Barry, Sorry I have not responded sooner, but an emergency came up when a contractor tore up the fiber-optic cable linking the processors at the IRIS station CCM. I've ended up replacing about 100 meters of cable, making new "pipe vaults" for the connections that have to survive flooding 10 to 12 meters deep. Hopefully the additional splice in the 2.4km cable will not push our optical power budget. Regarding LP transient noise from your VBB vertical. This is not an uncommon problem with broadband seismometers operating at extended periods, like in excess of 20 seconds. THe longer the operating period, the more sensitive the sensor is to steps in displacement, which show up in the velocity output as one-sided lumps with about the duration of the integrator period. I can only guess at some possible causes, so I will pass on my experience for everyone's benefit. The manufacturers of commercial broadband sensors actually suggest being tolerant of these transients during the "settling in" period of the instrument, which may last several weeks as all the parts come into thermal equalibrium and mechanical stresses dissipate. Of course, the same is true for the enclosure, pier, vault, etc., even the lay of the cable to the seis is of concern. Once an installed instrument has established itself as reliably quiet, the sudden incidence of transient noise can be attributed to either the electronics of the feedback, the mechanics of the sensor, the immediate environment (covers, cable, vault), or the regional environment. You mention seeing similar transients on a station in Norway. This would be pure coincidence. As you know, the only transient-like or one-sided impulsive phases would be direct compressive arrivals of teleseismic quakes: most of the energy from distant quakes that a broadband would record are long sequences of super-imposed sine waves that would follow the direct waves on most of the broadband sensors worldwide. Assuming that the electronics worked OK when it was installed, the main source of transients is dampness in the enclosure, which particularly affects the high impedances of the integrator. Intermittent conduction of contamination, like fingerprints, can be a problem. The STS-1 integrator components are actually mounted on ceramic standoffs to deal with this. Although seismometers are generally installed in dark, damp, moldy places, even in dripping caverns, no amount of dampness within the system can be tolerated; the enclosure should be sealed, using vapor tight connectors (or seal them with silica gel around the inside terminals). The enclosure(s) should be desiccated with several units of diatomacious earth (cheaper than silica gel) and an indicator strip should be installed to indicate that the desiccant is active. (I shocked some people by drilling 1.5" viewports in the covers of the Streckheisn electronics enclosure so I could install the indicator inside them). The desiccant can absorb about 1cc of water per unit; with 5 units in the 8"x10"x3" box, it stays active for about 5 years in the dampness of a cavern. In my opinion, there is little point using desiccant without the humidity indicator strips. (from Humidial Corp.) The connectors should also be (or be made) weather tight. Some have O ring seals, which are good only until the connector is dropped on the dirt floor. If I have any doubt, I use stretched self-vulcanizing tape over-wrapped with vinyl electrical tape that can be cut away if/when the connection has to be removed. The seismometer enclosure itself should also be sealed and desiccated if it is in a damp environment. The desiccant needs to be secured so that as it expands it doesn't touch any moving part. If the seis enclosure is tight enough to stay dry, it will also be spider and waterbug proof. Bugs can make interesting transients. I use wide, thin foam weatherstrip to seal the cover; it does breathe slightly along its length, which is preferable to absolute seals that might equalize barometric pressure through a pinhole with a little jet that blows the mass around. Another problem has occurred with commercial seismometers that use large capacitive transducers with close (0.2mm) clearances. ANY contamination, like an eyelash, will cause contact. A recurring problem has been peeling adhesives or coatings. These are often detected by strong local vibration, like a good stomp, that causes a transient. For the speaker coil/magnet transducer, small magnetic slivers on the poles can cause this. I use an inside-out strip of masking tape to repeatedly clean the magnet gap as well as to ensure that there are no dog hairs on the coil. For the home-made seis, another problem can arise from the flexures, especially if they are epoxied in place. They may be coming unglued from improper cleaning at assembly, or some residual excess glue might be in the flexing area. I haven't had this problem with the later versions of the large vertical. Of course, any spontaneous mechanical change of the moving system results in a transient; often these are bidirectional micropositioning due to temperature changes. You mention that you suspect changes in the slab under the seis, possibly due to temperature or drying out. This is always possible, but unless the cause is really obvious, like desiccation cracks, I would expect slow changes rather than transients. If there are large enough dimensional changes, the feet of the seis base may be moving on the slab; we always use glass plates under the feet to allow for smooth differential expansion; these are often epoxied to the pier or slab surface. Highly glazed ceramic tiles will also do; I used them for the top surface of the pier here. Other sources of strange transients from the environment can come from slight movements of the covers, cables, etc. The cable should always approach the seis laying on the pier, making S-turns or possibly a circle around the seis. Regards, Sean-Thomas __________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>