A DIGITAL ACCELEROGRAPH WITH AN OPTICAL SEISMOMETER

THE OBJECTIVE OF THIS PROJECT IS TO DEVELOP A NEW TYPE OF OPTICAL SEISMOMETER TO BE INCORPORATED INTO A RELIABLE, LOW-COST, DIGITAL ACCELEROGRAPH. THE OPTICAL SEISMOMETER USES NEWLY DEVELOPED NONCONTACT DISPLACEMENT-SENSING TECHNOLOGY TO MONITOR THE RELATIVE MOTIONS OF A SEISMIC MASS. THE NEW SENSOR WHICH USES LOW-COST, MASS-PRODUCED OPTICAL COMPONENTS, HAS A SENSITIVITY EQUIVALENT TO THAT OF EXISTING STATE-OF-THE-ART CAPACITIVE SEISMOMETERS WITHOUT THE REQUIREMENT FOR FORCE FEEDBACK. THE INTERFEROMETRIC DISPLACEMENT SENSOR HAS LINEARITY AND RESOLUTION OF LESS THAN 1 NM WITH A RANGE GREATER THAN 5 CM FOR A DYNAMIC RANGEEXCEEDING 150 DB. THE REMOTE OPTICAL DISPLACEMENT MEASUREMENT ALLOWS GREATER THERMAL ISOLATION THAN WITH EXISTING SENSORS. THE SENSOR DIRECTLY PRODUCES A 32-BIT OUTPUT WORD PROPORTIONAL TO DISPLACEMENT. THE NEW SEISMOMETER BEING DEVELOPED IN PHASE I IS COMPACT, ROBUST, AND INEXPENSIVE AND HAS LOW POWER CONSUMPTION. PLANS FOR PHASE II ARE TO INCORPORATE THE NEW SEISMOMETERS INTO A DIGITAL ACCELEROGRAPH SYSTEM. THE INHERENT HIGH RESOLUTION DIGITAL OUTPUT OF THE DISPLACEMENT EASES THE DATA CONVERSIONREQUIREMENTS. A FEW INEXPENSIVE DIGITAL LOGIC AND MEMORY INTEGRATED CIRCUITS ARE TO BE CONFIGURED TO PROVIDE ALL THE REQUIREMENTS FOR SEISMIC MONITORING. THE DEVELOPMENT PROJECT WILL PRODUCE AN ACCELEROGRAPH THAT HAS SIGNIFICANT ADVANTAGES OVER EXISTING EQUIPMENT IN COST, DYNAMIC RANGE, AND EASE OF USE.