PENYELIDIKAN GEOTEKNIK

Geofisika

Penyelidikan Geoteknik Untuk Rancangan Tambang Terbuka

Hidrologi dan Hidrogeologi

PemetaanGeologi

Pemboran Inti

Core Logging

Seismik Core SamplingGeolistrik Karakteristik akifer

Bulk Sampling

Pengujian Laboratorium

Passive Method

Active Method

Resistivity Method - Observation of electric fields caused by current introduced into the ground as a means of studying earth resistivity in geophysical exploration. Resistivity is the property of a material that resists the flow of electrical current.*

ELECTRICAL METHOD

Electrical methods employ a variety of measurements of the effects of electrical current flow within the Earth. The phenomena that can be measured include

current flow, electrical potential (voltages), and electromagnetic fields. A summary of the better-known electrical methods is given below. In this set of notes we will consider only one of these methods, the DC resistivity method, in greater detail.

DC Resistivity - This is an active method that employs measurements of electrical potential associated with subsurface electrical current flow generated by a DC, or slowly varying AC, source. Factors that affect the measured potential, and thus

can be mapped using this method, include the presence and quality of pore fluids can be mapped using this method, include the presence and quality of pore fluids and clays. Our discussions will focus solely on this method.

Induced Polarization (IP) - This is an active method that is commonly done in conjunction with DC Resistivity. It employs measurements of the transient (short-term) variations in potential as the current is initially applied or removed from the

ground, or alternatively the variation in the response as the AC frequency is changed. It has been observed that when a current is applied to the ground, the ground behaves much like a capacitor, storing some of the applied current as a

charge that is dissipated upon removal of the current. In this process, both capacitative and electrochemical effects are responsible. IP is commonly used to detect concentrations of clay, and electrically conductive metallic mineral grains.

• Self Potential (SP) - This is a passive method that employs measurements of naturally occurring electrical potentials commonly associated with shallow electrical conductors, such as sulfide ore bodies. Measurable electrical potentials have also been observed in association with groundwater flow and certain biologic processes. The only equipment needed for conducting an SP survey is a high-impedance voltmeter and some means of making good electrical contact to the ground.

• Electromagnetic (EM) - This is an active method that employs measurements of a time-varying magnetic field generated by induction through current flow within the earth. In this technique, a time-varying magnetic field is generated at the surface of the earth that produces a time-varying electrical current in the earth through induction. A receiver is deployed that compares the magnetic field produced by the current-flow in the earth to that generated at the source. EM is used for locating conductive base-metal deposits, for locating buried EM is used for locating conductive base-metal deposits, for locating buried pipes and cables, for the detection of unexploded ordnance, and for near-surface geophysical mapping.

• Magnetotelluric (MT) - This is a passive method that employs measurements of naturally occurring electrical currents, telluric currents, generated by magnetic induction from electrical currents in the ionosphere. This method can be used to determine electrical properties of materials at relatively great depths (down to and including the mantle) inside the Earth. In this technique, a time variation in electrical potential is measured at a base station and at survey stations. Differences in the recorded signal are used to estimate subsurface distribution of electrical resistivity.

seismic• Refraction Seismology -Refraction experiments are based on the times of

arrival of the initial ground movement generated by a source recorded at a variety of distances. Later-arriving complications in the recorded ground motion are discarded. Thus, the data set derived from refraction experiments consists of a series of times versus distances. These are then interpreted in terms of the depths to subsurface interfaces and the speeds at which motion travels through the subsurface within each layer. These speeds are controlled by a set of physical constants, called elastic parameters that describe the material. parameters that describe the material.

• Reflection Seismology - In reflection experiments, analysis is concentrated on energy arriving after the initial ground motion. Specifically, the analysis concentrates on ground movement that has been reflected from subsurface interfaces. In this sense, reflection seismology is a very sophisticated version of the echo sounding used in submarines, ships, and radar systems. In addition to examining the times of arrival of these, reflection seismic processing extracts information about the subsurface from the amplitude and shape of the ground motion. Subsurface structures can be complex in shape but like the refraction methods, are interpreted in terms of boundaries separating material with differing elastic parameters.

Rock properties

P wave and S wave

Modulus elastsitas

SEISMIC METHOD

Material P wave Velocity (m/s) S wave Velocity (m/s)

Air 332

Water 1400-1500

Petroleum 1300-1400

Steel 6100 3500

Concrete 3600 2000

Granite 5500-5900 2800-3000

Basalt 6400 3200

Sandstone 1400-4300 700-2800

Limestone 5900-6100 2800-3000

Sand (Unsaturated) 200-1000 80-400

Sand (Saturated) 800-2200 320-880

Clay 1000-2500 400-1000

Glacial Till (Saturated) 1500-2500 600-1000

Siklus HidrologiSiklus Hidrologi

air permukaan recharge

13/11/200313/11/2003 Luknanto@tsipil.ugm.ac.id Luknanto@tsipil.ugm.ac.id 1010

aliran air tanah

air permukaan

lapisan kedap air

CONTOH PENYELIDIKANGEOTEKNIK

• CORE SAMPLING

• PENGUKURAN MUKA AIR TANAH

• PEMASANGAN PIEZOMETER

Uji di laboratorium

• Parameter sifat fisik

• Parameter sifat mekanik

SIFAT FISIK

• DENSITAS

• POROSITAS

• KANDUNGAN AIR

• KEKERASAN• KEKERASAN

• ABRASIVITAS

• PERMEABILITAS

• KEMAMPUAN MERAMBATKAN GEL