Looking Beneath the Earth's Surface
The fusion of two technologies expedites a time-consuming search and discovery process.
Geophysics is a high stakes business. Deciding where to drill for oil, determining the probability of a volcanic eruption, and evaluating the risk a geologic fault poses to a nuclear power plant all require rapid and accurate interpretation of geophysical data. To address these concerns, Southwest Research Institute (SwRI) scientists have developed a robust, low-cost system for visualizing and processing geophysical data in real time, with the purpose of streamlining the path from data collection to interpretation and decision-making. Application of this new real-time data processing and visualization system makes possible the creation of better geophysical maps than were previously available and represents a significant advance in the way geophysical techniques are used to solve perplexing exploration problems.
Geophysical exploration involves learning more about the Earth's interior by taking measurements at the surface using such instruments as seismometers, gravity meters, and magnetometers -- developed to support exploration for mineral resources, such as oil and natural gas. To date, research efforts at the Center for Nuclear Waste Regulatory Analyses (CNWRA) at SwRI have concentrated on using magnetometers to collect the data needed to generate magnetic anomaly maps to evaluate potential volcanic hazards at nuclear facilities, including the proposed radioactive waste repository at Yucca Mountain, Nevada. The results of these efforts are now being applied to other client needs.
Anomalies in the Earth's magnetic field are produced by variations in the magnetic properties of rocks and other materials within the Earth. These anomalies occur in a variety of circumstances, such as across faults where there is a rapid change in subsurface rock properties or in the presence of man-made objects, such as buried containers, unexploded ordnance, or gas pipelines.
Magnetometers measure small changes in the intensity of the Earth's magnetic field -- on the order of one part in 50,000, or one nanoTesla. Mapping the magnetic field with this degree of accuracy enables geophysicists to image subtle, but important, variations in subsurface geology. For example, the flow and entrapment of oil is frequently controlled by relatively minor variations in subsurface properties, which can often be delineated by their magnetic signatures. Traditionally, these surveys have been time-consuming because the location of each measurement had to be surveyed, usually with a tape measure and compass or with optical survey tools, such as transits and theodolites. The magnetic measurements then had to be collected and recorded at each station. As a result, geophysicists often settled for comparatively low-resolution magnetic maps and, within the last 20 years, magnetic technology fell into disfavor. In oil exploration, magnetic methods were entirely displaced by high resolution, but extremely expensive, seismic exploration methods.
Implementing real-time visualization
The situation is changing with the introduction of real-time visualization techniques that rely on recent innovations in several technologies. SwRI scientists use cesium-vapor magnetometers that can measure the magnetic field at high rates of one to 50 samples per second. Such rapid data collection rates are useful because the magnetometer is interfaced with a real-time kinematic, differential Global Positioning System (GPS) capable of determining the mobile instrument position, once every second, to within several centimeters. This high accuracy is accomplished using a stationary GPS base station to radio-telemeter location corrections to a separate GPS instrument system carried by a mobile SwRI survey team.
While the survey crew is carrying a mobile GPS to track the locations of their measurements, the magnetic and GPS data are automatically telemetered via radio-modem to a base station where software, running on the Linux operating system, is used to visualize the magnetic data as it is collected. As it processes and stores the data, the base station also monitors the location of the magnetometer. With real-time visualization, the survey crew members gain quick insight into the magnetic anomalies they are mapping. Using this information, survey teams can concentrate their time and energy on mapping those 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 to geophysics.
Instrument system and operation
The SwRI mobile magnetic mapping system consists of three components: a mobile survey unit, a GPS base station, and a real-time visualization (RTV) base station. The usual instrument platform, weighing approximately 30 pounds, includes a magnetometer, carried by one person, and the GPS and telemetry equipment, carried by another. Magnetic anomaly maps also have been made by securing the instrumentation to a specially designed mountain bike so that the operator/rider can pedal across the survey area. Other plans being examined are to mount the instruments on an unmanned aerial vehicle and fly surveys at low altitudes. This flexibility in survey methods is possible because processing, monitoring, and storing the data can all be handled at the RTV base station.
It is important that the RTV base station is simple, reliable, and robust to ensure that no data are lost. The design consists of a 9,600 baud, two-watt radio modem, interfaced through a serial port connection to a field-hardened laptop computer running the Linux operating system. The computer and radio modem are both powered by a 12-volt battery. The radio-modem requires an antenna to provide line-of-sight communication with the mobile survey team's radio-modem. If required, a repeater can augment the signal relay. The RTV base station program waits for data at the serial port and selects for visualization and processing only those data strings tagged appropriately by the magnetometer and transmitter radio. Untagged or unrecognizable transmissions are discarded.
A number of events are programmed to occur as data arrives at the RTV base station. First, the GPS radio frequency (RF) data are associated with the corresponding magnetometer RF data transmission. This step involves interpolation of the GPS data to calculate the exact position of the magnetometer sensor. Field transmissions are then logged to a file that can be viewed using a text window on the laptop display screen. Any regional effects on the magnetic readings are removed using the International Geomagnetic Reference Field correction, and the values are displayed on a strip-chart showing the change in magnetic values over time. Finally, the positions of the survey team are transformed from latitude and longitude and plotted as x-y coordinates on a scalable, two-dimensional map using a Universal Transverse Mercator projection. These applied data corrections simplify the task of data interpretation and map generation for the survey team.
Use of the RTV base station has mitigated many of the difficulties that frequently accompany geophysical surveying. Previous surveys relied on downloading saved data from the magnetometer for later processing and map generation. This post-processing often occurred after returning home from the field before a magnetic map could be drawn and the quality of collected data evaluated. Delayed processing carried risks because the most interesting magnetic anomaly or information may have been just meters away from the selected survey area. However, there was no way to recognize this until the data had been processed.
Equipment and power failures remain very real technical problems when using batteries and multiple cable connections in remote and desolate environments. Real-time monitoring allows quick problem diagnosis and solution by the survey crew, as well as the data analysis team. This is particularly important if the surveyor has limited experience with the equipment and the survey conditions are rugged. For example, a common problem is the loss of differential correction transmissions from a GPS base station. When this happens, the mobile GPS receiver cannot accurately calculate positions, and the magnetic readings are useless. While this problem may not be immediately apparent to the mobile survey crew, those performing data analysis at the RTV base station can quickly alert them to the problem.
In practice, the SwRI RTV system smoothly reads radio transmissions, processes and feeds data to the displays, and writes to the data files in a seamless flow. It is even possible to switch to another virtual desktop and perform various editing or file management tasks while real-time processing is taking place.
Future enhancements include adding the capability of visualizing multiple surveys concurrently, adding color-coded symbols to the location map to indicate magnetic values, loading previous survey information into the database, performing real-time magnetic drift corrections to the data, and processing transmissions from other geophysical instruments.
Since the RTV system was developed, it has been used in Nevada and elsewhere, and has proved to be an effective tool for rapidly visualizing magnetic anomalies. Use of the system has also enabled the CNWRA to deliver high quality geological interpretations to the U.S. Nuclear Regulatory Commission more quickly than would otherwise have been possible. As a result of this field experience, SwRI is planning to add other geophysical sensors to the system with the goal of generalizing its applications far beyond the original objectives.
Published in the Spring 1999 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Maria Stothoff.