A Pc-Based Seismic Data Acquisition and Processing System
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? °61))10 A UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY II NE1111 III 3 1818 00019802 6 A PC-BASED SEISMIC DATA ACQUISITION AND PROCESSING SYSTEM by W. H. K. Lee, D. M. Tottingham, and J. 0. Ellis MS 977, 345 Middlefield Road Menlo Park, CA 94025 S. 00014$ V JAN 1 3 191i5 A,4 Open-File Report 88-751 rpt(rt Men-ii\p December 30, 1988 (03609;:e sov6i (0.s: This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards. Any use of the trade names or product names is for descriptive purposes only, and does not imply endorsement by the USGS. 1. INTRODUCTION A seismic data acquisition and processing system has been developed based on the IBM PC-AT technology. It was demonstrated informally at the PC session of the 1987 Fall AGU Meeting in San Francisco, and will be presented formally in the 1988 Fall AGU Meeting (Lee, Tottingham, and Ellis, 1988). Our goal was to build a simple and inexpensive system to digitize up to 64 analog seismic signals, detect and locate seismic events, and save the digital waveform data on mass storage. We believe that such a seismic data acquisition and processing system has many applications in seismology, especially for small local seismic networks. In this report, we will discuss our design philosophy and development history, the hardware requirements, and the currently available software. While discussing the actual hardware and software used in our system, we will refer to many commercial products. Alternative products are also mentioned whenever possible. The software for our PC-Quake system has been developed by both USGS staff and non-USGS scientists using commercially available software as much as possible. Therefore, not all of the software used in our system is in the public domain. However, these outside scientists have agreed to let us incorporate their executable code in our application software free of charge on an experimental basis. The required commercial -2- software can be purchased separately. In this open-file report, we release the executable software (excluding that which must purchased commercially), and the source code written by the USGS staff. These software are available on two standard 5.25 inch floppy diskettes. The PC-Quake system was developed to collect data for a non-USGS funded project. Users must be warned that the current edition, Version 2, PC-Quake software (released with this Open-File Report) still contains some "bugs", and we can not provide any technical support because resources within the USGS are limited. Although the programs in this disk have been tested, the U. S. Geological Survey cannot guarantee that they will give accurate results for all applications or that they will work on any PC computer system. A revised version of our PC-Quake system (including several additional programs for off-line data processing and analysis by our outside collaborators) and a documented user manual will be published under the auspices of the Working Group on Personal Computers of the International Association of Seismology and Physics of the Earth's Interior, and is scheduled for April, 1989. It will be available to the public (at a nominal cost) through the Seismological Society of America (Lee, 1989). 2. DESIGN PHILOSOPHY AND DEVELOPMENT HISTORY In the past 20 years, the first author has been involved in seismic networks, especially in data acquisition and processing (Lee and Stewart, 1981). Many computerized systems have been developed for seismic data acquisition and processing. The usual approach is to select the computer hardware first and then write the software. Because most designers would like their systems to do many things, it usually takes many years to complete a system. Unfortunately, the computer hardware is often obsolete by then. Until the advance of personal computers, all commercially available computers had proprietary, or "closed", architectures. It was difficult for others to build and integrate additional hardware, and to move software from one computer to another. We could not benefit much from previously designed systems because the hardware was different, and software peculiar to one system was generally not useful on another. Consequently, seismologists keep on developing new data acquisition and processing systems. This situation was drastically changed in 1981 when IBM introduced a personal computer with "open" architecture that became an industry standard. We now have standard hardware and software to build upon. At present, there are over 20 million IBM PC and compatible computers in use, and several million more are added each year. A few years ago, Al Lindh of the USGS bought an analog-to-digital board for an IBM PC from Data Translation, Inc. -4- After many months, he gave this DT2801 board to the first author, hoping that the board could be used for seismological purposes. Lee realized that it is not trivial to start something new, because he knew that many seismic data acquisition systems had been taken several man-years or several tens of man-years to complete. So he just let the A/D board sit. However, it started to bother him that a piece of hardware was idle, and he did not have the resources to make use of it. In early 1987, he met Jimmy Wong, who had just completed his master's degree in computer science, and had worked several years in data communication. Since Wong was looking for a job, Lee persuaded him to investigate this A/D card without pay until he found a job. After a few weeks of hard work, Wong demonstrated that with this A/D card, a PC could be used for seismic data acquisition and processing. However, by then, Wong found a job and had to terminate the collaboration. In April of 1987, John Rogers of the USGS needed some means to collect seismic data locally in Yakutat, Alaska because the USGS could not afford to telemeter the data to their central recording site at Palmer, Alaska. Lee suggested a PC-based system and provided the necessary hardware for Rogers. Based on what Jimmy Wong had developed and with minor help from Willie Lee, Rogers completed a prototype system and headed to Alaska in the summer of 1987. This system handled only 8 channels, and saved 30 seconds of digitized waveform data each time a seismic event was detected. Double buffering and DMA transfer were not supported in hardware, so the data acquisition stops for a few -5- seconds after each event. In the summer of 1987, the first author obtained funding to conduct a quarry source experiment from the Defense Advanced Research Projects Agency. The original plan was to use existing USGS seismic recording equipment to collect the data. However, it soon became evident that our existing recording equipment did not have the flexibility and real-time capability in the field. In order to do this with minimal funding, Lee recognized the necessity for developing a reliable seismic data acquisition system for field use quickly and inexpensively. Because funds were not available to purchase the necessary hardware for development and testing, Lee persuaded another USGS group (Dave Harlow and Randy White) to pool their resources together, and the necessary equipment was purchased. Without direct funding, the design philosophy is of necessity very different from the usual case. Instead of an elaborate system that does many things, the goal was a simple system that used standard hardware and required writing only a small amount of software. A design using IBM personal computers (PC's) was selected because they were the most common computer, low cost, many off-the-shelf peripherals, thousands of commercially available software, and "open" architecture. Such a system can grow in the future as the PC technology advances, and will not be obsolete in the near future. Based on Wong's and Rogers' work on the PC and after -6- consulting with the engineers at Data Translation, Inc., Lee recognized that a PC/AT would be needed to handle more than 8 channels and to perform additional functions. He quickly designed the present system using a more advanced A/D board (DT2821 by Data Translation, Inc.) and assigned most of the software development to his summer assistant, Dean Tottingham. By the end of the summer of 1987, Tottingham and Lee (with help from Will Kohler, John Rogers, Carlos Valdes, and Kip Wyss) had the present hardware system running on Version 1 of a program which they called MDETECT. Tottingham returned to school in the fall of 1987, and Lee continued to test the hardware and software. In early 1988, Lee established a 16-channel seismic network at the Kaiser Permanente Quarry, telemetering the analog data back to Menlo Park via two voice-grade phone lines (in a manner similar to the USGS Calnet). The newly developed system was put to actual test: an IBM PC/AT was used in conjunction with the DT2821 A/D board to digitize the incoming seismic signals at 100 samples per second and to display all the digitized traces on a monitor screen in real time. Seismic signals were monitored, and if an event was detected, the digitized waveform data were saved on the hard disk, first P-arrival times were automatically determined, and the event was located and displayed. The event triggering algorithm is based on short-term and long-term averages and is due to John Rogers. The automatic picking scheme is based on Allen (1978) as coded by Will Kohler. -7- Event location is based on a modified version of the HYPO71PC earthquake location program by Lee and Valdes (1985). In order to develop the PC-Quake system with limited available resources, we have used commercially available software for A/D operation, screen management, printing, and graphics development.