Seismic Glossary of Terms

Throughout the Napa Seismic website, a variety of seismological and geologic terms are used, especially in the N-Surveys sections. These terms or phrases are technically accurate, but to the average person without a geologic education, they can be confusing and daunting. Most concepts presented in geology are fairly simple, only requiring pattern recognition and logic to understand, but presented en masse without adequate explanation can make things confusing fast. Once the simple definition is known for each term though, it becomes much easier to digest and understand geologic content, including what is presented here on Napa Seismic.

Note – For a glossary of geologic terms (such as bedrock, alluvium, rhyolite) see the Wikipedia Glossary of Geology.

Below is a glossary of terms used throughout this website, and their explanations:

Ambient Noise Tomography

Ambient noise tomography is a method to generate an image of the subsurface by using the natural ambient noises of the Earth. Ambient noise means the naturally occurring wave phenomenon that are omnipresent at all times.

AVS30

Average velocity of the shear-wave traveling through the top 30 meters of the subsurface. In the USA, AVS100 is used, representing the top 100 feet.

For more information of AVS30, visit our site classification page.

Channel

A channel is a data input. For a seismograph, each channel is connected to a single geophone component (X, Y, or Z direction). Therefore a 3-component geophone that measures X, Y, and Z directions requires at a minimum a 3 channel geophone.

Coherency, Coherencies

For passive-seismic ambient noise tomography methods, 2 or more geophones record seismic data over an extended period of time. Since the data is accurately time stamped, the data from these geophones can be directly compared in terms of their similarities in wavelength, frequency, and velocity. How similar or dissimilar the data from one geophone to another is the coherency between them. A coherency of 1 means the data is exactly similar, a coherency of 0 means the data has zero similarity, and a coherency of -1 means the data is exactly opposite.

For SPAC methods, higher coherencies are better.

Dispersion Curve

A XY plot of wave velocity as a function of frequency.

Fibonacci Sequence

The Fibonacci sequence is a simple mathematical equation expressed in natural phenomenon throughout nature.

{\displaystyle F_{0}=0,\quad F_{1}=1,}
{\displaystyle F_{n}=F_{n-1}+F_{n-2},}

For n > 1

The first digits 0 through 1000 are:

0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, and more

Frequency

Frequency is the number of occurrences of a repeating event per unit of time. Frequency is measured in units of hertz (Hz) which is equal to one occurrence of a repeating event per second. 

Geology

Geology is the science of understanding the Earth, the rocks/materials of which it is composed, and the processes by which they change over time. As our understanding of the universe has expanded, geology has grown to encompass planetary geology, which is the study of the solid features of any terrestrial planet or natural satellite such as Mars or the Moon. Geology significantly overlaps with hydrologic and atmospheric sciences.

Geophysics

Geophysics is a subset of geology concerned with the physical processes and physical properties of the Earth and its surrounding space environment. Qualitative methods are used for analysis.

Geophone

A geophone is a device that converts ground movement into voltage, which is then converted into digital data by a A/D chip.

GPS – Global Positioning System

GPS is a radio-navigation method that provides geolocation and time information to a GPS receiver anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites.

HVSR – Horizontal to Vertical Spectral Ratio

HVSR is a geophysical technique for estimating the resonance frequency and amplification of ground motions influenced by a surface layer. To make these calculations, a 3-component receiver are required. To learn more about HVSR, visit our HVSR page.

Hydrology

Hydrology is the scientific study of the movement, distribution, and management of water on Earth and other planets.

Noise

Seismic noise is a generic name for a relatively persistent vibration of the ground, caused by known or unknown phenomenon, that is unwanted and often non-interpretable. For passive-seismic, most noise observed is human generated.

Common human generated sources of noise include traffic, industry, and construction.

Passive-Seismic

Passive seismic is the detection of natural earth movements, typically lower in frequency, with the purpose of discerning geological structure, site response, or resource exploration.

P-Wave (pressure wave)

A p-wave is an elastic body wave that travels faster than s-waves and surface waves. Being compressional, the oscillations of a p-wave’s particles are parallel to the direction of wave propagation. P-waves can propagate through solids, liquids, or gases.

Seismograph

A seismograph is a geophysical piece of equipment designed to record the ground motion of the Earth using 1 or more geophones. To do this, a seismograph uses an A/D microchip, which converts 1 or more analog signals (the type of data obtained by a geophone) to digital data.

  • Analog Signal – Analog signal is a continuous signal which represents physical measurements. Denoted by sine waves
  • Digital Signal – Digital signals are discrete time signals generated by digital modulation. Denoted by square waves.

Seismology

Seismology is the study of earthquakes and the propagation of elastic waves through the Earth or other planet-like bodies.

SPAC – Spacial Autocorrelation Method

The spacial autocorrelation method was first proposed by Japanese-American seismologist Keiiti Aki in 1957. Spacial means having a location in space and time, and autocorrelation

  • Spacings – A spacing is the distance between two geophones used for the SPAC method. A passive-seismic survey with multiple receivers can have many different spacings.
  • Pairs – A pair is 2 or more equidistant spacings but between different geophones. The SPAC method will average these pairs to improve the quality of data for that data point.

Surface Wave

A surface wave is a mechanical wave that propagates along the interface between differing media. As it pertains to seismology, elastic surface waves can travel along the surface of solids, manifesting as Rayleigh or Love waves.

  • Rayleigh Waves – Rayleigh waves include both longitudinal and transverse motions.
  • Love Waves – The particle motion of a Love wave forms a horizontal line perpendicular to the direction of propagation.

S-Wave (shear wave)

S-waves are a type of elastic body-wave and can travel though the body of a medium. Being transverse waves, the oscillations of an S-wave’s particles are perpendicular to the direction of wave propagation. S-waves can’t propagate in liquids with zero (or very low) viscosity however they may propagate in liquids with high viscosity.

Velocity

Velocity is the measure of an object’s speed and direction of motion.

Wavelength

Wavelength is the distance over which a wave’s shape repeats. Assuming a sinusoidal wave moving at a fixed speed, wavelength is inversely proportional to frequency of the wave: waves with lower frequencies have longer wavelengths, and higher frequencies have shorter wavelengths.

Last Updated – 8/29/20