Simultaneous acquisition is increasingly required for efficient operation of recent high-density and high-S/N seismic. Low frequency signals plays an essential role in recent imaging technologies such as full waveform inversion. Here, we developed new sweeps designed to simultaneously acquire low-frequency multi-shot data. Unlike other methods, our technique completely decomposes simultaneously acquired data, and therefore can efficiently obtainhigh-quality shot data.
The complete decomposition is achieved by amplitude modulation for repeated sweep sources. The signal for repeated sweeps exists at discrete frequencies compared with higher frequency resolution determined by the total time. Amplitude-modulated sweeps can be completely differentiated from the original sweep because modulated signals are assigned at empty frequency components. With the use of amplitude-controllable sources, our simultaneous method is available without changing the acquisition system and sampling rate. Besides, side-lobe free, low-frequency source can be obtained by so-called Wischmeyer-type half-cycle pseudorandom sweeps. The combination of half-cycle pseudorandom sweeps and amplitude modulation achieved simultaneous acquisition of low-frequency, high-quality data.
We acquired field crosswell seismic data using piezoelectric sources and hydrophones. The depths of the source and receiver wells are 50 m and the well distance is 64 m. We excited six amplitude-modulated pseudorandom sweeps at the same time, and each hydrophone receives mixed signals. The results showed that the shot gather obtained by simultaneous acquisition is equivalent to the gather obtained by single source. Amplitude modulated sweeps successfully produces each shot record without any cross talk. It is also confirmed that pseudorandom sweeps provide broad band data without side–lobe desirable for full waveform inversion
Our approach should contribute to reductions of acquisition time and cost for borehole and shallow marine surveys. Since amplitude modulation is applicable for amplitude-variable sources with arbitrary sweep designs, our method may be applied for broadband vibrotor acquisitions.