Geological investigations for any exploitable mineral resources can be performed using invasive (disturbing geological structures - such as trenches, pits, drillings, mines) or non invasive methods (geophysical methods). These are low cost and step by step more relevant, due to advanced technology of equipments production and performant software used for data processing. Geophysical measurements can reveal:
- Thickness of the cover deposits;
- Faults and fissuring zones
- Hypogene and supergene alterations;
- Metallic ores and mineralization;
- Sterile intercalations or allogene bodies inside the utile rock volume;
- Extension of the exploitable bodies;
In a complex geological investigations, combining two or more complementary geophysical methods, such as electrical, magnetic and seismic, always an increased degree of the geological knowledge will be obtained.
Higher costs can be expected when we use electromagnetic methods, such as MT, AMT, TDEM, radar or gravity measurements.
The geophysical method is chosen in connection with some criteria such as: resource’s nature, geological context, estimated depth, extension, precision demanded for estimation, time to perform investigations, maximum cost allowed.
Some examples related to geophysical investigations for natural resources are presented in the followings.
1. A Triassic basalt lava flow ( 1, 2, 3 ) was investigated using VES (Vertical Electrical Soundings), surface detailed magnetic measurements and wells. Submarine pillow lavas include flysch type sediments, and they are covered by carbonate deposits. The entire structure was folded in alpine tectonic events, have NW-SE direction, and they are affected by transversal faults on NE-SW directions.
2. Geophysical investigations to delineate a microgranite-porphyric body inside of a hill consist of VES Schlumberger and magnetic detailed measurements. Results show the development of the alteration zone at the upper part of the body, the presence of some directional faults with alteration zones along them, the presence of an important transverse fault, crossing the body in NE-SW direction and a consistent loess deposits toward the base of the hill ( 5, 6, 7, 8 ).
3. In the stack of Jurassic carbonate rocks belonging to Casimcea sincline – Central Dobrogea frequently clay layers are intercepted. ERT measurements performed in a small area near by a limestone quarry ( 9, 10, 11 )revealed the presence of a normal fault separating a downside compartment, with altered limestones and clay inclusions, and an uplifted side, made of compact limestones.
4. One of the most efficient methods to investigate river sand-gravel deposits are VES and ERT, due to their capability to delineate such depositional structures at depth or in surface. Vertical cross sections, resistivity tendency maps at different elevations/depths or 3D diagrams are relevant to describe the development of river aggregate deposits ( 13, 14 ).
The depositional characteristics, such as shape and dimensions, depend mainly on transport speed. In the upper part of river courses frequently torrential deposits, having linear shape, occure. Sometimes they have prolluvial character, often being recognized the supply channel and the distal part having a fan shape, where transport energy fades and material is granulometric sorted.
In the lower part of river courses, where erosion, resedimentation and meanders are present on large scale, the depositional structures are more complicated and interbedded layers frequently occure. For this reason, such an areas must be investigated with higher density of sampling points.
5. Electrical methods are also useful to investigate coal bearing Pliocene deposits from the Northern limit of Dacian Basin – Subcarpathian Hills. There are good enough resisitivity contrasts between productive coal layer and clays beneath and covering them. The cover rocks thickness, the coal layer thickness and structural characteristics ( flexures and breaks caused mainly by sin or post depositional landslides 15, 16, 17 ) can be easy revealed by this kind of geophysical measurements.
6. Metallic ore bodies can be investigated using different geophysical methods or techniques, related to their nature and characteristics. Electrical (resistivity, IP, SP, mise-a-la masse), electromagnetic, magnetic and radiometric methods are most used to prospect metallic mineralization.
7. Large dimensions and increased depths where oil and gas deposits or thermal water are placed involves, except seismics, some special methods, useful in preliminary investigation. They are able to delineate the major structural characteristics and sometimes even porous or fisural reservoirs . AMT, MT, CSAMT and similar methods, due to spectacular improvements of the equipments, became very strong instruments to prospect, explore or monitoring deep structures and processes.