Real-Time Geophysics Using Linux
Whether searching for gold or oil, or assessing the geologic hazards of a site, geophysics is a high-stakes business. Geophysical exploration involves learning more about the earth's interior by making measurements at the surface using instruments such as seismometers, gravity meters and magnetometers. At Southwest Research Institute, our team of geologists and geophysicists works worldwide to assess volcanic hazards using geophysical techniques (Figure 1).
We discovered that Linux provides a robust, low-cost system for visualizing and processing magnetometer data in real time. Application of this real-time, data processing/visualization system enables us to create better geophysical maps for use in volcanic hazard assessments. This technique illustrates the potential for Linux applications in a variety of geophysical surveys.
Our magnetic mapping system consists of three components: a mobile instrument platform, a GPS base station and a real-time visualization (R-T-V) base station (Figure 2).
The usual instrument platform includes a cesium-vapor magnetometer carried by one person and GPS and telemetry equipment carried by another (Figure 3), but alternatives are possible. We have made magnetic anomaly maps by securing this instrumentation to a mountain bike and pedaling across the survey area (Figure 4). Currently, we are planning to mount the instruments on an unmanned aerial vehicle (UAV) to fly surveys at low altitudes (Figure 5). This flexibility in survey design becomes possible because all data processing, data storage and data monitoring tasks can be handled at the R-T-V base station.
Anomalies in the earth's magnetic field are produced by variations in the magnetic properties of rocks within the earth's subsurface. Magnetic surveying is a particularly good method of mapping subsurface volcanic rocks because these rocks have strong magnetic properties. Creating a map of magnetic anomalies helps to locate buried volcanos and volcanic intrusions that are not seen in the surface geology (Figure 6). Therefore, accurate magnetic mapping represents a critical step in determining the volcanic history of a region as part of a volcanic hazard analysis.
A geophysical instrument called a magnetometer is used to collect the data needed to generate a magnetic anomaly map. Magnetometers measure small changes in the intensity of the earth's magnetic field. A geophysical survey crew transports the magnetometer across a study area, while continually taking magnetic measurements. During our surveys, the crew also carries a global positioning system (GPS) to track the locations of these measurements. Magnetic and GPS data are radio-telemetered to a base station where software, running under the Linux operating system, is used to visualize the magnetic data as they are collected. This base station also monitors the location of the magnetometer, then processes and stores these data. With real-time visualization, the survey crew gains quick insight into the magnetic anomalies being mapped. Using this information, survey teams can concentrate their time and energy mapping areas that reveal significant details about the subsurface geology. Thus, real-time visualization of magnetic measurements can greatly facilitate the search and discovery process inherent in geophysics.
The cesium-vapor magnetometer is very sensitive, capable of measuring a change of one part per million in the intensity of the earth's magnetic field. This magnetometer can sample at very high rates, 1Hz to 50Hz. Such high data-collection rates are useful because we interface the magnetometer with a real-time kinematic differential GPS, capable of determining the mobile instrument platform's position every second to within several centimeters. This high accuracy is accomplished by using a stationary GPS base station to radio-telemeter location corrections to the GPS carried by the mobile survey team (Figure 7). Magnetic readings become interleaved with GPS location readings as the magnetometer's data collection facility stores the data from both instruments.