That makes installing subsurface cables 12 to 17 times as pricey as overhead lines, according to National Grid (they also need replacing sooner).

根据国家电网(他们也需要尽早更换输电线路)所说：这使得安装地下电缆比架空线路昂贵12到17倍。

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In field practice, a subsurface is mapped by arranging a series of wave-train sources, such as small dynamite explosions, in a grid pattern…

第二句话对解释进行总的说明：“在实地作业中，通过将一系列波列源，诸如小规模炸药爆炸，排列成一个网格模式，从而将地下层面标绘出来。” 接下来是“地震反射法”的详细过程。

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