Challenges and Perspectives for Lowering the Vertical-Component Long-Period Detection Level

For observations of vertical component acceleration in the normal-mode band (0.3 mHz – 10 mHz) the detection sensitivity for signals from Earth’s body can be improved to levels below the Peterson Low Noise Model (PLNM). This is achieved by deterministic procedures which (at least partly) remove the accelerations originating from atmospheric mass fluctuations. The physical models used in such corrections are still too simple and fail at frequencies above 3 mHz. Anticipating improved atmospheric correction procedures, we explore the prospects of lowering the detection level. From recordings of excellent vertical component sensors operated under exceptional site conditions at the Black Forest Observatory (BFO) we select time windows of very low background signal, where all of the contributing broadband seismometers showed their best performance. Streckeisen seismometers of type STS-1, STS-2, and STS-6A, a Nanometrics Trillium T360, and the superconducting gravimeter (SG) SG056 manufactured by GWR Instruments take part in this comparison. Due to their low level of self-noise the STS-1 and the SG056-G1 benefit the most from a correction with the best currently available IBPM-model (improved Bouguer plate model) for atmospherically induced signals at frequencies below 1 mHz. As far as we know this is the first case where the background level of a broadband seismometer could be lowered below the PLNM. At signal periods beyond the normal-mode band (investigated up to 12 h) the gravimeters show the lowest level of selfnoise, directly followed by the STS-6A. In the band from 0.3 mHz to 10 mHz, the STS-1 has the lowest level of self-noise, which is at least 4 dB below the PLNM, directly followed by the T360 and the STS-6A. Sensors of lower self-noise than the currently manufactured STS-6A or T360 are needed before improved atmospheric correction procedures lead to a significantly lower vertical component detection threshold.