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The IRISDMC archives waveform (time-series) data from stations around the world. The following information highlights passive and active source data available through the DMC.
Time-series data is collected for many types of data, identified using a system of channel codes.
Passive Source
Most of the data archived at the DMC is from broad-band stations recording passive (or natural) source events. Some stations record continuously, some have triggered channels. Most of the passive source data are from permanent station installations and are archived in SEED format, but some data come to us from temporary installations and in formats other than SEED.
Active Source
Active source refers to the creation of seismic events using explosives or mechanical devices like air guns, from either experiments on land or while underway on properly equipped research ships.
Much of the active source data at the DMC is recorded using temporary deployments of instruments provided by the PASSCAL program, but we do have some active source data from other sources. Active source data can come in a variety of formats.
- Information about temporary deployments (including PASSCAL experiments)
Waveform Request Tools
IRIS provides a variety of Web Tools for requesting waveform data.
Request Waveform Data Using BREQ_FAST
Batch access to the IRISDMC archive can be obtained by electronically mailing a specially formatted file to the IRISDMC in Seattle. These are BREQ_FAST files and provide a base level access to the DMC needed by any user. Learn how to submit a BREQ_FAST Request »
WebRequest – An easy way to submit a request for data
WebRequest is a data request form that allows users to submit BREQ_FAST-formatted data requests to the DMC. Submit a WebRequest »
SOD – Standing Order for Data
SOD is a program that automates tedious data selection, downloading, and routine processing tasks in seismology. It allows you to define your desired data based on earthquakes, recording stations, and the resulting combination of information. SOD then retrieves the data that matches the criteria. Learn more about SOD »
JWEED
Using JWEED a person can obtain waveforms from any institution which has implemented DHI servers. JWEED now utilizes Web Mapping Services (WMS) for displaying maps. Learn more about JWEED »
Waveform Data Formats
SEED (Standard for the Exchange of Earthquake Data) format
SEED is an international standard format for the exchange of digital seismological data. SEED was designed for use by the earthquake research community, primarily for the exchange between institutions of unprocessed earth motion data. It is a format for digital data measured at one point in space and at equal intervals of time.
SEED helps seismologists who record, share, and use seismological data. By providing a standard, SEED makes transmitting, receiving, and processing earthquake data easier and more accurate. Before the introduction of SEED, ease and accuracy had been goals, but not really attained.
- More on SEED.
- The miniSEED format is described here.
- The dataless SEED format for metadata is described here.
Simple ASCII format
Waveform Records
Simple ASCII is a text column representation timeseries data that IRISDMC now provides. This representation makes it very simple to import into other utilities such as spreadsheets and mathematical analysis programs. More information on the Simple ASCII format »
SAC (Seismic Analysis Code) format
The SAC format comes in fixed binary and ASCII forms and is read by the SAC analysis utility: A general purpose interactive program designed for the study of sequential signals, especially time series data. More information on SAC »
Assembled Data
The IRISDMC distributes several pre-assembled data sets. Most of these data sets are from temporary deployments and in formats other than SEED. An assembled set is accompanied by a data report. We have assembled data from various sources including IRISPASSCAL (mostly active source), the USGS, and others.
What is an Audio Waveform?
As a matter of fact, a waveform is merely a graph that displays amplitude or level changes over time. Amplitude is measured in a bipolar manner, with positive and negative values; not to be confused with level, which can be the absolute value of amplitude changes or an average.
What makes this concept abstract is that waveforms typically contain tens of thousands of discrete changes within an unimaginably short period, crammed into a short block in a sequencer. As you probably already know, as you zoom in on a waveform, its contour becomes more and more visible.
—–Zoom——>
PCM and NYQUIST FREQUENCY
A waveform is a digitized recreation of very dynamic voltage changes over time. Here is how they are typically generated….
The discrete changes in an input signal are rectified in an instant through a process called “Pulse Code Modulation” (PCM). Simply put, PCM assigns a bit value to each sample at whatever sampling rate you’re running. Furthermore, the higher the bit depth, the more values the computer has to choose from and the more accurate the rectification is. Sampling Rate for recording purposes is pretty standard – 44100 Hz, the Nyquist Frequency for audio. Here is why it is standard…
- The highest audible frequency for humans is 20,000 Hz.
- Every frequency has a positive half and a negative half (compression and rarefaction).
- So, as long as we sample (analyze and generate) rapidly enough to catch both the positive and negative portions of the highest (most rapidly oscillating) audible frequency – we can confidently rectify just about every audible frequency.
- This means at a sampling rate of 40000 Hz, we will catch both the positive and negative portions 20,000 Hz. However, since sine waves are infinitely smooth and gradual, just one sample of each portion will not produce a very accurate waveform at very high and very low frequencies – but this gets pretty close. This is why the extra 4100 Hz in Nyquist Frequency exists. It accounts for any aliasing that may occur when sampling very low and very high frequencies.
Of course at this point you may be asking yourself “Why not just sample as frequently as possible? Why not just go to 50,000 Hz or 100,000 Hz?” The answer is negligence. We cannot hear the improvement past 44100 Hz. BUT THIS DOES NOT MEAN OTHER SAMPLE RATES ARE NOT USED. For example, when producing sound or music for film, the sample rate of the audio should match the video resolution rate – which is often over 90,000 Hz.
SYNTHETIC WAVEFORMS
Synthetic waveforms are MILES less complex than audio recorded from an acoustic or electrical source. This, frankly, is the beauty of them – they allow us tocreate sounds from scratch without getting ahead of ourselves. After all, every frequency part of a sound – harmonic or not – has the potential to send you down a path of unintentional, unfocused ideas that will do more harm than good in context. These are the four basic synthetic waveforms.
Sine
One harmonic, one frequency. So perfectly simple that it cannot technically exist acoustically or electrically. On the whole, even the purest sounding oscillators and self-resonating filters have a little bit of noise in their output. Y= Asinx describes it mathematically.
Square
Contains odd harmonics (odd whole number multiples of the fundamental). This means that if the fundamental frequency of a square wave is 200 Hz, it will also generate 600 Hz (3rd harmonic), 1000 Hz (5th harmonic), 1400 Hz (7th harmonic) and so on…..
Triangle
Waveform Generator
In short, Triangle is like a square wave in that it contains odd harmonics, except the harmonic content, is lower in amplitude than in a square wave. Meaning, the harmonics have less influence on the overall shape of the wave.
Traction Waveform Free
Sawtooth
Contains all harmonics. This means that this is the most complex of the four basic synthetic waveforms – but still nowhere close to as complex as real sound. In addition, ff the fundamental of a sawtooth is 100 Hz, that means it also contains 200 Hz, 300 Hz, 400 Hz ….etc. Each harmonic is a little lower in amplitude in comparison to the previous one.
To conclude, IT IS IMPORTANT TO REALIZE that triangle, square, and sawtooth waves ARE MADE UP OF SINE WAVES – AS IS EVERY SOUND IN THE UNIVERSE. In essence, one can synthesize a somewhat accurate square wave by adding odd multiples of the fundamental frequency to a sine wave. In the audio below, I have done just that! Listen to the sine wave gradually form into a square wave as I sweep in odd harmonics. The synthesized square wave plays in isolation at the end. I have isolated each partial on a different track and, in doing so, created a crude “spectrum” analysis of the sound.
Waveform Free
https://soundbridge.io/wp-content/uploads/2015/09/WF.-Sin-to-Sq.mp3