How to Read Helicorder Records
A helicorder is a device which records signals from a seismometer. The traditional helicorder records the seismic signal on a piece of paper which is wrapped around a rotating drum. A pen draws the signal on this paper. Each line of data represents a specific, constant time interval: for example, 1 hour per line. At the end of each interval, the pen jumps over a small interval and begins recording the next 1 hour interval. This process produces a striped recording like the one pictured at the right. When the helicorder record is complete, the paper must be replaced. The recording pen must also be regularly maintained. Digital helicorders are available for the display of seismic data on computer screens rather than on paper. One of the advantages of digital helicorders is that there is no need to maintain a physical recorder. Digital helicorders can also display data over the Internet. The MGS online helicorder records are generated by a program called heli-ewII, available from the USGS Earthworm web site.
Our helicorder displays are divided into two main categories:global (large, distant) and local (small, nearby) events. Signals generated by global events tend to be of lower frequency and higher amplitude than those caused by local events. Seismic events in and around Maryland tend to generate lower amplitude and higher frequency signals than large, global events. Global seismic events are generally detected by seismometers around the world. Most local events are usually detected only by seismometers nearby and in Maryland. By processing the raw data from our network with a band-pass filter we can separate global event signals from local event signals. You may choose to view local or global events from the helicorder menu pages. Please note that we began local event filtering on January 2, 2002, therefore there are no local helicorder recorders prior to this date.
The MGS online helicorder presents seismic data for an entire day. Each line of data on the record is a 15 minute recording of earth movements. The lines, or traces, alternate in color to make them easier to read. Typically, the traces are fairly flat, with only a small amount of waviness: (click here for an example). This kind of record indicates the a state of relative quiescence. Occasionally, the traces will show more motion, from wind, or perhaps someone walking nearby: or (another example). Sometimes you can see a distant quarry blast or other small event: (another example). You may even see an actual earthquake, like the one in the image below. Other isolated, large spikes and and patterns are usually caused by the seismometer being calibrated, transmission problems or other system problems.
The Soldiers Delight seismometer detects earth movements in the bedrock on which is is mounted. However, the event which caused that movement may have occurred much earlier than the time the seismometer records it. This is because vibrations take time to travel through the earth. The seismometer records earth motions in three directions: north-south, east-west, and vertical (up and down). The data for each direction is sent out from the seismometer to individual data channels: channel HHN is the north-south channel, HHE is the east-west channel, and HHZ is the vertical channel. These three directions make up the major components of the seismic signal. Measuring these three components separately gives precise information about the origin and magnitude of seismic events. There are 3 helicorder records for each day, one for each channel or component.
From the helicorder menu you can select which day's data , and which component of the data you would like to view. The seismograph's day begins at midnight, UTC time, which is 19:00 EST (7:00 p.m.). The helicorder records are near real-time. There is a 5 minute delay from the time the data are transmitted from the seismometer to the time they are displayed on the Internet. The delay is due to data transmission and processing. If you are viewing today's helicorder record, the image is automatically refreshed by your web browser every 5 minutes so that you will see the most recent data. Please note that our records begin on October 11, 2001, therefore selecting a date earlier than this will not work.
First line The date the data was obtained.
For example, SDMD HWN LD indicates data from the Soldiers Delight, MD seismograph, sampled at 100 data points per second, that has been filtered to show local events in the north-south direction, and the data is sent to the Lamont-Doherty seismic network .
Third line The title of the helicorder record. SDMD, the network name of the seismometer, is followed by the motion component displayed by the record. If the phrase 'BandPass' follows SDMD, the record shows local events. If the 'BandPass' phrase is absent, the record shows global events. For example, (SDMD Vertical Component) indicates the record represents data from the Soldiers Delight, MD seismograph, and shows unfiltered, globally significant, up-and-down motion.
|Hours/ Microvolt Axis (vertical axis)
The heligraph record begins at 00:00 Coordinated Universal Time(UTC), also know as Greenwich Mean Time (GMT). Recording continues for 24 hours, at which time a new helicorder record is started. There is a 5 hour difference between Eastern Standard Time (EST) and UTC. Although the record starts at midnight UTC time, it starts five hours before midnight EST.
EST The left vertical axis is the seismometer's local time (Eastern Standard Time, 24 hour mode).
MICROVOLTS The small numbers on the far right of the graph, next to the UTC axis, are the microvolt baseline for each corresponding line of data. The seismometer generates small voltages in response to earth motions for each of the seismometer axes. The absolute magnitude of these signals does not precisely indicate the location or strength of an earthquake. These factors must be calculated from data received from several seismometers located at a distance from one another. What this signal does tell us is the time the Soldiers Delight instrument sensed earth movement, the direction from which the movement was generated, and how strong the movement was around the instrument. These data are combined with data from other seismometers to determine the actual location and magnitude of an earthquake or event.
The heligraph record's vertical axis is a dual scale: 15 minute intervals, and also 10,0000.00 microvolts per division. The traces alternate in color (black, red, blue, green) to make interpretation easier. Each trace is a 15 minute block of data output by the seismometer. The traces represent + and - variations in the seismometer output around the baseline microvolts listed on the right side of the graph, for that 15 minute period. An earthquake will shake the seismometer and cause a distinct, alternating trace like that seen in the middle of the above record.
If a large seismic event causes a trace to exceed plus or minus 15 units on the hours axis, the trace is clipped, or truncated. The last line of data on the bottom of the record is the most recently plotted data.
|Minutes Axis (horizontal axis)
TIME This axis is divided into 10 second intervals, from 0 to 15 minutes. This is the amount of time between each 15 minute interval on the vertical axis. The image above shows how to interpret the horizontal axis.
|Record Traces (data)
The lines, or traces, indicate motion in the earth. An earthquake produces a trace similar to the large-amplitude waves seen in the center of the image above, although not always as large. Each trace is 15 minutes of data. The traces alternate in color to make them easier to read.
Depending upon the channel you have chosen to view, the record displays motion in north-south, east-west or vertical directions. You may view the following channels: HHN, for north-south movement, HHE for east-west movement, or HHZ, for up-and-down, (vertical) movement. These different channels often display very different information about the same event. An earthquake causes the ground to move up-and-down, as well as back-and-forth. The differences in these motions can tell a seismologist a lot about an earthquake. The time that an event is recorded at Soldiers Delight is not necessarily the time the event actually occurred. This is because it takes time for vibrations to travel through the earth. A distant earthquake may take hours to be detected at Soldiers Delight.
For information on earthquake intensity scales, see Earthquakes and Maryland.