GEOPHYSICS 6 Flashcards
Conventional marine seismic data acquisition method using a single vessel to tow one or more seismic source arrays and streamers in a straight line as the vessel records seismic data. With this method, the angle between the source and receivers is narrow.
Conventional marine seismic data acquisition method using a single vessel to tow one or more seismic source arrays and streamers in a straight line as the vessel records seismic data. With this method, the angle between the source and receivers is narrow.
single-azimuth towed-streamer acquistion
A conveyance, such as a sled with runners or pontoons, used to transport geophysical gear to a location. Skids are commonly deployed in acquisition of seismic data in marshes or other areas of soft, soggy terrain.
skid
A steel frame on which portable equipment is mounted to facilitate handling with cranes or flatbed trucks. The skid is robust, is usually designed with attachment points for hooks, chains, or cables, and has at least two lengthwise beams to facilitate sliding the equipment into place on the rigsite.
skid
To slide the rig over, such as to move it from one well slot to another on a fixed offshore platform. This operation is accomplished by disconnecting the rigid attachments from the platform to the rig, and energizing large-capacity hydraulic cylinders that push the rig over greased steel skid beams.
skid
The effective depth of penetration of an electromagnetic wave in a conductive medium. The skin depth is the distance in which the wave decays to 1/e (about 37%) of its value; it can be expressed as: δs = (2/σμω)1/2 = (2/σ)(ε/μ)1/2. Where δs = skin depthσ = electrical conductivityω = 2πf = angular frequency in radians/sf = frequency in Hzμ = μrμ0 = magnetic permeabilityμr = relative magnetic permeability of the conductorμ0 = relative magnetic permeability of free space = 4π × 10−7 newton per ampere squared (N/A2)ε = εrε0 = dielectric permittivityεr = relative dielectric permittivity of the materialε0 = dielectric permittivity of free space = 8.854 × 10−12 farads per meter (F/m).
skin depth
A process used in seismic processing to stack, or sum, traces by shifting traces in time in proportion to their offset. This technique is useful in areas of dipping reflectors.
slant stack
Also called interval transit time, The amount of time for a wave to travel a certain distance, proportional to the reciprocal of velocity, typically measured in microseconds per foot by an acoustic log and symbolized by t or DT. P-wave interval transit times for common sedimentary rock types range from 43 (dolostone) to 160 (unconsolidated shales) microseconds per foot, and can be distinguished from measurements of steel casing, which has a consistent transit time of 57 microseconds per foot.
slowness
A concave-upward, semicircular event in seismic data that has the appearance of a smile and can be caused by poor data migration or migration of noise.
smile
The mathematical description of refraction, or the physical change in the direction of a wavefront as it travels from one medium to another with a change in velocity and partial conversion and reflection of a P-wave to an S-wave at the interface of the two media. Snell’s law, one of two laws describing refraction, was formulated in the context of light waves, but is applicable to seismic waves. It is named for Willebrord Snel (1580 to 1626), a Dutch mathematician. Snell’s law can be written as: n1 sin 1 = n2 sin r, where n1=refractive index of first medium, n2= refractive index of second medium, sin i = sine of the angle of incidence, sin r= sine of the angle of refraction
snell’s law
Some authors use the term to describe P-waves in fluids, or as a synonym for seismic or elastic.
sonic
Pertaining to sound waves in the frequency range of 1 to 25 kilohertz.
sonic
A type of acoustic log that displays traveltime of P-waves versus depth. Sonic logs are typically recorded by pulling a tool on a wireline up the wellbore. The tool emits a sound wave that travels from the source to the formation and back to a receiver.
sonic log
Another term for probe, in electromagnetic methods, to measure the variation of a property versus depth, including electrical, electromagnetic and magnetotelluric properties. Probing differs from profiling in that the goal of probing is to provide a record of vertical changes, whereas profiling documents lateral variations.
sound / probe
A device that provides energy for acquisition of seismic data, such as an air gun, explosive charge or vibrator.
source
A geometrical arrangement ofseismicsources (a source array), with each individual source being activated in some fixed sequence in time.
source pattern
One of a number of locations or stations at the surface of the Earth at which a seismic source is activated.
source point
One of a number of locations or stations at the surface of the Earth at which a seismicsourceis activated.
sourcepoint
A display, also known as the f-k domain, of seismic data by wavenumber versus frequency rather than the intuitive display of location versus time for convenience during seismic processing. Working in the space-frequency domain provides the seismic processor with an alternative measure of the content of seismic data in which operations such as filtering of certain unwanted events can be accomplished more effectively.
space-frequency doman
The distance between successive shotpoints.
spacing
The distance between sources and receivers, particularly in logging tools.
spacing
Pertaining to a spectrum. The spectral content of a wavetrain or wavelet usually refers to its amplitude and phase as a function of frequency.
spectral
The distribution of gamma ray energies, or the number of gamma rays as a function of gamma ray energy.
spectrum
Generally, a display of entities or properties according to magnitude. In geophysics, spectrum refers to a display of characteristics of a wavetrain or trace as a function of frequency, wavenumber, or arrival time. A common display of spectrum is amplitude as a function of frequency.
spectrum
The apparent loss of intensity of a gravitational or magnetic field with distance. Spherical divergence decreases energy with the square of the distance.
spherical divergence