly incorrect or unreliable motion vectors can be repaired. Second, the enhancement and restoration algorithms should be robust against a limited amount of incorrect or unreliable motion vectors. Third, areas with wrong motion vectors that are impossible to repair can be protected against temporal restoration to avoid an outcome, which is visually more objectionable than the input sequence itself. In such a case, the unavailability of temporal information makes the spatial-only restoration more suitable than the temporal one.

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Audio electronics

Owen Bishop , in Understand Electronics (Second Edition), 2001

Compact discs

Compact discs, or CDs as they are more usually called, are digital recordings. The stages up to recording in the recording-reproduction chain are similar to those for digital, except that different systems of coding are used and the additional information added to the data stream is appropriate to a disc system. Samples from the left and right stereo channels are recorded alternately. This is possible because of the digital nature of the process. Among the many processes of coding and error-checking the digital data before it is finally recorded, we can shuffle batches of data so that the two channels, although both are converted to digital simultaneously, can then be stored, sorted and put on to the disc in alternate batches. The operation is reversed at playback and both channels are heard simultaneously.

The final digital signal is recorded on a disc of optically flat glass coated with a photo-resistant substance. Recording employs a beam from a low-power laser focused on to the underside of the disc (see overleaf). Although the laser has a power rating of only a few milliwatts its beam is focused on to a spot only 0.1   μm in diameter.

The intense energy of the beam at that point vaporizes the photoresist, leaving a pit about 0.6   μm in diameter and 0.1   μm deep. As the disc turns, the head produces a pit for every '1' in the digital signal, but leaves the disc unpitted when the digit is a 'O'. The head is near the centre of the disc at the start of the recording and is moved toward the periphery as recording proceeds. Thus the pits are arranged along a spiral track with 41 250 turns.

The rate of rotation of the disc is adjusted as the head moves outward, being gradually reduced to obtain a constant track speed of 1.2   m per second. Recording has to take place under scrupulously clean conditions as a single speck of dust could degrade several adjacent tracks.

A series of stages follow in which several copies of the master disc are made for use in mass-production. The discs eventually distributed in the shops consist of a transparent disc of polycarbonate, with dimples corresponding to the pits in the master. This disc is given a brightly reflective coating (usually of aluminium) which is then protected by having a further polycarbonate layer sealed over it.

Playback uses another laser, of much lower power than that used in recording. The optical system projects a finely focused laser beam on to the disc. The beam is partly refracted by the transparent plastic coating of the disc. Because of this, the beam may be relatively broad where it enters the plastic layer; the plastic layer helping to concentrate it into a small spot on the reflective surface. Since the beam is broad where it enters the disc, the effects of small scratches on the surface are minimized. The beam strikes the reflective layer and, if no dimple is present, it is reflected back into the optical system. On its way back it is internally reflected in the prism, which directs the beam to a photodiode sensor. If a dimple is present, most of the beam is scattered sideways and little of it is reflected back to the diode. In this way the output from the diode consists of a series of low and high level representing the digital information stored on the disc. The signals from the photodiode are then sent to decoding circuits which check it for errors, strip off the various information (such as track number and elapsed time) and decode it into a straight digital analogue of the original sound. This is then sent to the DAC which produces an analogue audio signal ready for amplifying and feeding to the loudspeaker.

Various methods are used to keep the playback head centred on the track. In one of these, the three-beam system, there is a tracking beam to left and right of the main beam, each with its photodiode. One beam is slightly ahead of the main beam and one is slightly behind. If the main beam is correctly centred, the two side beams skim the sides of the dimples equally and receive signals of equal intensity. But if the main beam wanders off track, one or other of the side beams receives a stronger signal. The timing of the signals from the two beams tell the logic whether the main beam is too far to the left or too far to the right. Whatever the error, the positioning mechanisms of the playback head is adjusted to correct it. Another system is used to ensure that the focusing of the beam is continually adjusted to give a spot of the minimum size.

CDs share with digital tape the advantage that the recorded digital information remains unchanged no matter how many times the disc is played. There is no wearing of the grooves to degrade the reproduction as there is in a vinyl disc. But CDs are not indestructible and particular care must be taken not to scratch the outer surface of the plastic coating. The use of CDs as computer memory is described on page 247.

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SARLAB PROJECT PRELIMINARY IDEAS ON SIDE LOOKING RADAR SYSTEMS FOR SPACELAB EARTH OBSERVATION PAYLOADS

J.P. Guignard , ... Thomson , in Space and Energy, 1977

Advanced SLR performances (e.g. options 2 to 4, fig. 3)

Based on the above, advanced SLR systems are defined for both earth resources and add-on payloads for later Spacelab missions.

All advanced options use digital imaging chain (for the sake of flexibility) with:

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on board preprocessing (presumming) and digital recording (video tape recorder)

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ground based digital correlator

Advanced "add-on" payloads consist of pressurized boxes.

Four preferred options are selected:

option 1 : the simplest one ("add-on" option)
option 2 : the main SARLAB option
option 3 : an intermediate option
option 4 : a very sophisticated one

These options are characterized in figure 2.

As far as the radar system is concerned, the 370 km altitude has been considered as a worst case condition.

A typical antenna layout is given in figure 4 (for option 2)

FIG. 4. RELATIVES SIZES OF ANTENNA AND 3-M LONG PALLET MODULE

It must be noticed that:

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the antenna should be mounted along the shuttle's fore and aft line

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the antenna should preferably be mounted on a pedestal. This pedestal can be moved on the pallet to positions which suit the accomodation requirements of adjacent pallets.

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the various resources offered by Spacelab (i.e. data, power, etc) are expected to be entirely adequate for SARLAB.

For the sake of argument, we can remark that:

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option 1 is based on the use of optical means as far as on-board recording and ground processing are concerned (however, presumming is carried out in digital form).

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option 2 to 4, deal with a completely digital imaging chain, featuring a first step towards the SARSAT imaging chain.

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option 2 provides significant geoscientific potential (dual polarization system) based on the use of a simple technology (2 antennas of 3 m length each)

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option 3 requires a more extended antenna technology study (9 m long antenna with dual polarization capability).

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option 4 is mainly consisting of twice the option 3 radars, respectively at X-band and L-band.

Finally, as far as the overall programme is concerned, option 2 is recommended as the most promising initial phase of such a programme.

The overall programme for option 2 is depicted in figure 5.

FIG. 5. OVERALL PROGRAMME FOR OPTION 2

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Exploration

James G. Speight , in Subsea and Deepwater Oil and Gas Science and Technology, 2015

4.2.3 Seismic Methods

The earliest seismic surveys, during the 1920s, were analog recorded and produced two-dimensional (2-D) analyses. Digital recording was introduced in the 1960s, and then, as computer technology burgeoned, so did geophysical signal processing. During the past 30 years, computer-intensive techniques have evolved. The data are achieved by placing sources and receivers along a straight line so that it can be assumed that all the reflection points fall in a 2-D plane formed between the line of traverse and the vertical. This is known as 2-D seismic. 3-D seismic is a method of acquiring surface seismic data by placing sources and receivers in an aerial pattern. One example of a simple 3-D layout is to place the receivers along a line and shoot into these receivers along a path perpendicular to this line.

The search for petroleum and natural gas relies heavily on the use of seismic technology, which is based on reading data initiated from energy sources, such as explosions, air guns (offshore use), vibrator trucks, or well sources. These sources produce waves that pass through the subsurface and are recorded at strategically placed geophones or hydrophones. The seismograph measures the shock waves from explosions initiated by triggering small controlled charges of explosives in the bottom of shallow holes in the ground. The formation depth is determined by the time elapsed between the explosion and detection of the reflected wave at the surface. In the offshore, seismic responses are usually read from streamers towed behind modern seismic vessels, recorded, and processed later by computers that analyze the data.

In a typical seismic exploration operation, a line of data receivers (geophones) are laid out for terrestrial programs or hydrophones for aquatic operations. Explosives or mechanical vibrators are commonly used on land and air-guns are used in aquatic or offshore environments. Seismic surveys are used by the offshore crude oil and natural gas industry to help determine the location of crude oil and natural gas deposits beneath the seafloor. In fact, prospecting for crude oil and natural gas using exploration seismology involves studying body waves such as compressional and shear waves propagating through the interior of the Earth. Generally, seismic methods are preferred (to methods that use magnetometers) as the primary survey method for oil exploration although magnetic methods can give additional information about the underlying geology and in some environments evidence of leakage from traps.

Seismic methods are based on determinations of the time interval that elapses between the initiation of a sound wave from detonation of a dynamite charge or other artificial shock and the arrival of the vibration impulses at a series of seismic detectors (geophones). The arrivals are amplified and recorded along with time marks (0.01 s intervals) to give the seismogram. The method depends upon (1) the velocity within each of the layers penetrated at depth is greater than that in the layers above; (2) the layers are bounded by plane surfaces; and (3) the material within each layer is essentially homogeneous. The major differences between earthquake seismology and petroleum exploration seismology are scales and knowledge of the location of seismic disturbances. Earthquake seismology studies naturally generated seismic waves, which have periods in minutes and resolution in kilometers. In exploration seismology, artificial sources are used that have periods of tenths of a second and tens of meters of resolution. Production seismology requires higher-frequency seismic waves and better resolution, often resolution in the order of a few meters.

The seismic method as applied to exploration of crude oil and natural gas involves (1) field acquisition, (2) data processing, and (3) geologic interpretation. Seismic field acquisition requires placement of strings of acoustic receivers (hydrophones) in the water in the case of marine exploration and geophones on the surface, in the case of land exploration. The end result of seismic data processing is the production of a subsurface profile similar to a geologic cross section, which can be illustrated in the form of a time scale or shown as a function of depth, either of the profiles can be are used for (structural and stratigraphic) geologic interpretation. Structural interpretation of seismic data involves mapping of the different subsurface strata by using seismic data as well as information from boreholes and outcrops. Stratigraphic interpretation investigates the various attributes within a common stratum and is used to interpret changes to infer varying reservoir conditions such as lithology, porosity, and fluid content.

The depths and media reached by seismic waves depend on the distance between the shot point and the receiving point(s). The first impulses or breaks in a seismograph are caused by waves that have traveled quickly between the shot point and the receiving point(s). At short distances, this is usually also the shortest path, but beyond a certain distance it is quicker for a refracted pulse to travel via a longer path involving underlying layers with a higher velocity. From a plot of the time of travel as a function of surface distance, data are obtained for determining both the velocity of the sound wave through the material and number of layers present. From the distances at which changes in velocity are indicated, the depth of each layer can be assessed. In general the deeper, older formations as a result of higher compression have a higher density and also a higher seismic velocity than the overlying material. Observed differences in velocity not only define the direction of slope of the rock surfaces but also provide information for computing the degree of slope present.

Commercial 3-D seismology began in the early 1980s on a limited basis. Recent innovations that were essential to the development of 3-D seismology are satellite positioning, new processing algorithms, and the interpretative workstation. The 3-D seismic survey provides a more accurate and detailed image of the subsurface and offers a significantly higher quality signal than the 2-D data that are (have been) commonly acquired. The 3-D technology also improves both spatial and temporal resolutions. New data processing techniques are pre-stacked 3-D depth migration, interpretation of multiple 3-D surveys in different times (4-D seismic), and reservoir characterization of horizons.

Seismic geophysical has significantly aided the search for crude oil and natural gas on the subsea continental shelves and other areas covered by water. A marine seismic project moves continually, with detectors towed behind the boat at a constant speed and at a constant depth. Explosive charges are detonated at a position and time determined by the speed of the boat, so that a continuous survey of the reflecting horizons can be obtained. The technique for mapping the 3-D structure beneath the seafloor (seismic reflection profiling) relies on the subsurface penetration and reflection of sound waves produced by a loud acoustic source—an explosion of dynamite historically and an air-gun in modern times—whose differential travel times can be detected using hydrophones. These acoustic reflections reveal different layers of sediments, rocks, and faults, among other subsurface features, that can be combined using computer techniques to provide an image of the subsurface seafloor structures.

To obtain the seismic data, an exploration vessel tows one (or two) air guns (usually 3–400 ft behind the vessel) along with several kilometers of streamers (cables to which are attached the necessary number of hydrophones). While the vessel is underway (at, for example, approximately 5 knots), the air guns are triggered at regular intervals (every 10 to 12 s), which produces a pulse of sound that is reflected from the seabed before being echoed back to the hydrophones.

The air guns used in this technology produce sounds on the order of 235–240 decibels (dB). The environmental (noise) threat posed by seismic reflection profiling depends on the number and frequency of surveys and the seasonal behaviors of cetaceans that may be affected in a given region. Such surveys need to be approached with caution and to avoid regions where affected species congregate.

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Introduction to Digital Signal Processing

Li Tan , Jean Jiang , in Digital Signal Processing (Second Edition), 2013

1.1 Basic Concepts of Digital Signal Processing

Digital signal processing (DSP) technology and its advancements have dramatically impacted our modern society everywhere. Without DSP, we would not have digital/Internet audio and video; digital recording; CD, DVD, and MP3 players; iPhone and iPad; digital cameras; digital and cellular telephones; digital satellite and TV; or wired and wireless networks. Medical instruments would be less efficient or unable to provide useful information for precise diagnoses if there were no digital electrocardiography (ECG) analyzers, digital X-rays, and medical image systems. We would also live in many less efficient ways, since we would not be equipped with voice recognition systems, speech synthesis systems, and image and video editing systems. Without DSP, scientists, engineers, and technologists would have no powerful tools to analyze and visualize the data necessary for their designs, and so on.

The basic concept of DSP is illustrated by the simplified block diagram in Figure 1.1, which consists of an analog filter, an analog-to-digital conversion (ADC) unit, a digital signal (DS) processor, a digital-to-analog conversion (DAC) unit, and a reconstruction (anti-image) filter.

FIGURE 1.1. A digital signal processing scheme.

As shown in the diagram, the analog input signal, which is continuous in time and amplitude, is generally encountered in the world around us. Examples of such analog signals include current, voltage, temperature, pressure, and light intensity. Usually a transducer (sensor) is used to convert the nonelectrical signal to the analog electrical signal (voltage). This analog signal is fed to an analog filter, which is applied to limit the frequency range of analog signals prior to the sampling process. The purpose of filtering is to significantly attenuate aliasing distortion, which will be explained in the next chapter. The band-limited signal at the output of the analog filter is then sampled and converted via the ADC unit into the digital signal, which is discrete both in time and in amplitude. The DS processor then accepts the digital signal and processes the digital data according to DSP rules such as lowpass, highpass, and bandpass digital filtering, or other algorithms for different applications. Notice that the DS processor unit is a special type of digital computer and can be a general-purpose digital computer, a microprocessor, or an advanced microcontroller; furthermore, DSP rules can be implemented using software in general.

With the DS processor and corresponding software, a processed digital output signal is generated. This signal behaves in a manner according to the specific algorithm used. The next block in Figure 1.1, the DAC unit, converts the processed digital signal to an analog output signal. As shown, the signal is continuous in time and discrete in amplitude (usually a sample-and-hold signal, to be discussed in Chapter 2). The final block in Figure 1.1 is designated as a function to smooth the DAC output voltage levels back to the analog signal via a reconstruction (anti-image) filter for real-world applications.

In general, the analog signal process does not require software, an algorithm, ADC, and DAC. The processing relies wholly on the electrical and electronic devices such as resistors, capacitors, transistors, operational amplifiers, and integrated circuits (ICs).

DSP systems, on the other hand, use software, digital processing, and algorithms; thus they have a great deal of flexibility, less noise interference, and no signal distortion in various applications. However, as shown in Figure 1.1, DSP systems still require minimum analog processing such as the anti-aliasing and reconstruction filters, which are musts for converting real-world information into digital form and digital signals back into real-world information.

Note that there are many real-world DSP applications that do not require DAC, such as data acquisition and digital information display, speech recognition, data encoding, and so on. Similarly, DSP applications that need no ADC include CD players, text-to-speech synthesis, and digital tone generators, among others. We will review some of them in the following sections.

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Ordnance Detection and Analysis

Richard D. Albright , in Cleanup of Chemical and Explosive Munitions (Second Edition), 2012

7.3.1 Brands Commonly Used for Ordnance

Professional UXO remediation services often rely on instruments that can digitally record the signals as well as map the anomalies using the Global Positioning System (GPS), such as the Ferex 4.021®, the Forester CAST® (digital recording option attached to the 4.021), Varian V92/Mk22 ®, the TM-4®, the GEM® 3, the EM31®, and the EM61®. The first four are analog magnetometers, which only detect ferrous metals or magnetic objects. The 4.021 uses a fluxgate-type sensor and the V92 uses an optically pumped cesium-type (atomic resonance) sensor probe. The TM-4 is a cesium or helium sensor type. The EM61 is a PI type detector, which can detect all metals. The Multi-sensor Towed Array Detection System (MTADS) is an improved PI system developed by the Naval Research Laboratory. The Geometrics G-858® is another useful instrument. Likewise, the GEM 3 is also useful. Newer systems are being developed constantly and any omission is due to lack of familiarity by the author and not an indication of disapproval. New computer programs are available that can show metal mass and depth. Thus, selection of an instrument may be less important than what program is used. The Naval Research Laboratory in Washington, DC, has developed a superb metal detection system. They can do work on federal property but have a civilian contractor, Black Hawk, for work on non-federal sites, the FUDS, or transferred ranges.

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