Demonstration of ACROSS sources phased array in a deep underground site

研制了一种新型地震主动监测方法ACROSS的硬件系统。核心硬件是一个旋转源。源以恒定频率旋转偏心质量，旋转产生离心力，离心力在地面上发射谐波弹性波。发射的波在时间上是极其相干和稳定的。因此，人们能够观察到通过断层带传播的波的一些时间变化。本文的研究结果如下：1)我们的实验场地建立了ACROSS系统。将一个ACROSS震源和三个钻孔式加速度计安置在地下深处的坚硬岩石上。为了使源的旋转和采样时间同步，源和观测系统通过光缆连接。由源产生的时序信号被传输到观测系统的AD转换器。2)观察微震动，定量评价噪声状况。3)多次驱动ACROSS源。发射波通过安装在距离震源1 m、300 m和1000 m处的3个加速度计进行观测。在一排时间序列数据中，人们无法检测到发射的谐波。然而，由于发射波的相干性和源接收机之间的同步性，发射波被准确估计。从这个实验中，我们发现产生的谐波在时间上是相当稳定的。在1 m位置观测到的波的时间稳定性为振幅0.1%，相位为100微秒。4)观测到15个频率响应分量，并对观测数据采用sompi-倒谱法进行处理。探测到P波到达时间。

A hardware system of new seismic active monitoring method, ACROSS, has been developed. The core hardware is a rotational source. The source rotates an eccentric mass with a constant frequency, the rotation generates centrifugal force and the centrifugal force emits harmonic elastic waves in the ground. The emitted waves are extremely coherent and stable in time. Therefore, one is able to observe some temporal variation of the waves that propagates through a fault zone. The results of this research are as follows ;1)ACROSS system was established in our experimental site. An ACROSS source and three bore-hole type accelerometers were settled on a hard rock in a deep underground. In order to synchronize between the rotation of the source and sampling timing, the source and observation system are linked by means of optical cables. Timing signals generated by the source were transported to the AD converter in the observation system.2)Micro-tremors were observed, and the noise condition was evaluated quantitatively.3)The ACROSS source was driven several times. The emitted waves were observed by using three accelerometers that were equipped at 1 m, 300 m and 1000 m far from the source. In a row time series data, one can not detect the emitted harmonic waves. However, because of the coherency of the emitted waves and the synchronization between the source and the receivers, the emitted waves were accurately estimated. From this experiment, we found that the generated harmonic waves were fairly stable in time. The temporal stability of the waves observed at 1 m site was 0.1% in amplitude and 100 micro-seconds in phase.4)Fifteen components of frequency response were observed and sompi-cepstrum method wad applied to the observed data. An arrival time of P wave was detected.