Lab 3 Exploration Seismology

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School

Queens University *

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151

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Geography

Date

Apr 3, 2024

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docx

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Uploaded by PrivateKoupreyMaster3671

Husein 1 Jenna Husein Seismic Exploration 251213113 Attended lab section 004 Performed on February 10 th 2022 Lab 3: Exploration Seismology Lab Demonstrator: Meryem Berrada

Husein 2 Purpose a) The purpose of this laboratory is to demonstrate the hammer seismic exploration technique, a method used to determine underlying soil and rock layers. Small-scale surveys utilizing hammer seismic systems are often used to locate the water table or the top of the underlying bedrock. b) Geological structure can be determined via seismic refraction, a geophysical exploration method in which vibrational energy propagates through the ground as elastic waves (like sound in the air). Only layers of rock with contrasting seismic velocities can be analyzed using this method. Depending on the density and mechanical strength of a material, seismic velocity varies. Seismic exploration is most commonly applied in sedimentary rocks, where layers of rock are nearly horizontal and seismic velocities differ. Oil and gas companies mainly use this technique to identify or outline subsurface geological formations that are likely to accumulate oil and natural gas (these formations are called traps). Additionally, seismic data can also be used to map subsurface minerals within more complex deposits (non-sedimentary rocks), but interpretation can be challenging. Theory and Procedure a) Based on Refraction Theory, a region's geological structure consists of two horizontal layers that are homogenous and isotropic. The elastic waves under these conditions propagate at definite velocities, V1 at layer 1 and V2 at layer 2. Geophones are positioned along the ground surface after an explosion charge has been detonated at the shot point. Only direct waves and refracted waves need to be measured. Direct waves travel nearly horizontally from the point of explosion to the geophones. These waves travel exclusively in the top layer and travel at a velocity, V1. x represents the distance between the explosion and the geophone, then T represents the time T it takes for the direct wave to reach that geophone. A straight line that passes through the origin and has a slope of (1/ V1) is plotted for the various geophones as a function of distance x. In the second layer, vibrational energy will hit the interface at the critical angle and will be refracted to travel along the top at velocity V2. b) A metal plate was placed on the ground, then geophones were placed at known distances from the metal plate, and the geophones were connected to a recording device. After that, the triggering device was installed (either a geophone near the metal plate, or a sensor attached to the hammer, depending on the system). Then, someone ran past the geophone while the others watched the computer screen. For safety's sake, the person using the hammer put on safety glasses and ear protection (everyone else must keep a safe distance from it). Lastly, we hit the metal plate with the hammer and checked the results on the screen.

Husein 3 c) This lab uses a sledgehammer, a metal plate, several geophones, and a recording device to conduct a refraction survey. Observation and Results a) slope = 1/v1 11.95623 this corresponds to soil and sand b)

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