GeophysicsNames ResistivitySeismic RefractionSeismic ReflectionGravity

made by injecting a DC current into the ground through two electrodes and measuring the resulting voltage at the surface at two other electrodes. The depth of measurement is related to electrode spacing. Resisitivity measures bulk electrical resistivity which is a function of the soil and rock matrix, percentage of saturation and type of pore fluids.made by measuring the travel time of a refracted seismic wave as it travels from the surface through one layer to another and is refracted back to the surface where it is picked up by geophones. Shock or impact is made at a point, seismic waves through the surrounding soil & rock. The wave speed relating to the density and bonding characteristics of the material. The velocity is determined. The magnitude of the velocity is than utilized to identified the material The travel time of a seismic wave is a function of soil and rock density and hardness.measures the travel time of seismic waves from the ground surface downward to a geologic contact where part of the seismic energy is reflected back to geophones at the surface while the rest of the energy continues to the next interface. The travel time of the seismic wave is a function of soil and rock density and hardness.Detect changes in the earth's gravitational field caused by local changes in the density of the soil and rock or engineered structures

Uses Resistivity measurements are primary used for soundings to determine depth and thickness of geologic strata; Also can be applied to profiling measurements for locating anomalous geologic conditions, detecting and mapping contaminant plumes, locating buried wastes and mineral exploration; Can be used for azimuthal measurements to determine fracture orientation.Primary application for seismic refraction is for determination of depth and thickness of geologic strata, structure and anomalous conditions; Depth can be calculated under each geophone to produce a detailed two dimensional top of rock profile; Detail is inversely proportional to geophone spacing; If compressional P-wave and shear S-wave velocities are measured, insitu elastic moduli of soil and rock can be determined; Can be used for azimuthal measurements to determine fractureorientation; Also has application for evaluation of man-made structures.Primary application is for determination of depth and thickness of geologic strata, structural and anomalous conditions.Standard gravity measurements are primarily applied to characterizing geologic structure using widely spaced stations (100's to 1,000's of feet apart). Microgravity measurements can be used to characterize detailed localized geologic conditions (such as bedrock channels, caves, and abandoned tunnels and mines) usually within the upper few 100 feet. Microgravityuses closely spaced stations (a few feet to about 50 feet) and a micro gravimeter (capable of reading to a few microgals).

Advantages Good vertical resolution (sounding);

May also be used for profiling; Measurements can be easily made to depths of few hundred feet or more; Various electrode configurations are available for different applications.Typical measurements are less than 100 feet but can easily made to greater depths, if necessary; Can resolve up to 3 to 4 layers; Can provide depth under each geophone; Both P and S waves can be determined; The source of seismic energy can be as simple as 10 pound sledge hammer.Provides a high resolution cross section (as compared to refraction) ofsoil/rock along profile line; The high resolution method uses frequencies of up to a few 100 Hz; Measurements can be made from about 50 feet to a few 1,000 feet deep; Measurements to these depths can often be made without explosives,often using a 10 pound sledge hammer as a seismic source; The survey line length (source to farthest geophone) is usually 1 to 2times the desired depth of investigation (much less than that required forrefraction measurements); Both P and S waves can be measured.Provides a means to characterize conditions in geologic and culturalenvironments, where other geophysical methods may fail; Does not require intrusive ground contact; Data can be interpreted to provide estimates of depth size and the natureof the anomaly; Can be used inside buildings and structures.

Disadvantages Requires intrusive contact with the ground;

Station measurements only;

Electrode array can be quite long, with outermost electrode spacing from 9to 18 times the depth of interest;

Susceptible to interference from nearby metal fences, buried pipes,cables, etc.;

Generally, cannot be used over asphalt or concrete;

Effectiveness decreases at very low resisitivity values (use electromagnetic measurements).The survey line length (source to farthest geophone) may be 4 to 5 timesthe desired depth of investigation;

Requires intrusive contact with the ground; Station measurement only; Sensitive to acoustic noise and vibrations; Seismic velocity of layers must increase with depth; Will not detect thin layers or layers with inverted velocities; Deeper measurements will require explosives as an energy source. The velocities of the primary (Vp) and secondary (Vs) waves are related toelastic constants and density of the medium by equations:

Where, K = bulk modulusG = shear modulusP = density The velocity of the S wave in most rocks is about one half that of the velocity of the P wave. The S wave is not propagated at all in fluidsRequires intrusive contact with the ground;

Station measurement only;

Sensitive to acoustic noise and vibration;

Can require extensive processing.Station measurements only;

Instruments carried by hand only;

Requires base station for drift corrections;

Requires accurate elevation measurements;

The process of making microgravity measurements is a relatively slow andtedious in the field and requires extensive processing and corrections;

Susceptible to cultural and natural vibrations.