# Real-Time Seismic Data from the Bottom Sea
## 2 Standalone Broadband Ocean Bottom System
### The Seafloor Unit  
The seafloor unit consists of an anchor, a seismometer containing 3 geophones (one in each orthogonal axis), and the associate electronics and the batteries (Figure 2). When the equipment reaches the seabed, the arm that supports the seismometer is tilted and separates the seismometer from the anchor and next the seismic sensor is positioned on the seabed, next to the anchor with the acquisition and power unit, by only the signal and power cable. The broadband seismometer is a Gurlap 6 TC OBS Gimbal with a sensitivity of 1630 V/m/s for each geophone and with a cut-off of at 0.033 Hz for detecting low frequency Earth movements. As illustrated in Figure 3, the bottom case is grooved to evacuate exceeding material like sand, mud, sludge, or other kind of sediments, getting a successful coupling with the seafloor. A centring of the Guralp 6 TC OBS masses is required when the seismometer is first deployed on the seafloor. This operation can be done from the land base at any time through the bidirectional communication link provided by the inductive and the RF link.  
Figure 1: Overview of the Real-Time Seismic Coastal Ocean Station.  
Figure 2: The prototype ocean bottom broadband unit.  
The seismic data of the Guralp 6 TC OBS is acquired and managed by an acquisition system (Figure 4), which consists of five main blocks: analogue-to-digital conversion module, micro-controller and data storage module, power regulation module, time base module, and inductive communication module as illustrated in Figure 4.  
The prototype broadband OBS is a modification of the autonomous one [8] with the addition of an inductive modem and its associated interface processor. A lithium-ion battery pack is built and used as the main power supply. The micro-controller and storage module is based on a LPC 4357 microcontroller and a 64 GB microSD memory card for data storage (approximately 1 year of raw seismic data). This module is in charge of configuration of the ADC module selecting the sampling rate and low power operations. The acquisition software designed acquires data continuously from input channels through a QSPI (Queued Serial Peripheral Interface) bus, performs local timestamping using the integrated RTC (Real-Time Clock), and stores the data in the microSD memory card. In addition to the seismic measurements, the internal temperature and humidity measurements are also performed. The data acquisition and storage system has been optimized to acquire seismic events in the domain of seismology. The data logger has a dynamic range of all the channels of 129 dB, slightly below the  
Figure 3: The Ocean Bottom Seismometer (OBS) with Güralp 6 TC seismometer at pressure test. The bottom case of the seismometer is grooved to evacuate exceeding material for a successful coupling with the seafloor.  
ADC dynamic range at the same sampling rate (130 dB@103 Hz). The resolution of the system is about 21.4 bits due to noise and the random noise is about 1.1 LSBs which corresponds to about 320 nV. A channel crosstalk of \(-147\) dB shows a good PCB design and EMI immunity.  
The acquired measurements are timestamped and organized into fixed length miniSEED files and stored on a microSD card. In normal mode, sub-samples seismic data acquired from the vertical axis of the seismometer is transmitted every ten seconds to the shore station through the inductive communication using a Sea-Bird UIMM module and the surface link. If required by the operators, the system can transmit historic packages of seismic data from all 3 axes at full resolution. Furthermore, this module carries out the following functions: time synchronization with a GPS signal, clock drift calculation, and low power operation modes. A lithium-ion battery pack is used as the main power supply and is enough for 6 months in service collecting data.