X. Roset

Ocean Bottom Seismometer: Design and Test of a Measurement System for Marine Seismology

The Ocean Bottom Seismometer (OBS) is a key instrument for the geophysical study of sea sub-bottom layers. At present, more reliable autonomous instruments capable of recording underwater for long periods of time and therefore handling large data storage are needed. This paper presents a new Ocean Bottom Seismometer designed to be used in long duration seismic surveys. Power consumption and noise level of the acquisition system are the key points to optimize the autonomy and the data quality. To achieve our goals, a new low power data logger with high resolution and Signal–to-Noise Ratio (SNR) based on Compact Flash memory card is designed to enable continuous data acquisition. The equipment represents the achievement of joint work from different scientific and technological disciplines as electronics, mechanics, acoustics, communications, information technology, marine geophysics, etc. This easy to handle and sophisticated equipment allows the recording of useful controlled source and passive seismic data, as well as other time varying data, with multiple applications in marine environment research. We have been working on a series of prototypes for ten years to improve many of the aspects that make the equipment easy to handle and useful to work in deep-water areas. Ocean Bottom Seismometers (OBS) have received growing attention from the geoscience community during the last forty years. OBS sensors recording motion of the ocean floor hold key information in order to study offshore seismicity and to explore the Earth’s crust. In a seismic survey, a series of OBSs are placed on the seabed of the area under study, where they record either natural seismic activity or acoustic signals generated by compressed air-guns on the ocean surface. The resulting data sets are subsequently used to model both the earthquake locations and the crustal structure.

New generation of ocean bottom seismometers

We present the newest development of an ocean bottom seismometer. The equipment represents the achievement of a joint work from different scientific and technological disciplines such as electronic, mechanic, acoustic, communications, information technology, marine geophysics, etc. This sophisticated equipment allows to record useful active and passive seismicity information, as well as other wavefronts, with multiple applications to the marine environment research. We are in the first series prototype hoping to improve many of the aspects that keep the equipment useful to work in deep-water areas.

Design, Characterization and Calibration of a Short-Period Ocean Bottom Seismometer (OBS)

First part of this paper presents an ocean bottom seismometers (OBS) designed and constructed for mid-term deployments in order to study the earth dynamics and internal structure. Many marine research institutes have developed such equipment, however there is no standard method for their characterization and calibration. The second part discusses the characterization tests based on the international standards carried out to present the specifications of the equipment built. Calibration of the constructed OBS is carried out through an oceanographic cruise using a widely used reference OBS. Data quality of the instruments is evaluated by direct inspection of the corresponding seismic record sections.

Geophone calibration by means of hyperbaric chamber

This work proposes the use of a hyperbaric chamber in order to verify and validate the calibration in a marine geophone sensor before and after the pressure underwater laboratory test. The focused objective is the behavior respect to frequency of coupling between the geophone and the sediment to characterize its transfer function. The calibration of the 3 axes of the geophone up on the sediment box are made with a shaker table. We can observe the variations of coupling sensibility of the sensors of the geophone work through the sediment after the test inside the hyperbaric chamber at 20 atmospheres of water pressure.

Calibration and modelling of a bottom sea geophone based on Virtual Instrument

This development is a characterization of a geophone sensor with a calibrator and a shaker table in order to obtain the significant parameters and to test the proposed equivalent model. The method is based on a Virtual Instrument and allows to compare the results of the real and simulated measurements by modelling. This application is a geologic prospection to choose the best structures for the geophone. This equipment allows to record useful active and passive seismicity information, by means of acoustic wave front, with multiple applications to the marine environmental research.

Virtual instrumentation for power supply harmonic measurement

The work reported here presents a virtual instrument (VI) for total harmonic distortion (THD) measurement index with a high accuracy, this is due to the use of high characteristics windows and DFT (discrete Fourier transform) interpolation algorithms are specially interesting in power switching supplies measurements. In the PWM switching regulators frequency characterization the stimulus signal amplitude has to be small enough to keep the converter in its linear region of operation, but it has to be also large enough to be distinguishable from the random noise. This VI allows the amplitude and frequency range obtention. The THD generated by a nonlinear load in the power distribution system can be measured by this VI. The THD generated in the power distribution system by a flyback DC-DC switching converters has been measured.

Low power and easy to use ocean bottom seismometer (OBS) for long period surveys

The OBS (ocean bottom seismometer) has been a key instrument for geophysical study of sea subbottom layers in the last decades. Now scientists are demanding highly reliable autonomous equipment capable of staying underwater for long periods of time and therefore handling large data storage. Power consumption of the acquisition system as well as a stable time base with temperature are the main issues in order to achieve high autonomy together with a good data quality. This paper presents a new OBS designed to be used in marine long term surveys. To achieve our goals, a new datalogger based on Compactflash memories with high data transfer capacity is designed to enable continuous data acquisition. The equipment is now under evaluation where lab tests have been carried out and tests under real environmental conditions are programmed for second semester of 2005.

Real-Time Seismic Data from the Bottom Sea

An anchored marine seismometer, acquiring real-time seismic data, has been built and tested. The system consists of an underwater seismometer, a surface buoy, and a mooring line that connects them. Inductive communication through the mooring line provides an inexpensive, reliable, and flexible solution. Prior to the deployment the dynamics of the system have been simulated numerically in order to find optimal materials, cables, buoys, and connections under critical marine conditions. The seismometer used is a high sensitivity triaxial broadband geophone able to measure low vibrational signals produced by the underwater seismic events. The power to operate the surface buoy is provided by solar panels. Additional batteries are needed for the underwater unit. In this paper we also present the first results and an earthquake detection of a prototype system that demonstrates the feasibility of this concept. The seismometer transmits continuous data at a rate of 1000 bps to a controller equipped with a radio link in the surface buoy. A GPS receiver on the surface buoy has been configured to perform accurate timestamps on the seismic data, which makes it possible to integrate the seismic data from these marine seismometers into the existing seismic network.

Near real-time determination of earthquake source parameters from the coastal ocean

In this paper we present a moored-buoy system for collecting near real-time seismic data from the coastal ocean. This easy to handle and sophisticated equipment allows the recording of useful passive seismic data in the costal ocean areas with maximum depth limited to the inductive maximum mooring depth (theoretical 7000 meters). Power consumption and noise level of the acquisition system are the key points to optimize the autonomy and the data quality. To achieve our goals, a new low power data logger with high resolution and Signal-to-Noise Ratio (SNR) has been designed to enable continuous data acquisition. The paper includes the laboratory validation results by means of automated measuring systems and calibration procedures. The seismic acquisition equipment includes an on board inductive communication system developed to transmit near real time seismic data to the shore stations.

Design of a Geophone sensor for Marine Deep Seismology

We present the newest development of a geophone sensor. The equipment represents the achievement of a joint work from different scientific and technological disciplines as electronic, mechanics, marine geophysics, etc. This handle equipment allows to record useful active and passive seismicity information, as well as other wave front, with multiple applications to the marine environmental research. We are in the second series prototype hopping to improve many of the aspects that keep the equipment handle and useful to work at deep-water areas.