How to get the data to produce gravity inversion maps
There are several ways of getting data and deriving gravity inversion maps. The methods are presented here. 4. 1 Gravity An appropriate method for producing gravity inversion maps is by modeling geological zones having anomalous density. It basically involves determining the top of the anomalous zone from non-potential fields data. For this case, for example, the westward continuation into the Gulf of the axial trough and linear magnetic anomalies of the Sheba Ridge is often used. A potential fields data is then used to derive the lower boundary of the geologic anomalous zones.
A lower boundary to an anomalous zone is formulated by predicting parameters representing the lower boundary within predetermined limits. Gravity and bathymetry profiles across the Carlsberg Ridge in the Indian Ocean are analyzed to investigate the isostatic compensation of this part of the Indian Ocean spreading ridge system. This is done by using a gravity inversion process on the potential fields data, such as measurements of gravity data and/or magnetic data. These may be in both vector and tensor form.
The potential fields data is compared to the predicted fields from the results of the inversion process to obtain a difference between the two. If the difference exceeds a predetermined value, the parameters representing the anomalous zone are adjusted to improve the fit. When the lower boundary limits are reached or the difference between the model and the data is less than the predetermined value or convergence is attained, the anomalous zone has been determined. 4. 2 Bathymetry The Gulf of Aden comprise two bathymetric channels that originate seaward of Perim Narrows and direct the dense RSW from the strait to the open ocean.
One can use 3-D seismic array to map crustal stretching across and along the margins. User receiver functions can be used to retrieve crustal thickness and crustal composition along the margins. Another option is utilizing the ocean wave-shoaling photographic imagery. The instrument used for the acquisition of sea-surface image sequences is a ground-based nautical X-band radar with horizontal polarization. The device utilized during the experiments is a software–hardware combination consisting of a commercial, navigational Furuno X-band radar antenna and radar device. In the case of datum determination, a tidal-gauge-measurement is used.
Figure 2: Bathymetry map Other cross-spectral techniques are employed to obtain an admittance function for isostatic studies. Synthetic topography and gravity profiles are computed for a cooling plate model are subtracted from the observed, to estimate the median valley signatures. The residual gravity anomaly over the ridge axis is mostly explained by the median valley topography with a uniform crustal thickness. Spatial profiles are developed using contours, seismic data or satellite imagery to create 3D maps. Then the profiles are created with a known parameter, a prerequisite for checking depth and depth inversions.
Depth inversion occurs when an observation has a shallower depth than the observation directly preceding it. The sonar instrument uses a transducer that is usually mounted on the bottom of a ship. The sound pulses are sent from the transducer straight down into the water where the sound reflects off the seafloor and returns to the transducer. It is claimed that acoustic penetration into the sea bed increases with decreasing frequency. The distance to the seafloor is calculated based on the time the sound takes to travel to the bottom and back to the surface.
Water depth is estimated by using the speed of sound through the water. The sound pulses are sent out regularly as the ship of opportunity moves along the surface, which produces a line showing the depth of the ocean beneath the ship. This continuous depth data is used to create bathymetry maps of the survey area. 4. 3 Age The age is mapped by shipborne magnetometers which allow delineation of zones of normal and reversed magnetic polarity. The magnetic zones form distinct stripes on the map as the oceanic plates grow. Heat flow measurements are another method of age determination and inversion map production.
The average value of heat flow measurements calculated from measurements and other collected data indicates that the age of basins and margins. Figure 3: A map showing age The thermal gravity anomaly can be conditioned using ocean isochrons from plate reconstruction models to provide the age and location of oceanic lithosphere (Chappell and Kusznir 2008). There is a relationship of water depth with age. The agreement in age from both heat flow and water depth data favors aids in age determination. The map production using all these characteristics helps in determining geologic events with respect to their dates.
4. 4 Crust thickness According to Lucazeau et al (2008), the high resolution 3-D forward modeling approaches reveal a possible crustal thickness and density distribution beneath. The use of satellite remotely sensed imagery give precise information about the sediments and sea bed. The seabed, basement and mantle boundaries are defined by a series of triangular facets, whose size varies as the amount of constraining data changes. The sediment and basement boundaries and the base of the crust are defined by larger facets than those defining the bathymetry. Figure 4: Crust thickness map
Another method is the use of a mechanical bed level detection in combination with DGPS. The bed level soundings are often performed by use of a vehicle moving through the surf zone. Alternatively the water depth can be measure by using the single-beam echo sounder. The sonar instrument is used to measure the crust thickness. The Instrument measures the vertical distribution of the turbidity levels in the water column, transition from water column to bed based on the scattering of light from the suspended particles and the bed material particles and transition from water column to air.
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