Info on geophysics

Airborne geophysics is a tool that has been used world-wide for geological mapping, mineral prospecting and groundwater exploration. The methods were developed mainly for mineral exploration, but recently system modifications and developments have also been directed toward applications in groundwater exploration and environmental investigations.

The airborne methods are cost-effective compared to ground geophysical methods because large areas of the land surface can be covered in a relatively short time. The most commonly used methods are airborne magnetics, electromagnetics, and gamma-ray spectrometry (radiometry). In general measurements are carried out from an aircraft equipped with geophysical instruments. The equipment can either be mounted in the aircraft or towed behind and below the aircraft. The collected data are processed by application of different techniques and the results are then plotted as images. These images carry information about the physical properties of the subsurface geology of the areas under investigation.

During the years 1997/98 and 1999/2000 the Government of Ghana supported by the World Bank carried out two airborne geophysical surveys in Ghana. The aim of these surveys was to provide basic data to promote investment in the mining sector and to support companies working in the water industry.

The figure below is showing the coverage of airborne geophysical surveys in Ghana.

Areas of the geophysical surveys carried out by High Sense (HS) in 1999-2000 and by the Geological Survey of Finland (GTK) in 1997 together with the area surveyed 1994-1996 by Aerodat (Ad), superimposed on The Geological Map of Ghana. The Fugro survey is being carried out in 2006 / 7

Airborne magnetic, radiometric and electromagnetic data are available at the Geophysics section of Geological Survey Department of Ghana.

The data show regional and local geologic anomalies that warrant further ground follow-up exploration targets - not only for the gold, bauxite, manganese etc but also for kimberlite pipes (hard rock diamond deposits). Only alluvial diamonds are mined at present in Ghana.

The types of data collected at GTK1, GTK2, GTK3 and GTK4 are Airborne magnetics, gamma-ray spectrometry and vertical coplanar dual frequency electromagnetics. The data collected in areas covered by HS1, HS2, HS3 and HS4 are airborne magnetics and gamma-ray spectrometry. Examples of data images are shown in the figures below.

The following geophysical anomalies and their possible use have been revealed in the data:

Magnetics:

Presence of several “bulls-eye” anomalies that indicate kimberlitic pipe (host rocks for diamonds ) signatures.

Linear magnetic features, their trends and patterns indicating dykes, faults, fractures, folds and shearing have also been mapped. These dykes, fractures, folds and shear zones are good targets for ground water, gold and other minerals.

Image of the magnetic field variations recorded in the southern part of the survey block denoted GTK1. The size of the surveyed area is approximately 5000 km2. Purple and red colours mark areas of low magnetization whereas blue indicate high values. Intermediate values are shown in yellow and green.

Electromagnetics:

In the electromagnetic frequency domain data both the low and high frequency components show medium to high conducting horizons that also might be indicative of gold and other metallic minerals.

Frequency domain electromagnetic data. Areas in red colour are highly conductive structures.

Radiometric (Potassium, Uranium and Thorium) anomalies:

High potassium as well as ratios of potassium-thorium, uranium-thorium values used in conjunction with magnetic data can indicate gold mineralization.

Radiometric data equally reveal the regional geology.

Example of gamma-ray spectrometry data from the area marked HS1. The data are presented in a Ternary colour composite scheme with red, green and blue colours representing the concentrations of potassium, thorium and uranium respectively.

Earthquakes in Ghana - Info on seismology

The West African Craton is a relatively stable region and therefore earthquakes are rare. Ghana lies in the south eastern margin of this craton. However, the southeastern part of Ghana, especially the Accra district and the coastal area to the east and west are subject to earthquakes. Major earthquakes in Ghana occurred in the years 1862, 1906 and 1939. Other significant ones occurred in 1858, 1863, 1907, 1911, 1918 - 1919, 1923, 1930, 1933 – 1935, 19 64, 1969, and recently 1997, 2003 and 2006.

Tectonics / active faults in Ghana

Most earthquakes all over the world are known to occur along major plate boundaries. Ghana lies in the West African Craton which is quite far from the major plate boundaries. The closest plate boundaries to Ghana is the spreading at the mid Atlantic ridge and the East African rift. Although Ghana is far from these plates a significant number of earthquakes have struck and continue to strike the southern part of Ghana.

Structural History of tectonics in Ghana

Interpretation of an offshore seismic reflection survey across the continental shelf south of Accra has revealed the presence of a sedimentary basin at least 5500m deep which is fault bounded on its northern flanks close to the coast and is synclinal in form.

The sediment troughs are defined by faults separating individual crustal blocks which continue to be active throughout the history of subsidence, the most important of these being the great boundary fault with a throw of several thousand metres.

It has been argued that the Akwapim escarpment which strikes north-eastwards from just west of Accra is a recent fault scarp. Other researchers have pointed out that more correctly it should be regarded as part of an Akwapim fault zone in which more recent faulting has occurred along an ancient line of thrust boundaries between Birimian, Togo and Dahomeyan (Benin) formations which originated during the pan-African orogeny.

The Akwapim fault zone stretches north-eastwards through Kpong and Ho into Togo and Benin. It meets the great boundary fault at Nyanyanu in the central region. In the acute angle between the two, there are a number of other faults the most important of which is the Weija fault, striking WNW. Other numerous faults have been revealed in the vicinity of Accra.

Evidence of tectonics in Ghana

Ghana is located well clear of the major earthquake zones of the earth but nonetheless earthquakes have been known to occur in the vicinity of Accra which has experienced major shocks in 1862, 1906 and 1939 (magnitude 6.4) and numerous smaller ones, the most recent in 1997, 2003 and 2006.

Thus there is good evidence of current tectonic activity associated with the Akwapim fault zone and, to a lesser extent, with the great boundary fault. It is not surprising, therefore, that as this region is under stress then earthquake activity is concentrated around Accra where the two fault systems intersect and where within the acute angle between them are numerous subsidiary faults; for it is here that the fracture strength of the ground must be weakest. Furthermore, the vector sum of stresses associated with basin subsidence fringing the continental margin may locally increase the stresses near the intersection.