Gravity Methods,Physics tutorial

Definition:  a nondestructive geophysical method is the gravity technique that calculated differences in the earth's gravitational field at important locations depending on the diverse earth materials having different mass densities (mass) that produce differences in the measured gravitational field. computers methods to determine the depth, and analytical that uses these variations geometry and density causing the gravity field variations.

Introduction to the Gravity Methods:

To provide a better understanding of the subsurface geology the goal of studying detailed gravity data is achieved the gravity method is a relatively non-invasive, inexpensive, non-destructive remote sensing technique. It is also passive - which is, energy not require be put into the ground in order to acquire data; thus, the method is complementary to a populated setting.  Walking traverses is permitted by small portable devices provide information about densities of rocks underground is given by the measurements of gravity. There is a broad range in density among rock types, and therefore geologists can make inferences as regarding the division of strata. At a measurement station on the surface. The gravity techniques takes calculating the gravitational attraction exerted by the earth the strength of the gravitational field is directly proportional to the mass and as a result the density of subsurface materials. It is suggestible check with other geophysics data such as magnetic and seismic for consistent manipulation of gravity anomaly.  The ambiguity in data interpretation of the subsurface geology is the general problem in geophysical surveying. It raised due to many different geologic configurations could imitate similar observed measurements. This basic constraint is brought about from the unavoidable fact that geophysical surveying attempts to solve complicated converse trouble.In geothermal energy investigations the gravity method has found extensive applications in as well as the monitoring of geothermal reservoirs under exploitation. Due to fairly cheap, fast in data collection with lowest amount logistics preparation. The process can deduce location of faults, porous areas for hydrothermal progress. It is still, more commonly used in detecting the location and geometry of heat sources.

Instrumentation:                                                                       

The variation in the earth's gravitational field is measured by a gravity meter. The difference in gravity is because of lateral changes in the density of the subsurface rocks in the vicinity of the measuring point. Due to the density variations are uniform and very small the gravimeters have to be very responsive so as to assess one part in 100 million of the earth's gravity field 980 gals or 980,000 mgals in standard units of mgals or microgals.

an absolute gravitational acceleration is not measured by most commonly used meters but differences in relative acceleration. There are various gravity meter providers where the accuracies of these meters can differ greatly. The common gravimeters on the market are the Worden gravimeter, the Scintrex and the La Coste Romberg gravimeter. While these meters are temperature controlled and contain small pen lights to study the meters, they are attached to rechargeable batteries. The meter usually has two batteries that give permission for 16 hours of readings. The Worden gravimeter is an totally mechanical and visual relying merely on an AA battery for enlightening the crosshairs. It uses a fixed length spring and mass attached to a calibration spring and veneer scale to calculate gravitational acceleration. semi-automated is the Scintrex Autograv but though a bit more luxurious, it has been exposed to have a upper stability and experience less tares a sudden jump in a gravity reading over more periods of time. The Lactose and Romberg form gravity meter has an advertised repeatability of 3 microgals 980,000,000 microgals is the Earth's gravitational field and is one of the preferred instruments for conducting gravity surveys in industry.

Data Acquisition:      

Gravity data possession is a relatively simple task that can be performed by one person. Hence, two people are generally essential to define the position latitude, longitude and elevation of the gravity stations. In order to determines the aim as example a dense body or fault gravity readings must be taken towards traverses which cross the address of the target. Its expected size will determine the distance between the range of readings, with larger station difference for large achievements and less separations for smaller ones. It is generally suggested to model the expected anomaly mathematically before conducting fieldwork. From this, along with the expected device accuracy, an estimate can be made of the anomaly size and the required station gapping.

Surveys are conducted by taking gravity readings at regular time intervals towards the traverse that crosses the expected location of the target. Though, in order to take into account the expected drift of the instrument, a local base station must be placed and has to be replacement every half to 1 hour or depending on the instrument drift characteristics to get the natural drift of the equipments. These repeated readings are performed due to even the most stable gravity meter will have their readings drift with time because of elastic creep within the meter's springs and also to help eliminate the gravitational effects of the earth tides readings have to be taken at a base station. The equipment drift is usually linear and less than 0.01 mGal/hour under regular performing circumstances. So gravity reduced as elevation increases, the elevation of each station has to be calculated with an error of greater than about 3 cm. Readings are taken by locating the device on the ground and leveling it. This may be usual with some device, as this is with the Scintrex. An important factor in calculating useful gravity values in detailed surveys is determining the earth tide effect as their gravitational effects may be greater than the gravity field differences because of the anomalous properties being required. The final phase of reading a gravity meter include seismic action or cultural movement as example those of vehicles or people. These will dislocate the readings the meter is actually a low-frequency seismometer and even though the Scintrex meter has an anti-seismic filter

The La Coste-Romberg meters are also mechanically damp to lessen the effects of earthquakes readings will still be interrupted if this happens, all operations should be dropped. Experience specifies which one should stay at least 1-2 hours later than a seismic event before resuming the survey. From the drift curve, a base reading corresponding to the time a particular gravity station was calculated by subtracting the base reading from the station reading. This gravity reading is not in mgals but in gravity meter units. One must multiply the gravity meter reading by the manufacturer supplied meter constants, calibration constants to obtain mgals.

Data Processing:                                                 

The duty of most fieldwork is to detect the topographic modifications and the effects of buildings around gravity station. Both of these effects will be used after in performing the gravity data. There are a number of methods to detect the elevation alterations and these usually include a mixing of recording elevation modifications in the field and computer computations using digital elevation models (DEM). The most common methods is by Hammer where one records an elevation amend in quadrants at set distances commonly from 0 to 1000 meters from the gravity station. A newly developed procedure uses a laser-placing gun to compute more accurate elevation changes within 100 meters of a gravity station. The best method is to use Hammer's method for close up to 200 meters station elevation modification and computer processes based on accurate minimum 10 meter 7.5 minute quadrangles DEM's.

The observed gravity readings calculated from the gravity inspection return the gravitational field due to all masses in the earth and the result of the earth's rotation. Numerous corrections have to be practiced to the field gravity readings. To analysis gravity data, one must eliminate all known gravitational results not related to the subsurface density changes. Each reading has to be corrected for elevation, the effects of tides, latitude and, if considerable local topography resides, a topographic correction. To understand the corrections, a gravity reading is first determined on the surface of a flat ground surface. Corrections are then practiced on account for deviations from this terms and condition. As the distance from the centre of the earth grows the pressure of gravity decreases. The correction to account for this is called the elevation correction. If the gravity reading is get on top of a hill, then there is a scarcity of mass on either side of the hill, compared to a horizontal ground surface. This is corrected with a topographic correction. It is vital to comprehend that mass higher than the reading site will also persuade the data and has to be accounted for. This may arise in areas of major topography or when surveys are conducted near large buildings. In a modern tool such as the Scintrex, the meter may automatically apply the tidal and drift corrections. The dissimilarity between observed gravity (gobs) and theoretical gravity (gth) at any point on the Earth's surface after decreasing the gravity readings to the geoidal surface that is making all the required corrections is considered as the Bouguer gravity anomaly or Bouguer gravity and the outcome are now due to lateral differences in density in the subsurface assumed to be caused by geologic structure being sought which can be used for manipulation.

Data Analysis and Interpretation:

The objective of the gravity method is to conclude information about the earth's subsurface. One can just bear out a qualitative examination of the grid of gravity values, contour maps or the gravity profiles to decide the lateral position of any gravity variations or one can achieve a more detailed analysis in order to enumerate the nature depth, geometry, density of the subsurface property that cause the gravity differences To find out the later, it is generally essential to take apart the irregularity of interest residual from the remaining background anomaly regional. Then the residual gravity anomaly is modeled to conclude the depth, density and geometry of the anomaly's resource. Most other regional-residual anomaly partition methods engage mathematical operations using a computer. One trouble with the mathematical techniques is that they do not exactly present the "true" residual gravity anomaly due to a detailed body. Therefore, they should not be used for quantitative elucidation of the subsurface but only for qualitative interpretation. The most familiar mathematical techniques are surface fitting and biased averaging. Once the left behind has been detached from the Bouguer gravity data modeling can be done over the characteristic of interest.

Micro-Gravity Monitoring:

Reservoir manufacturing calculations of mass and energy balance on producing geothermal reservoirs entails information about in- and out-flows from the reservoir. Such information is generally presented for surface flows, such as production, injection, and natural expulsion. Values for subsurface in- or out-flows are complicated to acquire. One method used is a history similarities process whereas reservoir presentation is computed for a variety of strengths of invasion and the matched against experienced concert. A more direct and independent scheme is by repeat micro-gravity over a producing field. As mass is eliminated from a geothermal reservoir the gravity field above the reservoir will adjust. For an influx it will increase while for a loss it will decrease. By measuring the surface gravity field at two points in time the change in gravity over the reservoir through the time interval can be resolved when such surveys are conceded out with suitable correctness, they agree to an approximation of mass loss or influx to be made not including any drill-hole information. Precision gravity surveys at Olkaria Geothermal field began in 1983 to examine gravity modified as a result of geothermal fluid withdrawal. A review of the experimented gravity data over each benchmark shows changes over the years all through monitoring. Maximum gravity changes demonstrate a constant trend in time, but diverse characteristic distributions from zone to zone. This information has been interconnected with production data (enthalpy and mass output) from nearby wells as well as supporting in identifying zones for re-injection.

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