Methods: Adoppting the elementary units content to divide layer, whole group. sample method and clue investigation.

方法采用初级单位含量分层, 整群, 随机的抽样方法及线索调查.

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An effective depth to time domain transforming method is presented to improve normal sample method.

提出了一种有效的对常规采样方法改进的深时转换方法.

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