Ultrasonic P- and S-Wave Attenuation and Petrophysical Properties of Deccan Flood Basalts, India, as Revealed by Borehole Studies

Abstract

Petrophysical properties and ultrasonic P- and S-wave attenuation measurements on 35 Deccan basalt core specimens, recovered from Killari borehole site in western India, provide unique reference data-sets for a lesser studied Deccan Volcanic Province. These samples represent 338-m-thick basaltic column, consisting four lava flows each of Ambenali and Poladpur Formations, belonging to Wai Subgroup of the Deccan volcanic sequence. These basalt samples are found to be iron-rich (average FeOT: 13.4 wt%), but relatively poor in silica content (average SiO2: 47.8 wt%). The saturated massive basalt cores are characterized by a mean density of 2.91 g/cm3 (range 2.80–3.01 g/cm3) and mean P- and S-wave velocities of 5.89 km/s (range 5.01–6.50 km/s) and 3.43 km/s (range 2.84–3.69 km/s), respectively. In comparison, saturated vesicular basalt cores show a wide range in density (2.40–2.79 g/cm3) as well as P-wave (3.28–4.78 km/s) and S-wave (1.70–2.95 km/s) velocities. Based on the present study, the Deccan volcanic sequence can be assigned a weighted mean density of 2.74 g/cm3 and a low Vp and Vs of 5.00 and 3.00 km/s, respectively. Such low velocities in Deccan basalts can be attributed mainly to the presence of fine-grained glassy material, high iron contents, and hydrothermally altered secondary mineral products, besides higher porosity in vesicular samples. The measured Q values in saturated massive basalt cores vary enormously (Qp: 33–1960 and Qs: 35–506), while saturated vesicular basalt samples exhibit somewhat lesser variation in Qp (6–46) as well as Qs (5–49). In general, high-porosity rocks exhibit high attenuation, but we observed the high value of attenuation in some of the massive basalt core samples also. In such cases, energy loss is mainly due to the presence of fine-grained glassy material as well as secondary alteration products like chlorophaeite, that could contribute to intrinsic attenuation. Dominance of weekly bound secondary minerals might also be responsible for the generation of microcracks, which may generate squirt flow in saturated samples. Hence, we argue that the Deccan basalts attenuate seismic energy significantly, where its composition plays a major role.