EAE2522 - Topic 3 - 1 Flashcards
<p><strong><span>What is the Seismic method?</span></strong></p>
<p>3 points.</p>
<p><span>Essentially the seismic method utilizes DENSITY contrasts in the sub-surface</span></p>
<ul> <li>Solid: <strong>Rock</strong>, compressible and rigid, density varies</li> <li>Liquid: <strong>Water/Oil</strong>, incompressible and not rigid, density varies</li> <li>Gas: <strong>Methane</strong>, highly compressible and not rigid, density varies</li></ul>
<div></div>
<div>Different Rocks, with different liquid/gas content will have different velocities.</div>
<p><span>EAE2522 4aa</span></p>
<p><strong><span>What is Acoustic impedance, IA?</span></strong></p>
<p><span>The product of density (RHOB) and seismic velocity (1/DT)</span></p>
<div>When a seismic pulse encounters a boundary between layers of different acoustic impedance, some of the energy is <strong>reflected </strong>while the remainder is transmitted. </div>
<div>The proportion of energy reflected is known as the <strong>reflection coefficient </strong></div>
<div><strong>(R)</strong>, and for a signal travelling from Bed 1 to Bed</div>
<p><span>EAE2522 4ab</span></p>
<p><span>Seismic phase response of:</span></p>
<p><strong><span>Clays and Silts</span></strong></p>
<p>4 points.</p>
<ul> <li>Closely spaced reflectors</li> <li>Moderate continuity</li> <li>Poor amplitudes</li> <li>Divergent for fine grained sediments</li></ul>
<p><span>EAE2522 4ac</span></p>
<p><span>Seismic phase response of:</span></p>
<p><strong><span>Clastics</span></strong></p>
<p>3 points.</p>
<ul> <li>Mounded - deep water</li> <li>Thin closely space - shallow water</li> <li>Olique progradational for sand rich clastics</li></ul>
<div></div>
<div>Similar: Volcanic rocks</div>
<p><span>EAE2522 4ad</span></p>
<p><span>Seismic phase response of:</span></p>
<p><strong><span>Carbonates</span></strong></p>
<p>4 points.</p>
<ul> <li>Large strong positive reflection coefficient</li> <li>High velocities</li> <li>High Amplitudes</li> <li>Good continuity</li></ul>
<div></div>
<div>Confused with: Tuffs and iginmrites </div>
<p><span>EAE2522 4ae</span></p>
<p><span>Seismic phase response of:</span></p>
<p><strong><span>Salt</span></strong></p>
<p>2 points.</p>
<ul><li>Low density </li><li>Diapiric appearance</li></ul>
<p><span>EAE2522 4af</span></p>
<p><span>Seismic phase response of:</span></p>
<p><strong><span>Basement Igneous or Metamorphic</span></strong></p>
<p>4 points.</p>
<ul><li>Low porosity </li><li>No unique reflection character </li><li>Strong top basement reflector - positive coefficent </li><li>Weak discontinous reflections</li></ul>
<p><span>EAE2522 4ag</span></p>
<p><span>Seismic phase response of:</span></p>
<p><strong><span>Igneous rocks</span></strong></p>
<div>High velocity</div>
<div>Positive coefficient </div>
<div>High Amplitudes</div>
<p><span>EAE2522 4ah</span></p>
<p><span>Seismic phase response of:</span></p>
<p><strong><span>Large plutons</span></strong></p>
<ul> <li>Dead intervals</li></ul>
<div></div>
<div>Similar: Large volcanic bodies</div>
<p><span>EAE2522 4ai</span></p>
<p><span>Interpretation</span></p>
<p><strong><span>Reflectors</span></strong></p>
<p>4 points.</p>
<div>Not every geological boundary generates a reflector </div>
<div>Not every Significant reflection is a significant Geological boundary</div>
<div>Properties of reflector may vary due to:</div>
<ul> <li>Density variations </li> <li>Fluids </li> <li>Type of fluids (gas, oil, water, coffee?) </li> <li>Pressure (lithostatic/overpressure) </li></ul>
<div></div>
Geological features don’t always appear as they are in seismic data<p style="text-align:right;"><span>EAE2522 4aj</span></p>
<p><span>Describe</span></p>
<p><strong><span>Gravity - mapping basins</span></strong></p>
<p>2 points.</p>
<ul> <li>Sedimentary basins often produce a negative or low gravity response.</li> <li>Basin and range region of the continental US can be determined from the negative gravity anomalies</li></ul>
<p><span>EAE2522 4ak</span></p>
<p><span>Gravity modelling</span></p>
<p><strong><span>Relic subduction zones</span></strong></p>
<p>2 points.</p>
<ul><li>Sedimentary basins and felsic batholiths often produce a negative or low gravity response. </li><li>Large mafic bodies produce gravity highs or positive anomalies (warm colours)</li></ul>
<p><span>EAE2522 4al</span></p>
<p><strong><span>Typical gravity solution makes what basic (yet unrealistic) assumptions?</span></strong></p>
<p>2 points.</p>
<ul> <li>The Earth is perfectly spherical </li> <li>Measurements are on or above the surface are equivalent to having all the mass concentrated in at a point at the centre of the Earth this is why we can use the Earths radius to make the calculation.</li></ul>
<p><span>EAE2522 4am</span></p>
<p><strong><span>What is the gravity (allowing for 'bulge')?</span></strong></p>
<p>3 points.</p>
<ul> <li>The mean Earth gravity is about 981,000 mGal (the well known 9.81 m/s²)</li> <li>978,100 mGal at the equator</li> <li>983,200 mGal at the pole</li></ul>
<p><span>EAE2522 4an</span></p>
<p><strong><span>How is Gravitational Acceleration related to geology?</span></strong></p>
<p>4 points.</p>
<ul> <li>Density can be conceptualised as a value that quantifies point masses needed to represent the material per unit volume.</li> <li>To represent a high density ore body, more point masses per unit volume are needed then for the lower density soil.</li> <li>Gravitational acceleration is determined by measuring the time rate of change of the speed of the ball as it falls.</li> <li>The acceleration the ball undergoes is proportional to the number of close point masses that are below it.</li></ul>
<div>I.e. The closer these point masses are, below the ball, the larger its acceleration.</div>
<p><span>EAE2522 4ao</span></p>
