OPERATION MANUAL - Scintrex

Optimizing your Field Operationsat times be troublesome even in the centre of large continents. These effectsare improved by selection of the Seismic Filter. For more information, seeAppendix B, “Theory of Operation”.Earthquake shock waves - The shock waves from an earthquake may affectmeasurements so much that they cannot be carried on while the shock waveslast. The effect of smaller tremors could be quite effectively reduced byselection of the Seismic Filter.Industrial traffic noise - Vibration due to industrial activity and/or thenearby movement of heavy vehicles generally introduces higher frequencyseismic noise (e.g. 4 to 100Hz). Gravity stations that are near such sources ofnoise should be avoided. Where they cannot be moved to a quieter location,measure longer (e.g. 120 seconds instead of 60 seconds).The standard deviation of 1 second measurement samples is a goodindication of the seismic noise, especially longer period microseismic noise.During quiet periods, SD is in the range of 0.010 to 0.030 mGal, whereas,during storms on open seas, it could increase to 0.150 mGal (measured inToronto) or even higher near the sea coast. It could go to 0.5 mGal and higherduring remote but strong earthquakes. The shorter period industrial noise isreduced by signal averaging over 1 second so that 1 second samples are lessresponsive to this noise. Noise spikes are removed by auto rejection of 1second samples.Minimizing Wind NoiseWind striking a gravity meter can cause both vibration noise and reduction ingravity by tilting the meter out of verticality. Tilting the meter so that it is notvertical tends to reduce the apparent gravity value, despite the statisticalaveraging of the gravity measurements and the automatic tilt correction.A windbreak should be devised to protect the meter from the effect of wind.A simple windbreak can be made from a sheet of canvas about 1.5m square,fastened on each of two opposing sides to two aluminum tubes or woodenrods (each, about 2m or 6.5ft long). The operator (or a helper) then holds therods outstretched and vertical on the upwind side of the gravity meter toshield it from the wind. Alternately, a large (golf) umbrella could also beused.The effect of wind noise may also be reduced by selecting the Seismic Filter.For details, see Appendix B, “Theory of Operation.”4-4CG-5 Manual - part # 867700 Revision 2