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Methods: Downhole Seismic

    The downhole seismic method is commonly used for site investigations where the following information is needed:
    WHAT IT MEASURES

    Downhole seismic is a surface method that uses the properties of acoustic and shear waves to generate a detailed vertical profile of the variation of seismic velocity (SH) with depth. These vertical profiles are used to calculate the elastic constants for foundation studies and to measure Vs30 for earthquake design ground motion (in accordance with CBC 2007 and IBC 2006).


    Both downhole and crosshole techniques can be used to obtain highly detailed vertical profiles of shear wave velocity; however, downhole has several advantages to crosshole:
    • Downhole requires just one borehole; whereas crosshole requires either two or three boreholes to be drilled to specifications
    • Downhole does not require a borehole deviation survey; whereas crosshole requires that each borehole be surveyed in order to know the exact distance between the boreholes at each measurement point
    • Downhole does not require a borehole hammer; whereas crosshole does
    • The travel path in downhole is vertical, which eliminates problems with refracting layers, which can diminish resolution of thin layers

    Spectrum utilizes the Seistronix RAS-24 twenty-four channel signal enhancement seismograph along with a three-component downhole seismometer to collect downhole seismic data.
    HOW IT WORKS

    Like seismic refraction, the downhole seismic method is based on seismic theory, which tells us that a seismic wave travels at the velocity of the material that it is currently traveling through, and when a wave reaches a boundary between two materials having different seismic velocities, that seismic wave will be refracted (or bent) either toward the normal to the interface or away from the normal to the interface (depending on whether the velocity increases or decreases at the boundary). Once the wave enters the new material, it will travel at the velocity of the new material. However, unlike seismic refraction, downhole seismic is also based on the assumption that the first arrival on the seismograph from a geophone at a given depth is from the direct wave, since the waves travel nearly vertically. Because the downhole seismic method makes use of the direct wave it does not have difficulty resolving hidden layers and does not require that layer velocities increase with depth. As a result, this method is highly reliable for identification of all seismic layers of interest, and is particularly useful for resolving thin layers or velocity reversals that the seismic refraction method will not detect.


    In a typical downhole seismic survey, a three-component downhole seismometer is lowered down a borehole, secured to the borehole wall and connected to a seismograph. A seismic “shot” is then made at the ground surface close to the borehole, which causes the direct wave to travel in a near-vertical path to the geophone in the borehole. Typically two different types of shots are made for a given depth – one shear wave shot and one p-wave shot. In order to create a shear wave shot a source is needed that generates shear waves. This shear wave source usually consists of a long plank weighted down by a vehicle or a shear wave “brick” secured to the ground. The plank or brick is generally established close to the borehole such that its length is equally distributed on either side of the borehole, and is oriented in a predetermined direction. Once the shear wave source has been established, hammer blows to the side of it are made – first on one side and then on the other side. This procedure is necessary because the shear wave is polarized: a blow to one side of the source will cause an excursion to the right on the seismograph and a blow to the other side of the source will cause an excursion to the left. Comparison of seismic records resulting from blows to either side of the plank or brick ensures that the shear wave is properly identified. In order to create a p-wave shot vertical hammer blows to an aluminum plate are made.


    Once a particular shot is made, the seismograph then records the time at which the first arrival from the seismic shot is received at the downhole seismometer. With a multi-channel seismograph shear wave and p-wave arrivals for a given depth can be routed to separate channels via a control box. The geophone is then lowered a small distance down into the borehole, secured, another shot is made and the process is repeated until the desired depth is reached. Once all the data have been collected for a given borehole, a time-depth plot of seismic first arrivals is made, which can be used to calculate the velocity of the materials through which the wave has traveled.


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