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2000 This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Introduction The Earthworm (EW) system 1, 2 is a seismic network data acquisition and processing system developed by the U. S. Geological Survey, Earthquake Hazards Team (USGS, EHZ) and Dr. Carl Johnson during the 1990's. It is used by the Northern California Seismic Network (NCSN) as well as many other networks including the Hawaiian Volcano Observatory, the Alaska Volcano Observatory, the Southern California Seismic Network, and others. The input to the EW system is comprised of many realtime electronic seismic waveforms fed to a multi-channel digitizer on a PC platform. For systems with more than 16 channels, the digitizer consists of one, two or four 64-channel multiplexer boards (National Instruments Corp. AMUX-64T) 3 attached to an A/D converter board located in the computer. When the system was initially developed the multiplexer boards were modified by adding passive filter circuits to each channel in order to eliminate electronic noise picked up in cabling. It was later discovered that a small amount of crosstalk occurred between any given channel and the channel immediately following it in the digitizing sequence. This crosstalk was measured at a ratio of approximately 49dB (280:1). Though small, during digitization of large amplitude signals it will cause what appear to be small earthquake arrivals at the wrong time on some channels. This can result in erroneous calculation of earthquake arrival times, particularly by automated algorithms, unless recognized by human analysts. Furthermore, even when crosstalk doesn't cause miscalculated arrival times, it still distorts the recorded data. Tests were performed to determine the best way to eliminate the crosstalk. It was discovered that two changes were necessary. First, the sample interval between successive channels in a scan needed to be increased from 4 microseconds to 10 microseconds. This is easily accomplished by changing the Earthworm configuration file. Second, the source impedance driving the input to the multiplexer channels needed to be lowered dramatically. The passive filters that were added to the boards had introduced an inherent impedance of 24 kilohms between the source (discriminator outputs) and the inputs. In order to provide a low impedance signal to the inputs while at the same …

A filter circuit board for the Earthworm Seismic Data Acquisition System