Seismic amplitude

The most basic form of real-time data display is the helical dmm recorder (often called hehcorders or drums ). A pen etches a trace on a smoked sheet (or draws an ink trace on a blank sheet) of paper wrapped around a cylindrical drum. Helical dmm recorders have played a vital role in volcano-seismic monitoring, allowing rapid visualization of seismic amplitudes and event identification of signal types. 

Continuous data are often downsampled to one sample per minute, which makes it easier to identify long-term trends in continuous seismic amplitude. The Real-time Seismic Amplitude Measurement system (Endo and Murray 1991) took the average amplitude of the seismic signal in each 1-min time window, and recorded these data into a file. RSAM data do not discriminate between different types of seismicity. 

Observatory has recorded an instrument corrected seismic amplitude and the peak frequency for each vertical-component seismic channel, on a 10-min timescale. 

R.E. Sheriff (1975) Factors affecting seismic amplitudes. Geophysical Prospecting 23, 125-138. 

A 3D seismic volume is represented by a cube S = S x,y,z) containing seismic amplitudes at each voxel position x,y,z), where x and y are positions in the two horizontal directions, measured in distance, and 2 is the position in the vertical dimension, measured in depth or two-way travel-time. The column S xq, yo, z) at position (xq, yo) is denoted a seismic trace. A seismic horizon is a piecewise continuous surface in 3D, consisting of positions 2 = g x, y) within the seismic volume. Throughout this work, seismic horizon segments are used as primitives for structural interpretation. Horizon interpretations are often placed on minimum, maximum or zero crossing events in the seismic cube, and are defined as piecewise continuous surfaces in 3D falling along these types of events. Minimum and maximum events in a seismic cube are commonly referred to as seismic extrema, and are chosen as the basis for the automated horizon interpretation presented in this paper. A set of seismic extrema can be described as a contour surface... 

All extrema within a 3D seismic volume can be represented by two sparse 3D cubes, containing only information about the minimum and maximum events in the seismic data. A vertical trace of the first cube contains the actual maximum or minimum seismic amplitude values along this trace, stored in the vertically closest voxel along the trace. This cube is referred to as the extrema value cube. The second cube, denoted the extrema position cube, contains sub-sample information about the exact location of the extrema, i.e., the vertical correction to the seismic sampling resolution. Both extrema cubes are sparse cubes, with value zero at voxel positions not falling on an extremum. The set of voxels containing extrema data is the same for the two cubes, but contains amplitudes and sub-sample positions respectively. 

Fig. 7. Direct watershedding of a 3D seismic amplitude volnme (left) produces a highly detailed segmentation (middle). This level of segmentation is for most purposes too detailed, and as is shown in Subsection 2.3, the level of detail may be reduced to give the segment size needed (right).

Murphy, J. R. and L, J. O Brien, 1977, The Correlation of Peak Ground Acceleration Amplitude with Seismic Intensity and Other Physical Parameters, Bull. Seismol. Soc. Am. 62 p 877. 

Monitoring by 3D repeatable seismic measures [3], also commonly named 4D seismic, is still, more or less, at the research stage (even in E P field) but early tests seem promising. This system is based on techniques such as the use of seismic sensors placed at regular intervals on the surface or in wells (permanently) [4]. Advanced multi covering seismic such as AVO (Amplitude Versus Offset) can help in better investigating petrophysical properties of reservoirs. 

The elastic radius delimits a zone inside which the displacements are irreversible. For a given medium, the size of this zone increases with explosion yield and decreases with depth. Observed from long distances compared with this radius, that is, beyond a few kilometres, an underground nuclear explosion can be represented, in seismic terms, by a single point emitting an isotropic seismic wave whose amplitude and frequency content are governed by the explosion yield, the depth of the zero point and the mechanical characteristics of the material surrounding this point. 

Because of the continuous presence of seismic background noise, only explosions whose yield exceeds a certain value are detected. This detectability limit varies with the distance from the explosion point and increases with the amplitude of the background noise. In the case of the Pacific Test Centre, the station at Rarotonga (Cook Islands), located at approximately 2000 km on a direct uninterrupted line from Mururoa, receives the T waves emitted by weak explosions and thus has a detectability close to one kiloton. The other stations are much further away and, apart from a few particularly sensitive stations such as the Yellowknife network in Canada, they have a much higher detectability limit. 

Fig. 4 also includes total (thermal) gas curves for the wells, showing the increased gas levels accompanying the undercompacted Rogaland and Hordaland Group claystones (causing increased amplitudes at the Top Paleocene seismic reflector above the structure), and the Jurassic hydrocarbon bearing sands. 

Fig. 19. Seismic traverse along line B-B (see Fig. 21) illustrating the major seismic horizons and the high-amplitude zone caused by major calcite cementation in the Lower Namur Sandstone, Gidgealpa Field. Note the lateral variation in seismic character of the high-amplitude zone caused by variable carbonate cementation. Horizons C, top Cadna-Owie Formation P, top Permian (Toolachee Formation).

Seismic mapping shows that carbonate cement distribution broadly follows the structural trend of the GMI anticline, in a SSW-NNE direction (Figs lA and 21). The carbonate cements extend over an area up to 7.5 km wide and at least 20 km long, covering an area of 150 km. Comparison of the carbonate cement isopach map with a depth structure contour map for the top Toolachee Formation (McIntyre et al., 1989) demonstrates that the cements concentrate near the crest of the Gidgealpa Field in the Lower Namur Sandstone. The high amplitudes produced by the carbonate cementation decrease downflank, and phase out completely in more off-structure regions of the fi eld. 

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