uzn
8th December 2017, 20:09
Well yes, the Earth is humming.
Scientists have isolated the earths humming at the bottom of the Indian ocean.
Abstract:
The Earth’s hum is the permanent free oscillations of the Earth recorded in the absence of earthquakes, at periods above 30 s. We present the first observations of its fundamental spheroidal eigenmodes on broadband ocean bottom seismometers (OBSs) in the Indian Ocean. At the ocean bottom, the effects of ocean infragravity waves (compliance) and seafloor currents (tilt) overshadow the hum. In our experiment, data are also affected by electronic glitches. We remove these signals from the seismic trace by subtracting average glitch signals; performing a linear regression; and using frequency-dependent response functions between pressure, horizontal, and vertical seismic components. This reduces the long period noise on the OBS to the level of a good land station. Finally, by windowing the autocorrelation to include only the direct arrival, the first and second orbits around the Earth, and by calculating its Fourier transform, we clearly observe the eigenmodes at the ocean bottom.
Conclusions :
We demonstrate the presence of eigenmodes at two ocean bottom seismometers for the first time. We do so by noise reduction and calculation of the autocorrelation of the seismograms. We successfully removed electronic glitches of varying periodicity from vertical OBS data, which reduced the noise level by 28 dB. Our technique filters the data and subtracts an average glitch wavelet, adjusted in position and amplitude. We use linear regression with an estimation of the double derivative of the pressure, the horizontal channels and an estimation of the gradient of the pressure as inputs to reduce the seismic noise level on the vertical seismometer channel and remove the long period glitches. By further removing compliance and tilt using frequency response functions based on the coherence between different seismometer and pressure channels, we were able to reduce the noise level by another 8 dB. The final noise level at the OBS is only around 182 dB in acceleration, which is close to the noise level of a quiet land station (Peterson, 1993).
On the denoised time series, we observe the hum signal on the ocean bottom, by windowing the autocorrelation around the direct arrival and the first and second orbits around the Earth, followed by a Fourier transform. We observe very clear peaks that coincide with the Earth’s theoretical eigenfrequencies between 2.9 and 4.5 mHz. The same procedure applied to reference land station TAM in southern Algeria shows a hum signal of similar amplitude on land and on the ocean bottom. We demonstrate the continuous presence of these modes over the whole 11 monthlong period of deployment of the OBS but observe no clear seasonal variation of the hum between 2.9 and 4.5 mHz.
Geophysical Research Letter:
http://onlinelibrary.wiley.com/doi/10.1002/2017GL074892/epdf?referrer_access_token=dopcYmDQNG6fhwrqkn6bd04keas67K9QMdWULTWMo8NeG9tQYGT7BxtW00NpeY6BT_NzTe12j 6MnPZEtxy-qUPm5ur7THWCVURRLNI5BTSeMqjgnkbuNHW-AuSA1kp3HeF2FbQwtobdbU5x8J2OLaso3dQSbCsdxbUKQGcxVtYbr9wtyKkVrj2lP0PkE1nrma2nbG5ADLmqYNOV9-m_SjGicSsrbVpZqUDo8JkSyvV1X-Mi1ougI-GPjiEZzIDaLaOVnOtXoE19KY9makD833DtzvRIGloECqBnXtNaS4Oo%3D
I hum an ohm to that ;)
Scientists have isolated the earths humming at the bottom of the Indian ocean.
Abstract:
The Earth’s hum is the permanent free oscillations of the Earth recorded in the absence of earthquakes, at periods above 30 s. We present the first observations of its fundamental spheroidal eigenmodes on broadband ocean bottom seismometers (OBSs) in the Indian Ocean. At the ocean bottom, the effects of ocean infragravity waves (compliance) and seafloor currents (tilt) overshadow the hum. In our experiment, data are also affected by electronic glitches. We remove these signals from the seismic trace by subtracting average glitch signals; performing a linear regression; and using frequency-dependent response functions between pressure, horizontal, and vertical seismic components. This reduces the long period noise on the OBS to the level of a good land station. Finally, by windowing the autocorrelation to include only the direct arrival, the first and second orbits around the Earth, and by calculating its Fourier transform, we clearly observe the eigenmodes at the ocean bottom.
Conclusions :
We demonstrate the presence of eigenmodes at two ocean bottom seismometers for the first time. We do so by noise reduction and calculation of the autocorrelation of the seismograms. We successfully removed electronic glitches of varying periodicity from vertical OBS data, which reduced the noise level by 28 dB. Our technique filters the data and subtracts an average glitch wavelet, adjusted in position and amplitude. We use linear regression with an estimation of the double derivative of the pressure, the horizontal channels and an estimation of the gradient of the pressure as inputs to reduce the seismic noise level on the vertical seismometer channel and remove the long period glitches. By further removing compliance and tilt using frequency response functions based on the coherence between different seismometer and pressure channels, we were able to reduce the noise level by another 8 dB. The final noise level at the OBS is only around 182 dB in acceleration, which is close to the noise level of a quiet land station (Peterson, 1993).
On the denoised time series, we observe the hum signal on the ocean bottom, by windowing the autocorrelation around the direct arrival and the first and second orbits around the Earth, followed by a Fourier transform. We observe very clear peaks that coincide with the Earth’s theoretical eigenfrequencies between 2.9 and 4.5 mHz. The same procedure applied to reference land station TAM in southern Algeria shows a hum signal of similar amplitude on land and on the ocean bottom. We demonstrate the continuous presence of these modes over the whole 11 monthlong period of deployment of the OBS but observe no clear seasonal variation of the hum between 2.9 and 4.5 mHz.
Geophysical Research Letter:
http://onlinelibrary.wiley.com/doi/10.1002/2017GL074892/epdf?referrer_access_token=dopcYmDQNG6fhwrqkn6bd04keas67K9QMdWULTWMo8NeG9tQYGT7BxtW00NpeY6BT_NzTe12j 6MnPZEtxy-qUPm5ur7THWCVURRLNI5BTSeMqjgnkbuNHW-AuSA1kp3HeF2FbQwtobdbU5x8J2OLaso3dQSbCsdxbUKQGcxVtYbr9wtyKkVrj2lP0PkE1nrma2nbG5ADLmqYNOV9-m_SjGicSsrbVpZqUDo8JkSyvV1X-Mi1ougI-GPjiEZzIDaLaOVnOtXoE19KY9makD833DtzvRIGloECqBnXtNaS4Oo%3D
I hum an ohm to that ;)