Abstract Earthquakes and other seismic sources produce waves with frequency content spanning many orders of magnitude. Recording a broader frequency band of interest has historically required deploying multiple instruments designed to work the best within limited, overlapping frequency ranges. Here, we detail a 300 m deep borehole deployment of a sensor package, including three new optical accelerometers that can potentially replace many dedicated instruments with a single, low-noise sensor. These instruments are designed with a flat frequency response from 0.005 to 1500 Hz, spanning the flat response segments of broadband sensors and geophones, as well as a low-noise floor and high sensitivity. The sensors have been functioning normally for over four years, fully grouted at depths of >100 m. Year-long power spectral density (PSD) profiles show that the optical accelerometers have a lower noise floor than a colocated geophone for all frequencies, with 20 dB noise reduction at 250 Hz. PSD comparisons to a broadband sensor installed at the surface show a 5–30 dB noise reduction for the optical accelerometer above 1 Hz, although this is likely due, in part, to the broadband sensor being subjected to much higher surface noise. At periods >5 s, the broadband sensor shows up to 20 dB lower noise than the optical accelerometer, which in turn has up to 50 dB lower noise floor than the colocated geophone. Finally, modeling the Brune displacement spectrum for theoretical seismicity within 1 km of the borehole shows that the optical accelerometers could potentially deliver a detection threshold improvement of one magnitude unit relative to the colocated geophone.

Recently Distributed Acoustic Sensing (DAS) measurement becomes popular for seismic observation due to its spatially high-density. We have conducted nine DAS measurements using a spare fiber of the optical seafloor cable seismic tsunami observation system off Sanriku, northeastern Japan from February 2019, and evaluated performance of the DAS data. For each observation, many earthquakes were observed and we compared DAS records to records from the cabled seismometer. It is found that system noise level of DAS measurement is the same as that of the seismometer. Seismic survey using controlled sources and DAS measurement on the seafloor cable was carried out in November 2020, and the signals from the controlled sources were clearly recorded. Clear reflection images were obtained using DAS data by elementary seismic data processing. In addition, a linearity of amplitude response was confirmed under the condition of small amplitude. DAS measurement can be quantitatively applied in an observation of earthquakes on the seafloor.

Vibration-monitoring instruments that employ standard, velocity-responsive geophones lack the frequency range and sensitivity required to characterize low-level vibration that can degrade the performance of precision laboratory instruments and adversely affect manufacturing processes. High-sensitivity accelerometers have a flat response over a broad seismic frequency range, producing stronger signals (higher signal-to-noise ratio) than standard geophones over much of the spectrum above and below the natural geophone frequency. Accelerometers can be used to examine the vibration susceptibility of sites that are being considered for vibration-sensitive manufacturing, testing, or calibration facilities. Unfortunately, ambient ground motion at proposed, inactive sites is commonly much lower than that at existing facilities and is much lower than would be expected when activity commences at the proposed site. In addition to ambient monitoring at current and proposed laboratory sites, we performed walkaway seismic tests to compare susceptibility to induced ground motion at shallow- and deep-bedrock sites in Central Texas. Ambient monitoring demonstrated that both shallow- and deep-bedrock sites satisfied recommended laboratory guidelines for displacement and acceleration. Walkaway surveys, performed using a weight-drop source at 10-m intervals from 10 to 100 m from the ground-coupled, triaxial accelerometers, showed that (a) accelerations induced over shallow and deep bedrock excee

The broadband capability of land receivers is reviewed. On the low-frequency side, two more octaves have been added to the signal using low dwell sweeps. These low frequencies can be recovered below the resonant frequency of the geophones by compensating for their attenuation. However, this inverse filter works well so long as there is adequate signal-to-noise ratio. To avoid amplifying instrument noise, new geophones have been developed with a higher sensitivity and a lower resonant frequency. MEMS accelerometers that display a linear response down to DC seem to be the ideal sensor to capture very low frequencies. Their limitations relating to the instrument noise may be compensated by a high spatial sampling. On the high-frequency side, progress is limited by the absorption that occurs at shallow levels and during propagation. The spurious frequency of geophones occurs above the high-cut used in most of the surveys. MEMS accelerometers benefit from a broadband response, higher sensitivity and lower instrument noise at high frequency. Therefore, the main issue is not related to the receivers but to the possibility of enhancing the signal-to-noise ratio at high frequency.

We developed several types of MEMS accelerometers using commercial MEMS elements for trial use in seismic surveys. Field experiments and earthquake observations were carried out for investigating the capabilities of the MEMS accelerometers. The results of these experiments and observations show that the properties of these MEMS accelerometers are similar and that they are about 1.5-3.0 times as sensitive as conventional geophones used in seismic surveys. The noise level of the MEMS 3-C accelerometer in natural earthquake observation was about 10-4kine (cm/s), and the useable frequency band extends to below 1Hz. For future works, we will further investigate the characteristic of MEMS geophones in low frequency band using earthquake records. In addition, we will reexamine the electronic circuit and the MEMS elements in order to attain high sensitivity.

Geophones have been the motion sensor of choice in oil and gas exploration surveys for many years and for good reason: they require no electrical power to operate, are lightweight, robust, and able to detect extremely small ground displacements. However, the seismic industry recently has developed considerable interest in microelectro mechanical systems (MEMS) accelerometers, which are similar to those used to sense accelerations for airbag deployment and missile guidance (among many other uses). The MEMS element is tied to an application specific integrated circuit (ASIC) that includes sensor signal conditioning, feedback control, and digitization blocks. The result is a custom-built unit for seismic applications that requires a power supply to operate.

The development of microdrilling technology, nominally defined as drilling technology for 1-in.-diameter boreholes, shows potential for reducing the cost of drilling monitoring wells. A major question that arises in drilling microholes is if downhole logging and monitoring in general--and downhole seismic surveying in particular--can be conducted in such small holes since the inner working diameter of such a seismic tool could be as small as 0.31 in. A downhole three-component accelerometer package that fits within a 031-in. inner diameter tube has been designed, built, and tested. The package consists of three orthogonally mounted Entran EGA-125-5g piezoresistive silicon micromachined accelerometers with temperature compensation circuitry, downhole amplification, and line drivers mounted in a thin-walled aluminum tube. Accelerometers are commercially available in much smaller package sizes than conventional geophones, but the noise floor is significantly higher than that for the geophones. Cross-well tests using small explosives showed good signal-to-noise ratio in the recorded waveform at various receiver depths with a 1,50-ft source-receiver well separation. For some active downhole surveys, the accelerometer unit would clearly be adequate. It can be reasonably assumed, however, that for less energetic sources and for greater well separations, the high accelerometer noise floor is not acceptable. By expanding the inner working diameter of a microhole seismic tool to 0.5 in

Strain-beam accelerometers constructed as integrated circuits are too insensitive for geologic seismographs (2002), but are widely used in geophones. Some other A seismometer is an instrument that responds to ground displacement and shaking caused by quakes, volcanic eruptions, and explosions. They are usually combined with a timing device and a recording device to form a seismograph. The output of such a device—formerly recorded on paper (see picture) or film, now recorded and processed digitally—is a seismogram. Such data is used to locate and character Ac…

hydrophones, or more often geophones or accelerometers, in the borehole record reflected seismic energy originating from a seismic source at the surface. In geophysics, vertical seismic profile (VSP) is a technique of seismic measurements used for correlation with surface seismic data. The defining characteristic of a VSP (of which there are many types) is that either the energy source, or the detectors (or sometimes both) are in a borehole. In the most common type of VSP, hydrophones, or more often geophones or accelerometers, in the borehole reco In geophysics, vertical seismic profile (VSP) is a technique of seismic measurements used for correlation with surface seismic data. The defining characteristic of a VSP (of which there are many types) is that either the energy source, or the detectors (or sometimes both) are in a borehole. In the most common type of VSP, hydrophones, or more often geophones or accelerometers, in the borehole record reflected seismic energy originating from a seismic source at the surface. There are numerous methods for acquiring a vertical seismic profile (VSP). Zero-offset VSPs (A) have sources close to the wellbore directly above receivers. Offset VSPs (B) have sources some distance from the receivers in the wellbore. Walkaway VSPs (C) feature a source that is moved to progressively farther offset and receivers held in a fixed location. Walk-above VSPs (D) accommodate the recording geometry of a deviated well, having each receiver in a different lateral position and the source directly above the receiver. Salt-proximity VSPs (E) are reflection surveys to help define a salt-sediment interface near a wellbore by using a source on top of a salt dome away from the drilling rig. Drill-noise VSPs (F), also known as seismic-while-drilling (SWD) VSPs, use the noise of the drill bit as the source and receivers laid out along the ground. Multi-offset VSPs (G) involve a source some distance from numerous receivers in the wellbore. A vertical seismic profile is constructed to identify a value known as a source wavelet. This is useful when it comes to a process known as deconvolution. Deconvolution allows for a more readable and more focused VSP. The idea is that the VSP reports any abnormal seismic activity and deconvolution allows for a more focused profile on these abnormal activities. VSPs are used to measure a seismic signal at depth and with that measurement the wavelength at the source of the seismic activity is easily found. With the measurement of the source wavelet, geophysicists can carry out deconvolution on the VSP and decrease the reports of all seismic activity and limit the reports to just abnormal or extreme changes in seismic activity. In recent years, using a VSP has…