Subsurface Geology and Petroleum Exploration

Subsurface Geology

• Subsurface geology studies the physical properties and location of rocks and soil below the surface.
• Shallow subsurface studies are called geotechnical reports, crucial for safe building construction and groundwater exploration in areas with geologic hazards.
• Deep subsurface studies are exploration reports, necessary for petroleum prospecting and development.
• Subsurface geologists use data to create resource maps, interpret geologic maps, understand structures like faults and folds, and make 3D models of geological settings.

Petroleum Exploration

• Subsurface geological methods are increasingly important in petroleum exploration and production.
• Growing demand for subsurface understanding leads to more specialists in this field.

Groundwater

• Subsurface studies are crucial for understanding and protecting groundwater aquifers to maintain water supplies.
• These studies help understand water flow, aquifer storage, and contaminant entry.

Subsurface Data Collection Methods

• Boreholes: Primary method for subsurface information. Drilling brings rock samples for on-site and laboratory study; tests can be done in open boreholes. Data includes material properties, rock variations, fault locations, and water/bedrock depths.
• Seismic Surveys: Analyze seismic waves reflected or refracted from subsurface layers. Active seismic uses generated waves (e.g., hammer, air-gun, or explosives); passive seismic uses natural waves. Surveys can be conducted on land or water, with varied depths (tens of feet to kilometers). Wave speed reveals layer composition and arrangement.
• Gravity and Magnetics: Often used together for subsurface characterization. Gravity surveys identify density differences in rock formations (useful for bedrock depth and basin basement depth). Magnetic surveys identify magnetic contrasts in rocks (useful for fault, fold, and magnetic mineral mapping). Ground or airborne surveys are possible, with ground data offering higher resolution. Combined analysis is needed.
• Ground Penetrating Radar (GPR): Images the subsurface using radar pulses. Similar to radio waves bouncing off an aircraft, radar signals bounce off subsurface layers, showing faults, contacts, and objects. Depth and resolution vary based on frequency and soil conditions. Typically depths are less than 100 feet.
• Well Cuttings: Samples produced from drilling operations. Samples at regular intervals establish a lithologic record and are plotted on a strip sample log.