Kondisi lapisan batuan bawah permukaan memiliki sifat fisis yang beragam. Tingkat kekerasan batuan bawah permukaan bumi merupakan salah satu sifat fisika yang dapat diketahui melalui pengukuran di permukaan bumi. Seismik refraksi merupakan salah satu metode geofisika yang digunakan. Telah dilakukan pengukuran seismik refraksi di dua tempat yang berbeda, BW17 dan BW27. Di wilayah BW17 didapatkan empat lapisan batuan. Lapisan pertama dengan kecepatan 405 - 734 m/s memiliki tingkat kekerasan very soft soil hingga firm cohesive soil, lapisan kedua dengan kecepatan 1172 ? 1721 m/s memiliki tingkat kekerasan stiff cohesive soil hingga very soft rock. Lapisan ketiga dengan kecepatan 1721 - 1954 m/s memiliki tingkat kekerasan very soft rock - moderately soft rock dan lapisa keempat dengan kecepatan lebih dari 2764 m/s memiliki tingkat kekerasan hard rock. Sementara di wilayah BW27 didapatkan tiga lapisan batuan. Lapisan pertama dengan kecepatan 480 - 536 m/s memiliki tingkat kekerasan very soft soil hingga firm cohesive soil, lapisan kedua dengan kecepatan 647 - 924 m/s memiliki tingkat kekerasan stiff cohesive soil hingga very soft rock dan lapisan ketiga dengan kecepatan lebih dari 1258 m/s memiliki tingkat kekerasan very soft rock hingga moderately soft rock.

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The subsurface rock layer has many physical properties. The hardness of the earth's subsurface rock is one of the physical properties that can be calculated from measuring on the earth surface. Seismic refraction is one of the geophysical methods that can be used for measurement. Seismic refraction measurement had been done in two different places, BW 17 region and BW 27 region. From the measurement, it is known that the BW 17 region has four rock layers. The first layer with velocity between 405-734 m/s has very soft soil to firm cohesive soil hardness. The second layer with velocity between 1172-1721 m/s has stiff cohesive soil to soft rock hardness. The third layer with velocity 1721-1954 m/s has very soft rock to moderate soft rock hardness. The other one with velocity more than 2764 m/s has hard rock hardness. Meanwhile, from the other measurement, the BW 27 region only has three layers. The first layer with velocity between 480-536 m/s has very soft soil to firm cohesive soil hardness. The second one with the velocity between 647-924 m/s has stiff cohesive soil to very soft rock hardness. The other one with velocity more than 1258 m/s has very soft rock to moderate soft rock hardness.