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Geophysics [ Question No: 1 To 16 ] :
The three main ways of measuring the electric resistivity of formation penetrated by borehole are ..................
When sonic logs are cross-multiplied with the density it produce the acoustic impedance log which help in generation of ......................... logs.
In gravity surveys the readings are function of :
In Bouguer gravity anomaly map, which of the following statements are/is correct. Select appropriate answer :
Which of the following is true statement in relation to proton precession magnetometer ?
Magnetic survey data needs to be connected for which of the following effects ?
The current geomagnetic polarity is named as :
The ionospheric magnetic field is because of :
Sampling the same place with seismic waves arriving from different directions and producing the anomalies is the basic principle of :
The difference in arrival times of P and S wave increases with distance while the ratio of P and S wave velocities :
Richter scale gives an idea of the amount of energy released during an earthquake and is based on the...............of seismic waves at a distance of.................km from the epicenter :
The surface waves for which the motion occurs in the direction of propogation and perpendicular in phased pattern are called :
The average rock density of continental crust and the oceanic crust respectively are :
The core mantle boundary occur at the depths of :
In seismology, the moment magnitudes are calculated by :
Kepler's Second Law of planetary motion states that orbital radius of a planet sweeps out :
Total number of questions : 16
Number of answered questions : 0
Number of unanswered questions : 16
Geophysics [ Question No: 1 To 16 ] :
The three main ways of measuring the electric resistivity of formation penetrated by borehole are ..................
Your Answer : (Not Answered)
Correct Answer : D
The three resistivity methods are the normal, lateral and induction logs.
The normal and lateral logs are conventional electrode tools that send current into the formation and measure potential at set spacings. The induction log uses coils to induce currents in the formation, and works even in boreholes filled with oil-based mud. All three measure formation resistivity, from which water and hydrocarbon saturation are estimated.
S.P., spontaneous potential and self potential are three names for the same log, which records natural voltages, not resistivity. Sonic and acoustic logs measure sound velocity, and the gamma ray logs measure natural radioactivity.
When sonic logs are cross-multiplied with the density it produce the acoustic impedance log which help in generation of ......................... logs.
Your Answer : (Not Answered)
Correct Answer : C
The acoustic impedance log is used to generate synthetic seismic logs (synthetic seismograms).
Acoustic impedance is density multiplied by velocity, and velocity comes from the sonic log. Changes in impedance between layers give reflection coefficients, and convolving these with a seismic wavelet produces a synthetic seismogram. This is compared with seismic sections to tie well depths to reflections.
Electrical, neutron and density logs are measured directly by their own tools; they are not built from the impedance log.
In gravity surveys the readings are function of :
Your Answer : (Not Answered)
Correct Answer : C
Gravity readings are a function of the density and mass of the rocks.
A gravimeter measures the vertical attraction of the earth at a point. By Newton's law of gravitation, this depends on the masses beneath the station and their distances, and so variations in reading reflect lateral differences in rock density, the density contrast between bodies and their surroundings.
Rigidity and bulk modulus are elastic properties that control seismic velocities, not gravity. Hardness has no effect on gravity, and specific gravity alone, paired with hardness, misses the dependence on the mass and geometry of the bodies.
In Bouguer gravity anomaly map, which of the following statements are/is correct. Select appropriate answer :
Your Answer : (Not Answered)
Correct Answer : D
The answer is All of the above are correct.
A Bouguer anomaly map shows the effect of density variations beneath the surface. Broad, long-wavelength anomalies come from deep density contrasts, such as crustal thickness changes or major basins, and reveal regional structure. Short-wavelength anomalies come from shallow sources, such as ore bodies, salt domes or buried channels, and are the targets of exploration.
The residual anomaly, left after the regional trend is removed, has no unique interpretation: many different bodies can produce the same anomaly. Geological knowledge of the area is needed to constrain it. Since each of the three statements is true, choosing any single one is incomplete.
Which of the following is true statement in relation to proton precession magnetometer ?
Your Answer : (Not Answered)
Correct Answer : C
The true statement is that it uses a proton-rich liquid such as water surrounded by a magnetising coil as detector.
In a proton precession magnetometer, a current in the coil around a bottle of water or kerosene aligns the spins of the hydrogen nuclei (protons). When the current is switched off, the protons precess around the earth's field at a frequency proportional to its strength, about 2 kHz, and the coil picks up this signal. The frequency gives the total field very accurately.
The protons are ordinary hydrogen nuclei, not radiation from uranium. The instrument measures the total field strength, so it is a scalar, not a vector, magnetometer, and it uses no permanent 'proton magnets'.
Magnetic survey data needs to be connected for which of the following effects ?
Your Answer : (Not Answered)
Correct Answer : C
Magnetic survey data must be corrected for the diurnal variation caused by ionospheric changes.
Electric currents in the ionosphere, driven by solar heating, make the earth's field at the surface change by tens of nanotesla over a day, and more during magnetic storms. A base-station magnetometer records this variation through the survey, and it is subtracted from the field readings so that the remaining anomalies come from the rocks.
Polar wandering and the apparent polar wander path concern palaeomagnetism over millions of years, not a survey made over days. Normalising a 'sample depth' is not a standard magnetic survey correction; the usual others are the removal of the regional (IGRF) field and, in aeromagnetics, levelling.
The current geomagnetic polarity is named as :
Your Answer : (Not Answered)
Correct Answer : D
The present polarity interval is the Brunhes Normal chron.
The earth's magnetic field has reversed many times. The current interval of normal polarity, the Brunhes chron, began about 780,000 years ago at the Brunhes-Matuyama reversal. It is named after Bernard Brunhes, who first found reversely magnetised rocks.
Before it came the Matuyama reversed chron, which was mostly reversed, not normal, and before that the Gauss normal and the Gilbert reversed chrons, both older than about 2.6 million years. None of these is the current polarity.
The ionospheric magnetic field is because of :
Your Answer : (Not Answered)
Correct Answer : C
The key gives Solar winds as the cause.
The ionosphere's own magnetic field comes from electric currents flowing in the ionised upper atmosphere. That ionisation and the currents are produced by the Sun: its ultraviolet and X-ray radiation ionises the gas, and the solar wind interacting with the magnetosphere drives further currents, especially at high latitudes. These are the external, Sun-driven part of the geomagnetic field.
Hydrogen fusion takes place in the Sun's core, not in the ionosphere, and 'helium bombardment on sodium' is not a mechanism for it. Fluctuations of the geomagnetic field are an effect of the ionospheric currents, not their cause.
Sampling the same place with seismic waves arriving from different directions and producing the anomalies is the basic principle of :
Your Answer : (Not Answered)
Correct Answer : B
The principle describes Seismic Tomography.
Tomography uses many seismic rays crossing the same region from different directions. Each ray's travel time is compared with that predicted by a reference model, and combining the anomalies along all the crossing paths locates where waves travel faster or slower, building a three-dimensional image of the interior, much as a medical CT scan does with X-rays.
Seismic inversion is the general mathematical step of deriving a model from data, not this crossing-ray method. Reflection and refraction surveys use waves returned from or bent along layer boundaries, usually from one shot direction at a time.
The difference in arrival times of P and S wave increases with distance while the ratio of P and S wave velocities :
Your Answer : (Not Answered)
Correct Answer : A
The ratio of P and S wave velocities remains same.
Both waves travel through the same rock, and the ratio of their speeds depends on its elastic properties, through Poisson's ratio. For most crustal and mantle rocks Vp/Vs is close to \(\sqrt{3}\), about 1.73. Because the S-wave is always slower by a nearly fixed proportion, the lag between P and S arrivals grows steadily with distance, which is how the S-P time is used to find the distance to an earthquake.
The ratio does not decrease logarithmically or linearly, nor increase periodically, with distance.
Richter scale gives an idea of the amount of energy released during an earthquake and is based on the...............of seismic waves at a distance of.................km from the epicenter :
Your Answer : (Not Answered)
Correct Answer : A
The Richter scale is based on the amplitude of seismic waves at a distance of 100 km.
Charles Richter defined local magnitude in 1935 as the logarithm of the maximum trace amplitude, in micrometres, recorded on a standard Wood-Anderson seismograph 100 km from the epicentre. Records at other distances are corrected to that reference, and each whole number step means a tenfold increase in amplitude.
Ground motion, velocity and the epicentre are not the measured quantity, and 200, 300 and 400 km are not the reference distance.
The surface waves for which the motion occurs in the direction of propogation and perpendicular in phased pattern are called :
Your Answer : (Not Answered)
Correct Answer : B
These are Rayleigh waves.
In a Rayleigh wave, a surface wave, each particle moves in a vertical ellipse in the plane containing the direction of travel: it has a component along the direction of propagation and a component perpendicular to it, out of phase, so the ground rolls like an ocean wave.
Love waves (L-waves) are surface waves too, but their particle motion is horizontal and purely perpendicular to the direction of travel. P-waves and S-waves are body waves, with motion parallel and perpendicular to propagation respectively, not surface waves.
The average rock density of continental crust and the oceanic crust respectively are :
Your Answer : (Not Answered)
Correct Answer : C
The average densities are 2·7 gm/cm\(^{3}\) and 3 gm/cm\(^{3}\).
Continental crust is broadly granitic, rich in quartz and feldspar, with an average density of about 2.7 g/cm\(^{3}\). Oceanic crust is basaltic and gabbroic, richer in iron and magnesium, at about 3.0 g/cm\(^{3}\). This contrast is why continents stand high and oceanic crust sinks back into the mantle at subduction zones.
Option A reverses the two and inflates the first. 8 and 12 gm/cm\(^{3}\) are values for the core, not the crust. Option D gives gm/m\(^{3}\), a unit a million times too small.
The core mantle boundary occur at the depths of :
Your Answer : (Not Answered)
Correct Answer : C
The core-mantle boundary lies at a depth of about 2900 km.
The boundary, also called the Gutenberg discontinuity, is at about 2,890 km. There P-wave velocity drops sharply and S-waves disappear, because the outer core below it is liquid. This is what produces the S-wave shadow zone beyond about 103° from an earthquake.
About 5,150 km is the depth of the inner core boundary, so 5500 km falls within the inner core region. 3900 km lies inside the outer core, and 1200 km is within the lower mantle.
In seismology, the moment magnitudes are calculated by :
Your Answer : (Not Answered)
Correct Answer : C
Seismic moment is Slip × Area × Elastic Modulus.
The seismic moment of an earthquake is the product of the rigidity (shear modulus) of the rock, the area of the fault that ruptured and the average slip on it:
\[ M_{0} = \mu A D \]
The moment magnitude is then calculated from the logarithm of \(M_{0}\). Slip times modulus leaves out the rupture area. The other two options use the P-S arrival time or amplitude, which belong to locating an earthquake or to the older Richter magnitude, not to the moment magnitude.
Kepler's Second Law of planetary motion states that orbital radius of a planet sweeps out :
Your Answer : (Not Answered)
Correct Answer : C
Kepler's Second Law says the line joining a planet to the Sun sweeps out equal areas in equal intervals of time.
Because the orbit is an ellipse, the planet moves faster near perihelion, where the radius is short, and slower near aphelion, where it is long, so that the area swept per unit time stays constant. The law is a consequence of conservation of angular momentum.
'Equal area in equal orbits' and 'larger area in smaller orbits' talk about different orbits, but the law concerns one planet in its own orbit. 'Smaller area in larger time interval' contradicts it outright.
Marks
Total number of questions : 16
Number of answered questions : 0
Number of unanswered questions : 16